Download Astronomical Facts `n Stuff

Astronomical Facts ‘n Stuff
Mini Encyclopedia of Astronomical terms, abbreviations, objects & facts.
Contents
Version History ......................................................................................................................................... 2
Glossary of Astronomical & Astrophotography Terms ............................................................................... 3
Astronomy Acronyms & Abbreviations .................................................................................................. 150
Spectral Star Classes ............................................................................................................................ 159
Stellar Evolutionary Paths .................................................................................................................... 160
The Solar System .................................................................................................................................. 161
Planetary Information .......................................................................................................................... 163
Asteroids from the Main Belt ................................................................................................................ 164
Asteroids from the Kuiper Belt ............................................................................................................. 166
Modern Constellations .......................................................................................................................... 167
The Greek Alphabet .............................................................................................................................. 170
Meteor Showers .................................................................................................................................... 171
Significant People in Astronomy ........................................................................................................... 173
SI Units and Specifics ........................................................................................................................... 176
Unofficial Units and Specifics ................................................................................................................ 177
Formulas .............................................................................................................................................. 178
The Periodic Table ................................................................................................................................ 196
The Electromagnetic Spectrum ............................................................................................................. 197
Lens & Mirror Characteristics................................................................................................................ 198
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Version History
Version
Date
1.0.0
20 Jul 2012
Original document release
1.0.1
12 Aug 2012
Added Lens & Mirror characteristics, Electromagnetic spectrum, Solar System quick info, more terms.
1.1.0
6 Sep 2012
Added more definitions, Added more Constellation Information, Added Significant People, Changed page format to landscape.
1.2.0
30 Sep 2012
Separated Asteroids & Kuiper Belt from Solar System, Added Meteor Showers, Added Formulae, Added Planetary Info.
1.2.1
3 Oct 2012
Added more, and tidied up terms.
4 October 2012
Changes made
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Glossary of Astronomical & Astrophotography Terms
..A
Abbe
See “Eyepiece”
Abell Cluster
A galaxy cluster that belongs to the Abell catalogue. This is a listing of more than 4,000 galaxy clusters that meet certain
criteria, one of which is having at least 30 galaxies within a set magnitude range. The catalogue is divided into five groups of
richness according to how many galaxies the cluster contains. Class 0 clusters contain between 30 and 49 galaxies, and class 5
clusters contain more than 299 galaxies.
Aberration
Any optical defect and/or design error which causes any of the processed light to deviate from reaching the focal point, therefore
reducing the quality of the image.
Aberration of Starlight
The apparent displacement of a star's position as a consequence of Earth's motion through space and the finite speed of light.
Ablation
The vaporisation of the surface layers of a body entering the atmosphere as a consequence of the heating that results from the
compression of air ahead of it.
Absolute Brightness (Absolute Magnitude)
A measure of the true brightness of an object. The absolute brightness or magnitude of an object is the apparent brightness or
magnitude it would have if it were located exactly 32.6 light-years (10 parsecs) away.
Absolute Magnitude
A scale for measuring the actual brightness of a celestial object without accounting for the distance of the object. Absolute
magnitude measures how bright an object would appear if it were exactly 10 parsecs (about 33 light years) away from Earth.
Absolute Zero
This is the coldest temperature theoretically possible (-273.15 degrees Celsius), where the motion of atoms in a material would
stop completely, leaving them only with a small amount of quantum mechanically energy.
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Absorption
The process by which light transfers its energy to matter. For example, a gas cloud can absorb starlight that passes through it.
After the starlight passes through the cloud, dark lines called absorption lines appear in the star’s continuous spectrum at
wavelengths corresponding to the light-absorbing elements.
Absorption Line
A dark line in a continuous spectrum caused by absorption of light. Each chemical element emits and absorbs radiated energy at
specific wavelengths, making it possible to identify the elements present in the atmosphere of a star or other celestial body by
analysing which absorption lines are present.
Accretion
The accumulation of dust and gas onto larger bodies such as stars, planets, and moons
Accretion Disk
A rotating disk of gas surrounding a compact object (such as a neutron star or black hole), formed by material falling inward.
Achromatic Lens
A lens with two or more elements, usually of differing glass types, designed to produce an image substantially free from false
colour and bringing most of the viewed colours to a sharp focus.
Active Galactic Nucleus
A very bright, compact region found at the centre of certain galaxies. The brightness of an active galactic nucleus is thought to
come from an accretion disk around a supermassive black hole. The black hole devours matter from the accretion disk, and this
in-fall of matter provides the firepower for quasars, the most luminous type of active galactic nucleus.
Active Galaxy
A galaxy possessing an active galactic nucleus at its centre.
Actual Field of View
A characteristic of eyepiece/telescope combinations. The actual field of view is the angle, measured on the sky, from one edge of
the eyepiece field of view to the other. It is approximately equal to the apparent field of view divided by the magnification.
Adaptive Optics or Active Optics
Guiding on steroids. In most A/O units a lens is moved in real time to correct for movements in the guide star utilising
refraction. This differs from traditional guiding where corrections are sent to the mount over periods of ~0.5 to 5 seconds. At
long focal lengths adaptive optics can be better than normal guiding.
Advanced Camera for Surveys
An optical camera aboard the Hubble Space Telescope that uses CCD detectors to make images. The camera covers twice the
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area, has twice the sharpness, and is up to 10 times more efficient than the telescope’s Wide Field and Planetary Camera 2. The
ACS wavelength range spans from ultraviolet to near-infrared light. The camera’s sharp eye and broader viewing area allow
astronomers to study the life cycles of galaxies in the remotest regions of the universe.
Aerogel
An advanced material used by the Stardust spacecraft to capture small particles of cometary dust. It is 99.8 per cent air and
was effective at slowing down the cometary particles gently so that they weren’t damaged.
Aerosol
Refers to small particles suspended in the air, as opposed to large particles like snowflakes and raindrops. Aerosols make the air
look hazy by scattering light.
Afocal Coupling
The technique of imaging through a camera lens held up to the eyepiece of a telescope. It is used for cameras with nonremovable lenses.
Afterglow
The fading fireball of a gamma-ray burst — a sudden burst of gamma rays from deep space — that is observable in less energetic
wavelengths, such as X-ray, optical, and radio. After an initial explosion, an expanding gamma-ray burst slows and sweeps up
surrounding material, generating the afterglow, which is visible for several weeks or months. The afterglow is usually extremely
faint, making it difficult to locate and study.
Airglow
The faint background glow in the night sky caused by gas in the ionosphere. Because of airglow the night sky is never completely
dark as seen from Earth's surface.
Airy Disk
The Airy disk is the brightest spot formed by a star image as seen through a telescope. It is surrounded by alternating rings of
light and dark (these are due to diffraction - any light passing through an aperture is diffracted, and the effect is inversely
proportional to the size of the aperture.) An optical system of good quality increases the relative brightness of the central Airy
disk compared to the surrounding diffraction rings. (Defocusing a star image will accentuate the diffraction rings and is
sometimes useful for assessing a telescope’s optics.).
Albedo
The reflective property of a non-luminous object. A perfect mirror would have an albedo of 100% while a black hole would have
an albedo of 0% (The albedo of Earth is 0.36, that of the Moon is 0.07).
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Alfven Waves
These are transverse waves that travel through electrically conducting fluids or gases in which a magnetic field is present, like
the Sun’s plasma.
All-Sky Camera
This has a lens (or element of its design) that enables it to image the majority, if not all, of the sky.
Almanac
A book of tables detailing the positions, times and other data about celestial objects, usually produced annually.
Alpha Particle
A cluster of two protons and two neutrons; a helium nucleus.
Alpha Process
A process by which lighter elements capture helium nuclei (alpha particles) to form heavier elements. For example, when a
carbon nucleus captures an alpha particle, a heavier oxygen nucleus is formed.
Alt-Azimuth Mount
A type of telescope mount that is simpler to construct than an equatorial mount, but requires simultaneous movement about the
vertical (altitude) and horizontal (azimuth) axes to track a celestial object.
Altitude
The angular distance from the observer's horizon, usually taken to be that horizon that is unobstructed by natural or artificial
features (such as mountains or buildings), measured directly up from the horizon toward the zenith; positive numbers indicate
values of altitude above the horizon, and negative numbers indicate below the horizon.
Aluminising
The process of coating a telescope mirror with a thin layer of vaporised aluminium.
Amplifier Glow
Known as 'amp glow' this is the heat given off by the electronics in the camera showing up on the image due to heating of the
sensor.
Analemma
Observed every day from the same location at the same time, the Sun follows a figure-of-eight path through the sky. Known as
an analemma, this pattern is due to the tilt of the Earth’s axis as it orbits the Sun.
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Anastigmatic
Describes an optical system which is corrected for spherical aberration, coma, and astigmatism. The point is, that in many
common optical designs, spherical aberration and coma tend to be more serious problems than astigmatism, so there isn't much
point in correcting astigmatism unless the other two are already dealt with.
Andromeda Galaxy
The large spiral galaxy located some 700,000 parsecs from the sun; the most distant object visible to the unaided eye.
Angstrom
Unit of length convenient for measuring wavelengths of electromagnetic radiation: 1 A = 10-10m.
Angular Diameter
The apparent size of an object, usually expressed in degrees, minutes, or seconds of arc.
Angular Distance
The apparent distance between two objects on the celestial sphere measured in degrees, minutes or seconds of arc.
Angular Momentum
A measure of the mass, radius, and rotational velocity of a rotating or orbiting body. In the simple case of an object in circular
orbit, the angular momentum is equal to the mass of the object times its distance from the centre of the orbit times its orbital
speed.
Angular Resolution
The ability of an instrument, such as a telescope, to distinguish objects that are very close to each other. The angular resolution
of an instrument is the smallest angular separation at which the instrument can observe two neighbouring objects as two
separate objects. The angular resolution of the human eye is about a minute of arc. As car headlights approach from a far-off
point, they appear as a single light until the separation between the lights increases to a point where they can be resolved as two
separate lights.
Annular Eclipse
A solar eclipse where the Moon passes directly in front of the Sun but is too far from Earth to completely cover the solar disc. At
the peak of an annular eclipse, the Moon is surrounded by a ring of sunlight.
Anomaly
The angle at the Sun between a planet and its perihelion.
Ansae
Literally handles. Originally a description of the appearance of Saturn's rings before they were recognised as being a ring
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system. Now used to describe (i) the extension of Saturn's rings outside the disc of the planet, and (ii) extensions from the
central star of some planetary nebulae (due to bipolar outflow of material).
Antapex
(plural antapexes or antapices) The point opposite the solar apex.
Antarctic Circle
The line of latitude on Earth’s surface that is 23.5 degrees north of the South Pole. The Antarctic Circle marks the northernmost
point in the southern hemisphere that experiences the midnight sun.
Anthelion
(plural anthelia) A faint, white halo rarely seen in the sky opposite the sun on the parhelic circle.
Antimatter
A type of matter in which each particle (antiproton, antineutron, etc.) is opposite in charge and certain other properties to a
corresponding particle (proton, neutron, etc.) of the same mass of the ordinary type of matter from which the solar system is
made. Particles of antimatter are known to exist, but it is not known why matter is dominant in this region of the universe or
whether regions exist in which antimatter is common.
Apastron
For an orbit around a star, the farthest point from that star.
Aperture
The size of the opening through which light passes in an optical instrument such as a camera or telescope. A higher number
represents a smaller opening while a lower number represents a larger opening.
Aperture Fever
An amateur astronomer who is eager to the point of obsession, to have telescopes of ever larger clear aperture, is said to have
aperture fever.
Aphelion
The point in the orbit of a solar-system object where it is farthest from the sun.
Aplanatic
This describes the type of telescope which has been made so that its optical systems show no signs of aberrations from spherical
aberration and coma.
Apoapse (also Apocentre)
The position in an orbital path that is the greatest distance from the primary body.
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Apoapsis
The point in an orbit when the two objects are farthest apart. The opposite of periapsis.
Apochromat
A type of refractor objective made from usually three lenses of different materials, selected to bring three colours of light to the
same focal point, thereby bringing the colours that are in between to very nearly the same focal point.
Apo(chromatic)
A lens system that brings all three primary colours to the same point of focus, and is thus considered free of chromatic
aberration. (Some consider that a true apochromatic lens ought to eliminate spherical aberration also.)
Apocenter
The furthest point of an astronomical object in an elliptical orbit to its centre of attraction (the principal focus of the ellipse).
Apogee
Moment of farthest distance from the Earth of a body in orbit around the Earth.
Apollo Asteroid
An asteroid whose orbit brings it closer than 1 astronomical unit to the sun.
Apparent Field of View
The angular size of the field as seen through a particular eyepiece without a telescope. (Varies with different designs, and is
often specified on the eyepiece. Roughly; Orthoscopics 40 degrees, Plossls 50 degrees, Superwides 60-70 degrees, Ultrawides
70+ degrees).
Apparent Horizon
Where the sky appears to meet the Earth. Because of perspective effects, different observers generally have different apparent
horizons. Because of refraction, even the sea horizon usually lies above the geometric horizon.
Apparent Magnitude
The apparent brightness of an object in the sky as it appears to an observer on Earth. Bright objects have a low apparent
magnitude while dim objects will have a higher apparent magnitude.
Apparition
The period or time when an object is visible and well placed for observation.
Appears
Local time at which the satellite appears visually. The first figure indicates the visual brightness of the object. The smaller the
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number, the brighter and more eye-catching it appears to an observer. The units are astronomical magnitudes [m]. Azimuth is
given in degrees counting from geographic north clockwise to the east direction. The three-character direction code is given as
well. In case the satellite exits from the Earth shadow and comes into the glare of the Sun, the elevation above horizon is given
in degrees for this event. If this figure is omitted, the satellite is visible straight from the horizon.
Appulse
The apparent close approach of two celestial bodies as seen from Earth, such as a star and a planet, or two planets for example.
Apsides
Bodies in an elliptical orbit all reach a point when they are furthest or closest to their parent object, eg. a planet around a star.
Apsides is a collective term for these points. Earth’s apsides are its perihelion and aphelion.
Arc (measurement of)
Angles on the celestial sphere, measured in degrees, minutes and seconds. Arc may be an expression of the angular distance
between two celestial objects or the angular size of an object.
Arcminute
A small unit of angular measurement, spanning one-sixtieth of a degree; an arcsecond is one-sixtieth of an arcminute.
Astronomers measure the separation between stars in the night sky in terms of degrees.
Arctic Circle
The line of latitude on Earth’s surface that is 23.5 degrees south of the North Pole. The Arctic Circle marks the southernmost
point in the Northern Hemisphere that experience the Midnight Sun.
Arcsecond (arcsec)
Measurement of angular separation: a 1-inch stick would subtend an angle of 1 arcsec at a viewer's eye at a distance of about
6.5 miles.
Arecibo
Short name for the National Astronomy and Ionospheric Centre (NAIC) at Arecibo, Puerto Rico; often refers only to the NAIC
1000-ft (305-m) zenith antenna, the world's largest radio astronomy collector.
Areography
The proper name for the geography of Mars.
Argument of the Pericentre
The angle, measured around the orbital plane, between the ascending node and the pericentre.
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Armillary Sphere
An instrument consisting of graduated metal circles used to represent the motions of celestial bodies around the earth.
Array
An orderly arrangement or impressive display. For radio telescopes, an array is a group of individual radio dishes that work
together. The VLA (Very Large Array) has 27 telescope dishes arranged in a “Y” pattern.
Artificial Star
This is simply an artificial point of light used to test the optics of a telescope in the absence of, or instead of, a suitable real star
for collimation and other adjustments.
Ascending Node
The position in the orbit of a planet (or the Moon) where it crosses the plane of the ecliptic, moving northward.
Aspheric
Not spherical.
Aspides
The points in the Moon's orbit where it is either closest to Earth (perigee) or farthest from Earth (apogee).
Asterism
A pattern formed by stars that aren’t necessarily in the same constellation, for example the Plough and the Summer Triangle.
Asteroid
A small planetary body in orbit around the Sun, larger than a meteoroid but smaller than a planet. Most asteroids can be found
in a belt between the orbits of Mars and Jupiter. The orbits of some asteroids take them close to the Sun, which also takes them
across the paths of the planets. Beyond the orbit of Neptune a gigantic supply of such bodies seem to exist.
Astigmatism
The distortion of an image resulting from a failure of light rays from different parts of the same zone to focus in the same plane.
Asteroseismology
The study of the internal structure of stars by analysis of the way they pulsate.
Astrobiology
The study of life throughout the Universe, its origin, evolution, ecology and destiny.
Astrograph
A photographic telescope. An astrograph was used by Clyde Tombaugh to discover Pluto in 1930.
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Astrolabe
An astronomical and navigational instrument for gauging the altitude of the Sun and stars.
Astrological Sign
One of twelve sections of the zodiac that are 30 degrees long and that correspond to the positions of the constellations as they
were about 2,600 years ago when the astrological system was established. Do not confuse an astrological sign with the
astronomical constellation of the same name, as they only partially overlap.
Astrology
A non-scientific system based on superstition, that purports to explain or predict human actions by study of celestial positions.
Astrometry
Branch of astronomy that focuses on measurements, especially those relating to positions and movements.
Astronomical Horizon
The intersection of a horizontal plane through the eye with the celestial sphere. Because the celestial sphere has an infinite
radius, two observers at different heights above sea level, but placed on the same vertical line, have the same astronomical
horizon. Also, because of dip, the astronomical horizon always lies above the sea horizon, but it usually is hidden by trees, hills,
and buildings on land. These objects then determine the observer's apparent horizon.
Astronomical Twilight
The times are the moments of beginning/end of the astronomical twilight, i.e., the moments the Sun reaches a depression of 18°
below the horizon. If the Sun is below this angle, no brightening of the sky can be observed.
Astronomical Unit
Approximately equal to the mean earth-sun distance, which is about 150,000,000 km or 93,000,000 miles. Formally, the AU is
actually slightly less than the earth's mean distance from the sun (semi-major axis) because it is the radius of a circular orbit of
negligible mass (and unperturbed by other planets) that revolves about the sun in a specific period of time.
Astronomy
Astronomy, from the Greek words astron (ἄστρον), "star", and -nomy from nomos (νόμος), "law" or "culture", is the scientific
study of the universe and the celestial bodies that reside in it, including their composition, history, location, and motion.
Astrophotography
A specialized kind of photography, comprised of concepts and techniques that pertain to the production of photographic images
of astronomical objects.
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Astrophysics
The branch of astronomy that deals principally with the physics of the universe, including luminosity, density, temperature, and
the chemical composition of stars, galaxies, and the interstellar medium.
Atmosphere
A layer of gases surrounding a planet, moon, or star. The Earth's atmosphere is 120 miles thick and is composed mainly of
nitrogen, oxygen, carbon dioxide, and a few other trace gases.
Atmospheric Distortion
The blurring of an image due to the layer of gases surrounding the surface of Earth. As starlight travels through the atmosphere,
pockets of air act like little lenses and bend the light in unpredictable ways. This distortion causes stars to appear to twinkle.
Atom
The smallest possible unit of a chemical element. When an atom is subdivided the parts no longer have properties of any
chemical element. An atom consists of a nucleus (made of protons and neutrons) with orbiting electrons.
Atom Epoch
Fourth epoch in the history of the Universe, lasting from about 100 sec to 106 years, in which matter came to dominate radiation
as the principal constituent of the Universe.
Atomic Mass Unit
Convenient unit for measuring the mass of an atom or molecule: 1 atomic mass unit is defined as I /12 the atomic mass of the
most abundant carbon isotope, 12C. A=12 for 12C.
Atomic Nucleus
Concentrated, positively charged matter at the centre of an atom; composed of protons and neutrons.
Atomic Number
The number of protons in the nucleus of an element. It is the atomic number that defines the identity of an element. Z=6 for
12C.
Atomic Theory
A model that offers a logical explanation for the law of multiple proportions and the law of constant composition by stating that
all elements are composed of atoms, all atoms of a given element are identical, but the atoms of one element differ from the
atoms of any other element; that atoms of different elements can combine to form compounds and a chemical reaction involves
a change not in the atoms themselves, but in the way atoms are combined to form compounds.
Atomic Weight
The number of protons and neutrons in an atom, averaged over the abundances of the different isotopes.
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Attitude
The orientation of a spacecraft or satellite with respect to its direction of motion.
Aureole
Another term for a Corona.
Aurora
A phenomenon produced when the solar wind (made up of energized electrons and protons) disturbs the atoms and molecules in
a planet’s upper atmosphere. Some of the energy produced by these disturbances is converted into colourful visible light, which
shimmers and dances. Auroras have been seen on several planets in our solar system. On Earth, auroras are also known as the
“Northern Lights” (aurora borealis) or “Southern Lights” (aurora australis), depending on in which polar region they appear.
Aurora Australis
Also known as the southern lights, this is an atmospheric phenomenon that displays a diffuse glow in the sky in the southern
hemisphere. It is caused by charged particles from the Sun as they interact with the Earth's magnetic field.
Aurora Borealis
Also known as the northern lights, this is an atmospheric phenomenon that displays a diffuse glow in the sky in the northern
hemisphere. It is caused by charged particles from the Sun as they interact with the Earth's magnetic field.
Auroral Corona
An auroral corona usually appears during energetic auroral displays. They are seen as rays of auroral emission coming straight
at you, where the perspective often makes it look as if they are emanating from a single point in the sky.
Auto-Guider
Or Autoguider. A CCD/CMOS device that is optically attached to a guidescope or off-axis guider and electronically attached to the
control of the telescope mount. It monitors the position of a guide object on the CCD array and adjusts the telescope's drives so
as to keep the object in the same position, thus correcting for any errors in the drive or in polar alignment. It enables longexposure photography or imaging through the main OTA without the astronomer having to make manual corrections to the drive
in response to what he sees in a guidescope.
Autumnal Equinox
The moment when the Sun crosses the celestial equator travelling in a southward direction, on or about September 22. In the
Northern Hemisphere, it marks the first day of Autumn. The term is also applied to the Sun’s position in the sky at that moment.
It is one of two points where the ecliptic and celestial equator intersect, the other being the vernal equinox.
Averted Vision
A technique for observing faint objects through a telescope by viewing slightly to the side, allowing light from the object to fall on
an area of the eye more sensitive to faint light.
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Axis
Also known as the poles, this is an imaginary line through the centre of rotation of an object.
Azimuth
Azimuth direction of the object is given in degrees counting from geographic north (0°) clockwise (to the east) direction. East is
90°, south 180°, and west 270°. The three-character direction code is given as well. For example, NNW stands for north-northwest.
..B
Back Focal Length
Classically, the distance from the last optical element of a system encountered by the light passing through it, to the focal plane.
Latterly, from the point where the beam emerges from within the assembly of optical parts, to the focal plane.
Bahtinov Mask
A patterned 'mask' that is put in front of the scope to assist in achieving perfect focus (removed when focused), named after
inventor Pavel Bahtinov.
Bailly's Beads
Bright points of light seen along the edge of the Moon just before and just after a total eclipse of the Sun. They are caused by
sunlight shining though valleys (or between peaks) at the Moon's limb.
Barlow Lens
An intermediate concave lens (or lenses) placed in front of the eyepiece of a telescope to increase the effective focal length of a
telescope. Sometimes called negative lenses. A 2x Barlow, will double the power of an eyepiece (ie. a 20mm eyepiece becomes
a 10mm).
Barred Irregular Galaxy
This is an irregular galaxy which exhibits a bar structure.
Barred Spiral Galaxy
A galaxy with a “bar” of stars and interstellar matter, such as dust and gas, slicing across its centre. The Milky Way is thought to
be a barred spiral galaxy.
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Barrel Distortion
An aberration of optical systems, in which magnification decreases with distance away from the optical axis. With such a system,
squares are imaged with their sides bulged out, looking sort of like barrels.
Barycentre
The centre of gravity of the Earth-Moon system. The Earth is 81 times the mass of the Moon and the barycentre of the EarthMoon system actually lies inside the Earth.
Baryon Particles
Any of the subatomic particles that interact through the strong nuclear force. Most commonly, these are protons and neutrons.
Their presence in the universe is determined through their gravitational and electromagnetic interactions.
Basin
Basins or impact basins are the results of massive impacts on a planetary surface. They appear as vast craters. Some lunar
basins have been covered or partially filled by lava, which has later erupted from beneath the surface.
Bayer Designation
This is given to stars in a constellation to indicate their order of brightness using the Greek alphabet; where Alpha (a) denotes
the brightest star in a constellations and Beta (b) denotes the second brightest and so on.
Bayer Mask
Coloured filters in front of camera pixels to give coloured image.
Beam Splitter
This is an optical component of a bino viewer that splits the light coming into the eyepiece holder into two beams for viewing.
BeppoSAX
A space-based X-ray observatory built and operated by the Italian Space Agency and the Netherlands Agency for Aerospace
Programs. BeppoSAX has been instrumental in identifying and locating gamma-ray bursts.
Best Seen Between / hmax
This is the best visibility time interval of the object. The calculation takes into account the magnitude of the object (required
elevation above horizon), and the elevation of the Sun. The time is given in local civil time (LCT), i.e., the time zone and
definitions as selected by you. hmax is the maximum altitude over the horizon, that the object reaches during this time period.
Bi-Polar Outflow
When stars are born they begin to form jets at their poles known as bi-polar outflows. These jets are thought to originate from
material that collapsed onto the star as it was forming. They may, in fact, be crucial to the process.
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Bias Frames
A bias frame (may also be known as 'offset') is an image taken with no actual exposure time. These images are used to adjust
the light frames (the actual pictures) to mitigate the noise inherent in the chip itself. If a camera won't take a 0-length image
(shot with the shutter closed) then a 'pseudo bias' can be taken by leaving the lens cap on, setting the camera for its fastest
shutter speed, standing in a dark closet and pressing the shutter. Take a few of them. Not to be confused with a dark frame
which uses an exposure of the same length as the light (picture) frames.
Big Bang
The initial singularity that started the space and time of our Universe, now thought to have occurred 13 to 15 billion years ago.
Big Bang Theory
A cosmological model, in which the universe was once compressed to infinite density and has been expanding ever since.
Originally a term used by unbelieving scoffers, now as widely accepted as the model itself.
Billion
One-thousand million, 109, in the USA only; elsewhere a million million, or 1012.
Binary (star system)
A system of two stars that revolve around a common centre of gravity.
Binding Energy
Energy derived from the conversion of mass to energy when neutrons and protons are combined to form nuclei.
Binning
The technique of grouping together pixels on a CCD camera chip to make larger pixels. Reduces noise, but decreases resolution.
Binocular
Two telescopes aligned side by side, so you can look at something with both eyes at once. They are traditionally labelled with
two numbers, separated by "x". So a binocular labelled "25x90" is called a "Twenty Five by Ninety; a 25 times magnification,
with an objective lens of 90mm in diameter.
Bino-Viewer
An optical gadget that splits the single beam of light coming out of a telescope into two parts, so you can observe with both eyes
at once. Bino-Viewers generally do not work with all telescopes or all eyepieces.
Biosphere
The part of the Earth, or other planet, and its atmosphere capable of supporting life
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Bit Depth
A CCD camera's analog signal has to be converted to digital values in order for a computer to work with them. This is done by
an Analog-Digital converter. The bit depth determines the number of values that can be assigned to a pixel: 256 for 8-bit and
65536 for 16-bit.
Blackbody
Body capable of absorbing energy of all wavelengths falling on it; it is also capable of radiating all frequencies in a particular ratio
to its absorbing properties. The value of the ratio depends only on the temperature of the body.
Black Dwarf
A non-radiating ball of gas that results either when a white dwarf radiates all its energy or when gas contracts gravitationally but
contains too little mass to begin nuclear fusion.
Black Hole
The collapsed core of a massive star. Stars that are very massive will collapse under their own gravity when their fuel is
exhausted. The collapse continues until all matter is crushed out of existence into what is known as a singularity. The
gravitational pull is so strong that not even light can escape.
Black Hole Stars
Astronomers know that in the early Universe, black holes with masses many times that of the Sun roamed space. But they’re
not sure where these monsters came from. Enter a new theory – black hole stars. Black hole stars were supermassive stars in
the early Universe. At their centres were massive black holes, formed by crushing gravity. As a supermassive star became more
massive and grew, the black hole at its centre would have gorged on the star’s gas and would also have grown. Eventually the
star would begin to die as it cooled down and, in doing so, would expose the massive black hole at its heart.
Blazar
These are powerful jets of radiation coming from the heart of a quasar (see ‘Quasar’) that we are viewing ‘head on’.
Blue Moon
Refers to either:
The third full moon in a season with four full moons.
The second full moon of a calendar month.
The moon tinted towards blue as it appears in the sky, caused by dust or smoke in the atmosphere
Blueshift
The shortening of a light wave from an object moving toward an observer. For example, when a star is traveling toward Earth, its
light appears bluer.
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BL Lacertæ objects
Objects which are strong emitters of infra-red radiation. They are very luminous and remote and are thought to have similar
properties to quasars.
Bode's law
The physical laws describing the properties of a numerical scheme that roughly gives the radii of the orbits of the seven
innermost planets and the radius of the asteroid belt. More numerology than science.
Bohr atom
Nils Bohr's model of the hydrogen atom, in which the energy levels are depicted as concentric circles of radii that increase as
(level number) 2.
Bok Globule
A Bok Globule is a dense cloud of dust and gas in space thought to be important in the role of star formation, named after the
astronomer Bart Bok.
Bolide
An asteroid or comet that explodes, crashes onto the Earth or another body in the Solar System, generating a huge fire-ball.
Bolometer
An instrument used to measure heat radiation.
Bolometric Magnitude
The magnitude of a celestial object corrected to take account of the radiation in parts of the spectrum other than the visible,
therefore it is the total sum of all emissions received from an object from the whole electromagnetic spectrum.
Boundary Layer
A relatively thin layer of fluid next to a boundary (such as a solid surface). For example: The layer of air next to the Earth's
surface (the “planetary boundary layer”). Ordinarily, only the boundary layer is appreciably affected by the properties of the
surface. The rest of the fluid is unaffected, so the part of the atmosphere above the boundary layer (which is typically a few
hundred meters thick) is called the “free atmosphere.”
Bracketing
Bracketing is an imaging method where you take several exposures (of the same object) with slightly different settings either
side of the planned settings, to see what works best.
Brandon
See “Eyepiece”
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Brown Dwarf
An object substantially (~13 x) larger than Jupiter but with a mass no more than 40 per cent that of the Sun. These objects are
not big enough for gravitational collapse to heat them to the point that nuclear reactions can be triggered. Brown dwarfs may be
very common in the universe and could even have planets in a habitable zone.
Bulb (or B setting)
This is a setting usually seen on old SLR cameras (and now seen in a different guise in the manual mode on some DSLRs), which
allows the shutter of the camera to be left open for an extended period of time. This is especially crucial for long-exposure work
where the shutter may have to be open for several minutes to capture enough light from a celestial object.
Burster
A sporadic source of intense X rays, probably consisting of a neutron star onto which new matter falls at irregular intervals.
..C
Calcium-K
Calcium-K is a specific wavelength (roughly 393.3 nanometres) of ultraviolet light, emitted by calcium atoms that are missing
one electron. Specific solar telescopes filter all of the Sun’s light apart from this particular wavelength.
Caldwell Catalogue
A listing of 109 bright star clusters, nebulae and galaxies that weren’t included in the Messier catalogue. The catalogue was
compiled by Sir Patrick Caldwell-Moore.
Captured rotation
Rotation of an object that spins at the same rate as that object takes to orbit another object. Sometimes referred to as
synchronous rotation. The Moon is a good example of an object that has captured rotation.
Carbon-14 (14C)
A radioactive isotope of carbon produced in the upper atmosphere and present in living plants and animals that can be used in
carbon-14 dating because it decays to nitrogen (14N) and a beta ray with a half-life of about 5,730 years.
Carbonaceous Chondrite
A meteorite containing controls, with a high abundance of carbon and other volatile elements.
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Carbon Cycle
A chain of nuclear reactions, involving carbon at its intermediate stages, that transforms four hydrogen atoms into one helium
atom with a resulting release in energy. The carbon cycle is only important in stars hotter than the sun.
Carbon stars
Red stars of spectral type 'R' and 'N' which have carbon-rich atmospheres.
Carey Mask
A patterned 'mask' that is put in front of the scope to assist in achieving perfect focus (removed when focused).
Cassegrain (telescope or focus)
Cassegrain telescope is a reflecting telescope that uses two curved mirrors of particular shapes to form the image: The primary
mirror is a concave paraboloid, just as in a Newtonian. The secondary mirror is a much smaller convex hyperboloid, positioned a
little way toward the primary from the primary's focal plane. The secondary reflects the converging beam of light back toward
the primary, which typically has a hole in the centre, so that the focal plane of the combined mirrors is easily accessible behind
the primary.
Catadioptric
A telescope that uses a system incorporating both lenses and mirrors to form an image. Common types: Schmidt-Cassegrain
(SCT) and Maksutov-Cassegrain (MCT, mak) telescopes.
Celestial Co-Ordinates
A system by which the position of a body on the celestial sphere is plotted with reference to a reference plane and a reference
direction. The four systems in use are Ecliptic Co-ordinates, Equatorial Co-ordinates, Galactic Co-ordinates, and Horizon Coordinates.
Celestial Equator
An imaginary line that divides the celestial sphere into a northern and southern hemisphere. Our (the Earths) equator, infinitely
projected into space.
Celestial Meridian
The line of zero right ascension in the equatorial coordinate system.
Celestial Object
A natural object which is located outside of Earth's atmosphere, such as the Moon, the Sun, an asteroid, planet, or star.
Celestial Poles
The North and South poles of the celestial sphere.
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Celestial Sphere
An imaginary sphere of great (or infinite) radius that is centred on the earth and is used for practical purposes in astronomical
observing. Since stars are very distant from us, they make up a background that is essentially unchanging from year to year; of
course, over a period of years, the closer stars will move very slightly and factors such as precession cause a change in the
appearance of the stars in our skies over many years. But we create a map grid and some of these map grids include equatorial
coordinates (right ascension and declination), ecliptic coordinates (ecliptic longitude and latitude), and galactic coordinates
(galactic longitude and latitude) - Which refer to the earth's rotation, the earth's revolution about the sun, and the Milky Way
galaxy's plane, respectively.
Cell
The mechanical support that grips a lens or mirror, and which is in turn fastened to the telescope tube, or to some other piece of
telescope structure.
Centaurs
These are an unstable orbital class of minor planets that behave with characteristics of both asteroids and comets. Centaurs
have transient orbits that cross or have crossed the orbits of one or more of the giant planets, and have dynamic lifetimes of a
few million years. It has been estimated that there are around 44,000 centaurs in the Solar System with diameters larger than 1
km. Any centaur that is perturbed close enough to the Sun is expected to become a comet.
Central Meridian
The imaginary line through the poles of a planet that bisects the planetary disc.
Cepheid Variable
This is a variable star whose light pulsates in a regular cycle. The period of fluctuation is linked to the brightness of the star.
Brighter Cepheid’s will have a longer period.
Chandra X-Ray Observatory
A space-based X-ray observatory; also known as the Advanced X-ray Astrophysics Facility (AXAF). Chandra is designed to
observe X-rays from high-energy regions of the universe, such as hot gas in the remnants of exploded stars. The satellite was
launched and deployed in July 1999.
Chandrasekhar Limit
The mass beyond which a white dwarf must inevitably collapse into a neutron star, about 1.4 solar masses.
Chaos
A distinctive area of broken terrain.
The unordered state of matter in classical accounts of cosmogony.
The hypothetical first epoch in the history of the Universe, lasting 10" sec: a period about which we cannot yet even speculate.
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Charge-Coupled Device
An electronic detector that records visible light from stars and galaxies to make photographs. These detectors are very sensitive
to the extremely faint light of distant galaxies. They can see objects that are 1,000 million times fainter than the eye can see.
CCDs are electronic circuits composed of light-sensitive picture elements (pixels), tiny cells that, placed together, resemble mesh
on a screen door. The same CCD technology is used in digital cameras.
Charles' Law
For a given mass of gas at constant pressure, the volume varies directly with the temperature (on the absolute scale).
Charm
An arbitrary name that corresponds to a property that distinguishes certain elementary particles, including types of quarks, from
each other.
Chemical Compound
A pure substance consisting of atoms or ions of two or more different elements. The elements are in definite proportions. A
chemical compound usually possesses properties unlike those of its constituent elements. For example, table salt (the common
name for sodium chloride) is a chemical compound made up of the elements chlorine and sodium.
Chemical Evolution
The chemical (i.e., pre-biological) changes that transformed simple atoms and molecules into the more complex chemicals
needed for the origin of life. For example, hydrogen atoms in the cores of stars combine through nuclear fusion to form the
heavier element helium.
Chemically Peculiar Stars
Stars manifesting anomalies in the relative abundances of elements, which may arise from mechanical rather than nuclear
effects; so-called manganese stars, for example, show a great overabundance of manganese and gallium, usually accompanied
by excess mercury.
Chroma (or Chrominance Noise)
This is the blotchy variation in colour sometimes seen in DSLR images.
Chromatic Aberration
The introduction of false colour into an image, caused by a lens bending different wavelengths of light unevenly, which focuses
the colours in different places.
Chromosphere
The chromosphere is the layer of the Sun’s atmosphere above the visible ‘surface’ of the photosphere and beneath the outer
tenuous corona. Meaning ‘sphere of colour’, it is the place where dynamic prominences and other similar events occur.
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Chondrite
A type of stony meteorite that contains numerous small spherules of silicate (silica, silicon dioxide) minerals. A subset of this
type of meteorites, the carbonaceous chondrites, contains several per cent organic carbon.
Circumpolar Star
A star or constellation that doesn’t set (i.e. doesn’t disappear below the horizon) over the course of the night, due to its
proximity to one of the poles. The constellation of Cassiopeia is circumpolar.
Civil Twilight
The times are the moments of beginning/end of the civil twilight, i.e., the moments the Sun reaches a depression of 6° below the
horizon. On clear weather, no significant dim-out can be distinguished compared to the time of sunset/sunrise.
Clear Aperture
The diameter of the beam of light that goes into a telescope. In most cases, that is the unobstructed diameter of the front lens,
or the corrector, or the primary mirror, depending on telescope type.
Close to Moon/Sun
The satellite is closer than 1.5 degrees from the center of the Moon or the Sun, but the satellite does not cross in front of the
Moon/Sun. The direction and distance to the center line on Earth is given. For the Sun, move to the indicated center line
position and observer with proper equipment.
Close to
The Moon or main object appears close to the listed star or planet. These events may be useful for reasons of 'near miss' or to
make it easier to find the fainter object in the sky. Usually, such constellations give a nice view.
Closed Universe
A possible state of the universe. In this state, the expansion of the universe will eventually be reversed; it is characterized by
positive curvature, being finite in extent but having no boundaries. Recent observations indicate that this is unlikely to be the
true state of our universe.
Clock-Face Direction
In a simple clock-face coordinate system with the clock face superimposed on the satellite itself, with 12:00 o’clock being at the
top and 9:00 o’clock being at the left, the satellite will seem to move toward the given direction. This number is helpful when
observing with binoculars.
Cluster (Astronomical)
Group of stars numbering from a few to hundreds of thousands of stars. Galactic clusters, sometimes called open clusters,
contain up to a few hundred members and occur rather close to the plane of the Galaxy. Globular clusters contain tens of
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thousands of stars distributed about their centre in a spherical manner and are found far from the plane of the Galaxy as well as
in it toward the centre of the Galaxy.
CNO Cycle
A nuclear-fusion-reaction sequence in which hydrogen nuclei are combined to form helium nuclei, and in which other nuclei, such
as isotopes of carbon, oxygen, and nitrogen, appear as catalysts or by-products. The CNO cycle is dominant in the cores of stars
on the upper main sequence. Same as carbon cycle.
Coated
This is most commonly used to describe a lens or prism in which the surfaces where the beam of light goes from air into glass, or
from glass into air, have been coated with something that reduces the amount of light that gets reflected, and hence, wasted –
or worse, reflected into the focal path and disturbing the image. Beware: this is not the same as multi coated or fully coated.
Coelostat
A system of two moveable mirrors used in solar telescopes. The mirrors follow the Sun and keep its image in the same location
as Earth rotates.
Coherent Radiation
Radiation in which the phases of waves at different locations in a cross-section of radiation have a definite relation to each other;
in non-coherent radiation, the phases are random. Only coherent radiation shows interference.
Cold Dark Matter
This is the favoured model to describe dark matter (a mysterious matter that doesn’t emit any light). Its constituent particles
move slowly (which is why it’s called ‘cold’), and therefore it was easy for dark matter to clump together in the early Universe.
Collecting Area
The area of a telescope’s primary light-collecting mirror. A telescope’s light-gathering power rises with an increase in its
collecting area.
Colliding Galaxies
A galactic “car wreck” in which two galaxies pass close enough to gravitationally disrupt each other’s shape. The collision rips
streamers of stars from the galaxies, fuels an explosion of star birth, and can ultimately result in both galaxies merging into one.
Collimate
The term used for adjusting a telescope to gain maximum optical performance by means of alignment of the optical parts.
Collisional Process
An event involving a collision of objects; for example, the excitation of a hydrogen atom when it is hit by an electron.
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Colour (Color [USA])
.. Of an object, a visual property that depends on wavelength. The perception of colour stems from the varying spectral
sensitivity of different types of cone cells in the retina, to different parts of the spectrum, colours may be defined and quantified
by the degree to which they stimulate these cells. The colour of an object is a complex result of its surface properties, its
transmission properties, and its emission properties, all of which factors contribute to the mix of wavelengths in the light leaving
the surface of the object.
An arbitrary name assigned to a property that distinguishes three kinds of quarks.
Colour Fringing
Another name for chromatic aberration.
Colour Index
The difference B-V between the blue (B) and visual (V) magnitudes of a star. If B is less than V (that is, the star is brighter in
blue than in visual light), then the star has a negative colour index, and is a relatively hot star. If B is greater than V, the colour
index is positive, and the star is relatively cool.
Colour-Magnitude Diagram
A Hertzsprung-Russell diagram in which the temperature on the horizontal axis is expressed in terms of colour index.
Colure
A great circle that passes through the celestial poles and either the equinoxes (equinoctial colure) or solstices (solstitial colure).
Coelostat
A device, usually consisting of two mirrors, that is designed so as to reflect the light from a celestial object into a fixed
instrument, where it forms a non-rotating image.
Coma
A comet's atmosphere (composed of dust and/or various gases) surrounding its nucleus. The coma is rather tenuous, and stars
can be occasionally easily seen through it, shining from behind, and yet, the coma is usually thick enough that it masks our view
of the true nucleus of the comet, as seen from the earth. As a comet's nucleus is usually quite small, it is not able to retain its
coma for long periods of time, and the coma material gradually drifts away into space. Much coma material is thrown back into
what we see as the comet's tail. But all coma material originates in the comet's nucleus, and solar sublimation due to heating
causes gases to move outward, often in jets, taking dust material with them to form the coma and tail.
Comatic Aberration (Coma)
An optical defect in a lens, which means light rays that enter the edge of the lens at an angle converge so as not to be brought to
a sharp focus. The result is a smearing of detail towards the edge of the field of view. Coma is an optical aberration, where
stars at the edge of a field of view appear to broaden out into triangle or fan shapes, caused by an imperfection in the lens or
mirror.
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Comet
A gigantic ball of ice and rock that orbits the Sun in a highly eccentric orbit. Some comets have an orbit that brings them close
to the Sun where they form a long tail of gas and dust as they are heated by the Sun's rays.
Comet Tail
A tail is made up of dust and gas from a comet’s coma. A tail forms when the solar wind separates dust and gas from the coma,
pushing it outward and away from the Sun in either a slightly curved path (for dust) or a straight path (for gas).
Commensurability
This is the property of two objects orbiting the same body whose periods are in a rational proportion. For example, the orbital
period of Saturn around the Sun is very nearly 5/2 the orbital period of Jupiter.
Comparison Spectrum
A spectrum of known elements on earth usually photographed on the same photographic plate as a stellar spectrum in order to
provide a known set of wavelengths or zero Doppler shift.
Concave
In lenses, it is the side that goes in and causes light rays to spread apart (diverge). In mirrors, it is also the side that goes in, but
it causes light rays to meet (converge).
Conjunction
There are at least three definitions of conjunction. Bodies are said to be in conjunction when they have the same ecliptic
longitude (this is the strict definition) or when they have the same Right Ascension or when they are at their closest. Planets are
said to be "at conjunction" when they are in conjunction with the Sun. For extended bodies (e.g. Sun, Moon, planets), the
body's position is taken to be its centre.
Conjunction, Inferior
A conjunction when one of the inferior planets (Mercury or Venus) appears to lie very close to the Sun, or in line with the Sun, as
seen from Earth, but with the planet between Earth and the Sun. Obviously, a superior planet (Mars, Jupiter, Saturn, Uranus,
Neptune or Pluto) cannot be at inferior conjunction!
Conjunction, Superior
A conjunction when one of the inferior planets (Mercury or Venus) appears to lie very close to the Sun, or in line with the Sun, as
seen from Earth, but with the planet on the far side of the Sun.
Conservation Law
A statement that the total amount of some property (angular momentum, energy, etc.) of a body or set of bodies does not
change.
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Conservation of Energy and Mass
A fundamental law of physics, which states that the total amount of mass and energy in the universe remains unchanged.
However, mass can be converted to energy, and vice versa.
Constellation
A grouping of stars that make an imaginary picture of mythological figures in the sky. The International Astronomical Union
designated 88 constellations in 1922. The majority of constellations visible from the northern hemisphere are named after the
original 48 constellations in the Almagest, an astronomy work written by the ancient Greek astronomer Ptolemy sometime in the
2nd century, A.D.
Contact Binary Stars
Two gravitationally bound stars (a binary star system) that have both filled their ‘Roche lobes’. These are teardrop-shaped
regions around each star in the binary system, within which matter is gravitationally bound to each star. Any matter beyond a
star’s Roche lobe will fall onto the other star in the binary system.
Contact Eclipsing Binary
Eclipsing binary stars transit each other during their orbit around each other and their combined brightness varies as they do so.
In some cases the two stars’ outer atmospheres fill the Roche lobe making contact with each other to make a ‘contact eclipsing
binary’.
Continuously Habitable Zone
Region around a star in which a planet can maintain appropriate conditions for the existence of life (including the retention of a
significant amount of liquid water) for a period sufficient to allow the emergence of life.
Continuous Spectrum
A spectrum with radiation at all wavelengths but with neither absorption nor emission lines.
Continuum
The continuous spectrum that we would measure from a body if no spectral lines were present.
Contrast
This is the difference in brightness of an object against its background, or the difference in brightness across its surface. Since
most objects are inherently low in contrast anyway, any other light interference will further reduce contrast and obscure detail.
Convection
The transfer of heat through a liquid or gas caused by the physical upwelling of hot matter. The heat transfer results in the
circulation of currents from lower, hotter regions to higher, cooler regions. An everyday example of this process is boiling water.
Convection occurs in the Sun and other stars.
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Convection Cells
These are huge ‘bubbles’ (some the size of Jupiter) under the surface of the Sun, caused by the rising and sinking of super-hot
gas in the Sun’s interior. Smaller convection cells cause ‘granules’ on the surface. These can be observed with a specially
filtered H-alpha telescope.
Convection Zone
The region below a star's surface where energy flows outward by the rising of hot gas known as convection.
Convex
In lenses, it is the side that bulges out and causes light rays to meet (converge). In mirrors, it is also the side that bulges, but it
causes light rays to spread apart (diverge).
Corona
The outer part of the Sun's atmosphere. The corona is visible from Earth during a total solar eclipse. It is the bright glow seen
in most solar eclipse photos.
Coronagraph
A type of telescope with which the corona can be seen at times other than that of a total solar eclipse.
Coronal Mass Ejection
A vast cloud of hot plasma, mainly composed of electrons and protons, that is ejected from the surface of the Sun.
Corrector Plate
This is a lens plate that fits on the front of Schmidt-Cassegrain and Maksutov-Cassegrain telescopes to correct optical
aberrations.
Cosmic Abundances
The relative proportions of chemical elements in the Sun, the solar system, and the local region of the Milky Way galaxy. These
proportions are determined by studies of the spectral lines in astronomical objects and are averaged for many stars in our cosmic
neighbourhood. For example, for every million hydrogen atoms in an average star like our Sun, there are 98,000 helium atoms,
360 carbon atoms, 110 nitrogen atoms, 850 oxygen atoms, and so on.
Cosmic Background Radiation
The primordial radiation field that fills the universe. It was created in the form of gamma rays at the time of the big bang, but
has since cooled so that today its temperature is 3 K and its peak wavelength is near 1.1 millimetres (in the microwave portion of
the spectrum). Also known as the 3-degree background radiation. Also called cosmic microwave background radiation, CMBR.
Cosmic Ray
Atomic nuclei (mostly protons) that are observed to strike the Earth's atmosphere with extremely high amounts of energy.
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These are energetic particles that originate outside Earth’s atmosphere. The most energetic cosmic rays are of an unknown,
extragalactic origin and travel at nearly the speed of light.
Cosmic String
A tube-like configuration of energy that is believed to have existed in the early universe. A cosmic string would have a thickness
smaller than a trillionth of an inch but its length would extend from one end of the visible universe to the other.
Cosmogony
The study of celestial systems, including the solar system, stars, galaxies, and galactic clusters.
Cosmological Constant
A term added to the field equations by Einstein in order to allow solutions in which the universe was static; that is, neither
expanding nor contracting. Although the need for the term disappeared when it was discovered that the universe is expanding,
the cosmological constant is retained in the field equations by modern cosmologists, but is usually assigned the value zero.
Cosmological Distance
A distance far beyond the boundaries of our galaxy. When viewing objects at cosmological distances, the curved nature of
spacetime becomes apparent. Possible cosmological effects include time dilation and redshift.
Cosmological Principle
This principle states that the distribution of matter across very large distances is the same everywhere in the universe and that
the universe looks the same in all directions. According to this principle, our view of the universe is like the view from a boat on
an ocean, which is essentially the same for any other person on any other boat on any other ocean. Measurements of matter and
energy in the universe on the largest observable scales support the cosmological principle.
Cosmological Redshift
A Doppler shift toward longer wavelengths that is caused by a galaxy's motion of recession, which in turn is caused by the
expansion of the universe.
Cosmology
A branch of science that deals with studying the origin, structure, and nature of the universe.
Cosmos
The Universe.
Counterglow
The English name for the skyglow opposite the Sun, very difficult to observe, caused by very thinly spread interplanetary
material. Usually known by its German name, the Gegenschein.
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Crater
A bowl-shaped depression formed by the impact of an asteroid or meteoroid. Also the depression around the opening of a
volcano.
Craterlet
A small crater ranging from a few millimetres across to a few metres.
Crayford
A type of telescope focuser in which the focus tube is moved by a roller, and also slides on rollers, as well, and makes for precise
adjustments.
Crepuscular Rays
When the lower atmosphere is hazy, light passing through gaps between broken clouds can produce bright “beams” that are
made visible by scattering by aerosol particles. These are usually seen best when the Sun is low, or at twilight
Crescent
The phase of a body that is less than one-half illuminated.
Critical Mass
The mass of an isotope above which a self-sustaining chain reaction can occur.
Crown Glass
Originally the main material used to make flat planes of glass for windows, it is composed of soda-lime glass. It can be used to
make lenses and prisms. Crown glass bends and disperses, or spreads out, light less than flint glass.
Cubewano
Is a low-eccentricity Kuiper belt object that orbits beyond Neptune and is not controlled by an orbital resonance with Neptune.
Cubewanos have orbits with semi-major axes in the 40–50 AU range and, unlike Pluto, do not cross Neptune’s orbit. That is,
they have low-eccentricity and sometimes low-inclination orbits like the classical planets.
Culmination
An object culminates when it reaches greatest and least altitudes ( upper culmination and their lower culmination respectively).
For non-circumpolar objects, the lower culmination is below the horizon. Most objects (the Moon sometimes being a notable
exception) culminate when they reach the observer's meridian.
Curvature of Field
An aberration of optical systems, in which the surface on which images in sharp focus are formed is not flat, but curved. Also
called Field Curvature.
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Cytherean
An adjective used to describe things related to the planet Venus. The correct form is really ‘Venereal’, but this is rarely used.
Astronomers use ‘Cytherean’ because the island of Cytherea crops up in the mythology of Aphrodite (also called Venus).
..D
D Lines
A pair of lines from sodium that appear in the yellow part of the spectrum.
Danjon Scale
A method of measuring lunar eclipse brightness, ranging from 0-4, darkest to brightest respectively. The Danjon scale is
represented by the letter L.
Dark Adaptation
Your eye does not reach its full power of detecting low light levels (its full dark adaptation) until you have been in a dark
environment for quite a long time, perhaps several hours. To obtain full dark adaptation, the darkness of the surroundings must
be light free.
Dark Dust Cloud
A region of interstellar space that contains a rich concentration of gas and dust. Such a cloud is often irregular in shape but
sometimes has a well-defined edge. Visible light cannot pass through these clouds, so they obscure the light from stars beyond
them.
Dark Energy
A mysterious force that seems to work opposite to that of gravity and makes the universe expand at a faster pace.
Dark Frames
These are images taken before, after or during your capture of light frames and are taken with the dust cover on the scope so no
light gets in. These are used to adjust the light frames to correct for things like amplifier glow, hot pixels and gradients. They
should be taken at the same temperature that the light frames were taken at and have exactly the same exposure settings (ISO,
length etc.).
Dark Matter
A term used to describe matter in the universe that cannot be seen, but can be detected by its gravitational effects on other
bodies.
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Dark Matter Halo
A giant halo of dark matter found around a galaxy. These have been detected by the effect their gravity has on the rotation of
spiral galaxies.
Dark Nebula
This is an interstellar cloud of gas and dust which is silhouetted against a brighter background such as a field of stars or a
glowing nebula.
Daughter Isotopes
Isotopes formed by the radioactive decay of another isotope.
Dawes Limit
The Dawes limit has to do with the physiology of human vision as well as the optical quality of telescopes. It is an empirically
determined standard of how well an excellent small telescope can be expected to perform, in excellent conditions, when
observing double stars that are not too faint, in which the two components are of the same brightness, or very nearly so.
Day, Sidereal
The interval between two successive meridian passages (or culminations) of the same star. A sidereal day is equal to 23 hours,
56 minutes and 4.091 seconds. This is the true rotational period of the Earth.
Day, Solar
The mean interval between successive meridian passages of the Sun. It is equal to 24 hours, 3 minutes and 56.55 seconds. The
solar day is slightly longer than the sidereal day because the Sun seems to moves eastward against the stars, on average at
roughly one degree per day.
dB Decibel
A unit of power ratio; the gain or loss in power in dB is equal to 10 times the logarithm of the power ratio. 1dB is approximately
the smallest change in volume of sound which a normal ear can detect.
Decay Constant
For an atom that undergoes radioactive decay, the decay constant is the proportionality factor between the time rate of decay
and the total number of atoms present; it is the inverse of the mean lifetime of an atom.
Deceleration Parameter (q0)
A particular measure of the rate at which the expansion of the universe is slowing down.
Declination
One element of the astronomical coordinate system on the sky that is used by astronomers. Declination, which can be thought
of as latitude on the earth projected onto the sky, is usually denoted by the lower-case Greek letter delta and is measured north
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(+) and south (-) of the celestial equator in degrees, minutes, and seconds of arc. The celestial equator is defined as being at
declination zero (0) degrees; the north and south celestial poles are defined as being at +90 and -90 degrees, respectively.
Deconvolution
A process used in image processing to help reverse the effects of optical distortion.
Decretion Disk
This is a circumstellar disk formed from gas ejected from the central star that now follows a near Keplerian orbit around it. This
type of disk can be found around many Be stars.
Deep-Sky Objects
This is the common name of a range of faint objects including galaxies, nebulae and star clusters, in other words objects which
lie beyond the solar system. These objects are also often fondly referred to as ‘faint fuzzies’ by amateur astronomers.
Degenerate Gas
A gas in which either free electrons or free neutrons are as densely spaced as allowed by laws of quantum mechanics. Such a
gas has extraordinarily high density, and its pressure is not dependent on temperature, as it is in an ordinary gas. Degenerate
electron gas provides the pressure that supports white dwarfs against collapse, and degenerate neutron gas similarly supports
neutron stars.
Degree
A unit used in the measurement of angles, heavily used particularly in astronomy. Due to ancient Babylonian mathematics, we
still divide a circle into 360 even units of arc and call each of these units one degree. The entire sky, therefore, spans 360
degrees. Up to about 180 degrees of sky is visible from any given point on earth with an unobstructed horizon (as measured
from, say, east to west, or north to south). The degree is used to make measurements of distance, or position (as with
declination) in astronomy. In turn, a degree is composed of 60 minutes of arc, and also of 3600 seconds of arc.
Degree of Arc
One degree of arc is 1/360 of a full circle. The apparent sizes of objects as seen from Earth can be measured in degrees of arc.
The angular diameter of the full moon or the Sun as seen from Earth is one-half of a degree.
Delta
Distance of the celestial body from Earth in Astronomical Units (AU). For the Moon, Delta is the topocentric distance of the
Moons mass centre from the observer in Earth radii (ER). It is also the fourth letter in Greek alphabet.
Density
The amount of matter contained within a given volume. Density is measured in grams per cubic centimetre (or kilograms per
litre). The density of water is 1.0, iron is 7.9, and lead is 11.3.
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Density-Wave Theory
The explanation of spiral structure of galaxies as the effect of a wave of compression that rotates around the center of the galaxy
and causes the formation of stars in the compressed region.
Decending Node
The position in the orbit of a planet (or the Moon) where it crosses the plane of the ecliptic, moving southward.
Detatched Object
Detached objects are a dynamical class of bodies in the outer Solar System beyond the orbit of Neptune. These objects have
orbits whose points of closest approach to the Sun are sufficiently distant from the gravitational influence of Neptune, that they
are essentially unaffected by Neptune and the other planets: this makes them appear to be "detached" from the Solar System.
In this way, they differ substantially from the majority of the known Trans-Neptunian Objects.
Detection
In electromagnetics, an operation converting the vector electromagnetic wave to a scalar time series proportional to either the
amplitude or the power of the wave, with or without an accompanying angular time series. A crucial aspect of detection is: the
signal-to-noise ratio after detection is the square of the signal-to-noise ratio before detection.
Determinism
The doctrine according to which like causes always produce like effects and, conversely, events are entirely explainable by their
antecedent causes.
Deuterium
An isotope of hydrogen with a proton and a neutron in the nucleus (mass of 2 amu).
Dew Cap (Shield)
A cylindrical extension of the telescope tube beyond the upper end, whose purpose is to retard the formation of dew on optical
elements at or near that end.
Diagonal Mirror
(i) The small flat mirror used near the upper end of a Newtonian telescope, to direct the converging beam of light over to the
side of the tube.
(ii) Star diagonals, that use mirrors and not prisms.
Dichotomy / Half-Phase
An inner planet or moon is half lit from the Sun, i.e., its phase is 50%. This does not correspond to the event of greatest
elongation, due to different eccentricities of the planet and Earth.
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Differential Flexure
The result of one part of a telescope system being rigid and another moving slightly in relation to it, very noticeable when applied
to guiding during imaging.
Differential Gravitational Force
A gravitational force acting on an extended object, such that the portions of the object closer to the source of gravitation feel a
stronger force than the portions farther away. Such a force, also known as a tidal force, acts to deform or disrupt the object,
and is responsible for many phenomena, ranging from synchronous rotation of moons or double stars to planetary ring systems
to the disruption of galaxies in clusters.
Differential Rotation
The rotation of a body such as a gaseous planet or the Sun so that different parts are rotating at different speeds. For example,
a star or planet which rotates faster at its equator than it does at its poles.
Differentiation
The sinking of relatively heavy elements into the core of a planet or other body. Differentiation can occur only in fluid bodies, so
any planet that has undergone this process must once have been at least partially molten. Also, a process whereby a stem cell
acquires the characteristic features of a given cell type.
Diffraction Grating
A very closely ruled series of lines that, through their diffraction of light, provide a spectrum of radiation that falls on it.
Diffraction Limited
A term given to mean the level of performance that would be apparent if there was no atmospheric disturbance to distort the
view. Unless you are beyond the Earth’s atmosphere, there is always some atmospheric disturbance present.
To say that a telescope is diffraction limited, is to say that no conceivable improvement in its optical quality can make its images
noticeably sharper. It is so good, that when used in excellent conditions, then only the wave nature of light itself prevents the
telescope from delivering perfectly sharp images.
Diffraction-Limited Optics
The theoretical limit of the resolving power of a telescope’s optics. This limitation arises because the diffraction, or bending, of
light as it passes through the aperture of a telescope causes a star to be observed as a bright disc (the Airy disc) with bright
rings around it, rather than a point of light.
Digital Image
A visible image that is recorded by an electronic detector and subdivided into small picture elements (pixels). Each element is
assigned a number that corresponds to the brightness recorded at its physical location on the detector. Computer software
converts the numerical information into a visual image. The Hubble Space Telescope records digital images.
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Dioptre
A measurement of the optical power of lenses and curved mirrors in telescopes and binoculars. The dioptre is the reciprocal of
the lens or mirror’s focal length in metres.
Dioptre Adjustment
This is the ability to adjust the power of the eyepieces on a pair of binoculars to suit your eyes.
Dip
The dip of the apparent (or sea) horizon is its angular distance below the astronomical horizon.
Direct Motion
Another term for prograde motion.
Disk
A flattened, circular region of gas, dust, and/or stars. It may refer to material surrounding a newly-formed star; material
accreting onto a black hole or neutron star; or the large region of a spiral galaxy containing the spiral arms. Also, the apparent
circular shape of the Sun, a planet, or the moon when seen in the sky or through a telescope
Dispersion
Visible light is actually made up of different colours. Each colour bends by a different amount when refracted by glass. That’s why
visible light is split, or dispersed, into different colours when it passes through a lens or prism. Shorter wavelengths, like purple
and blue light, bend the most. Longer wavelengths, like red and orange light, bend the least.
Distance Modulus
The difference m-M between the apparent and absolute magnitudes for a given star. This difference, which must be corrected
for the effects of interstellar extinction, is a direct measure of the distance to the star.
Distortion
An aberration of optical systems, in which magnification changes systematically with distance away from the optical axis, causing
distortions of images.
Diurnal
Another name for daily.
Dob(sonian)
A type of reflector. A simple alt/az mounted scope that is easy to use and offers very good value for money.
Doppler Effect
Named after C. J. Doppler, the apparent change in wavelength of sound or light caused by the motion of the source, observer or
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both. Light waves emitted by a moving object as received by an observer will be blueshifted (compressed) if approaching,
redshifted (elongated) if receding. A similar effect occurs with sound waves. How much the frequency changes depends on how
fast the object is moving toward or away from the receiver.
Doppler Imaging
A technique that uses high-resolution spectrographic data of a star to produce a map of the star’s surface.
Doppler Shift
A change in frequency resulting from relative motion along the line between the transmitter and the receiver. If the source and
the receiver are approaching each other, the frequency received is higher than the frequency transmitted by a factor, depending
on the actual relative velocity. Knowledge of this shift is used to determine the relative velocity.
Double Bond
Two shared pairs of electrons.
Double Star
A grouping of two stars. This grouping can be apparent, where the stars seem close together, or physical, such as a binary
system.
Doublet
Two simple lenses used in combination, placed close together or in contact. If they are cemented together, they constitute a
"cemented doublet". If they are merely closely adjacent, they are a "separated doublet".
Drake Equation
An approach to estimating some of the factors in guessing at the number of communicating civilizations in the galaxy.
Drawtube
The moving part of a telescope's focuser.
Drifting (Signal)
Refers to a signal with an apparent time rate of change in its typical frequency. All signals drift to some extent. In a SETI
system, the dominant drift should be largely the result of only the time rate of change in the Doppler shift.
Duct
If the ray curvature within a thermal inversion is stronger than the curvature of the Earth's surface, rays can be continuously
guided along the surface of the Earth without ever escaping to space. This region in which rays are trapped is called a duct. An
observer within the duct sees a superior mirage of distant objects within the duct. Ducting occurs when the temperature
gradient within the inversion is steeper than about 10 K in 100 meters (1 degree in 10 m).
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Dust
Not the dust one finds around the house—which is typically fine bits of fabric, dirt, and dead skin cells—but rather the irregularly
shaped grains of carbon and silicates measuring a fraction of a micron across that are found between the stars. Dust is most
evident by its absorption, causing large dark patches in regions of our Milky Way Galaxy and dark bands across other galaxies.
Dwarf Elliptical Galaxy
A member of a class of small spheroidal galaxies, similar to standard elliptical galaxies except for their small size and low
luminosity. Dwarf galaxies are probably the most common in the universe, but cannot be detected at distances beyond the Local
Group of galaxies.
Dwarf Nova
A close binary-star system containing a white dwarf; material from the companion star falls onto the other at sporadic intervals,
creating brief nuclear outbursts.
Dwarf Planet
This classification was introduced in 2006 for celestial objects that orbit the Sun, are nearly spherical in shape, but are not
satellites and have not cleared their neighbourhood of other celestial bodies. This new class includes Pluto (formerly a planet),
Ceres (the largest object in the Main Belt) and Eris (a trans-Neptunian object that was found to be larger than Pluto), the planet
that started the debate.
Dwarf Spheroidal
A dwarf galaxy that contains few stars and hardly any gas. Most of the Milky Way’s satellites are dwarf spheroidals.
Dwarf Star
Most of the stars in the Universe are dwarf stars. Their masses are between a tenth and one hundred times the mass of the Sun.
Dynamo
A device that generates electricity through the effect of motion in the presence of a magnetic field. The solar dynamo explains
sunspots and the solar activity cycle.
..E
Early-Type Star
Is a legacy term that refers to hotter and more massive stars, in contrast to late-type stars that are cooler and less massive.
The term originated from historical stellar models that assumed stars began their early life at a high temperature then gradually
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cooled off as they aged. It can be used to refer to the higher temperature members of any particular population or category of
stars, rather than just all stars in general.
Earth
The third planet from the Sun (sol) and one of four terrestrial planets in the inner solar system. Earth, the only planet where
water exists in large quantities, has an atmosphere capable of supporting myriad life forms. The planet is 150 million kilometres
(93 million miles) away from the Sun. Earth has one satellite - the Moon.
Earthshine
Sunlight illuminating the moon after having been reflected by the earth.
Eccentricity
A measure of the flatness of an ellipse, defined as half the distance between the foci divided by the semi-major axis. Elliptical
orbits have an eccentricity >0 and <1, parabolic paths have an eccentricity =1, and hyperbolic paths have an eccentricity >1
Eclipse
The total or partial blocking of one celestial body by another.
Eclipse Mapping
A technique used to indirectly map the surface of an eclipsing binary star, to look for star spots and make other studies by using
Doppler imaging.
Eclipsing Binary
A star with a companion that periodically passes in front of it, and eclipses it as seen from Earth.
Ecliptic
The apparent path of the sun against the sky background (celestial sphere); formally, the mean plane of the earth's orbit about
the sun. This is the only place in the sky where solar and lunar eclipses occur.
Ecliptic Co-Ordinate System
The system of specifying positions in the sky that uses the ecliptic—the Sun’s apparent annual path through the fixed stars—as
the fundamental reference plane. Ecliptic co-ordinates are useful when specifying positions in the solar system and especially
positions relative to the Sun.
Ecliptic Latitude
The angular distance of an object relative to the ecliptic, expressed in degrees. Distances north of the ecliptic are positive;
distances south are negative. The ecliptic latitude of the Sun is always zero.
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Ecliptic Longitude
The angular distance of an object eastward from the vernal equinox, measured in degrees along the ecliptic. The ecliptic
longitude of the Sun is zero when the Sun is on the vernal equinox and increases through the year by very nearly one degree per
day.
Ecliptic Plane
Or plane of the ecliptic, is the plane defined by the Earth's orbit around the Sun. Hence, the position of the Sun as viewed from
Earth defines the intersection of this plane with the celestial sphere. The ecliptic plane is used as a reference plane for describing
the position of other Solar System bodies. It differs from the celestial equator because of the axial tilt of the Earth.
Ecosphere
The portion of the atmosphere from sea-level to about 4000 meters in which it is possible to breathe without technological
assistance.
Effective Temperature
Of a star or planet is the temperature of an ideal black body that would emit the same total amount of electromagnetic radiation.
Egress
When a celestial body emerges from a shadow or from a transit.
Einstein Ring
This is a rare form of gravitational lens that occurs when a distant galaxy’s light is magnified by another galaxy directly in front of
it, as seen from Earth. The result is a distorted image of a background galaxy in the shape of a perfect ring.
Ejecta
Material that is ejected. Used mostly to describe the content of a massive star that is propelled outward in a supernova
explosion. Also used to describe the material that is blown radially outward in a meteor impact on the surface of a planet or
moon.
Electric Field
A force field set up by an electric charge.
Electromagnetic Force
The force created by the interaction of electric and magnetic fields. The electromagnetic force can be either attractive or
repulsive, and is important in countless situations in astrophysics.
Electromagnetic Radiation
Another term for light. Light waves created by fluctuations of electric and magnetic fields in space.
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Electromagnetic Spectrum
The full range of frequencies, from radio waves to gamma waves, that characterizes light.
Electromagnetism
The science dealing with the physical relationship between electricity and magnetism. The principle of an electromagnet, a
magnet generated by electrical current flow, is based on this phenomenon.
Electron
A tiny (1/1830 the mass of a proton), negatively charged particle that orbits the nucleus of an atom. The charge is equal and
opposite to that of a proton in the nucleus, and in a normal atom the number of electrons and protons is equal, so that the
overall electrical charge is zero. It is the electrons that emit and absorb electromagnetic radiation, by making transitions
between fixed energy levels.
Electron Carrier
In a chain of chemical reactions, molecules that accept electrons from an electron donor and pass these to an electron acceptor.
Electron Configuration
The arrangement of an atom's electrons in space.
Electron Degeneracy
Occurs when electrons are compressed into a very tiny volume. Electron degeneracy is the force that supports a white dwarf
against its own gravity, preventing it from collapsing. A neutron star is supported by neutron degeneracy. If a star is massive
enough, not even neutron degeneracy can support its weight. The result is a black hole.
Electron Volt (eV)
A unit of energy that is equal to the energy that an electron gains as it moves through a potential difference of one volt. This
very small amount of energy is equal to 1.602 x 10–19 joules. Because an electron volt is so small, engineers and scientists
sometimes use the terms MeV (mega-[million]) and GeV (giga-[billion]) electron volts.
Element
(i) A substance composed of a particular kind of atom. All atoms with the same number of protons (atomic numbers) in the
nucleus are examples of the same element and have identical chemical properties. For example, gold (with 79 protons) and iron
(with 26 protons) are both elements, but table salt is not because it is made from two different elements: sodium and chlorine.
The atoms of a particular element have the same number of protons in the nucleus and exhibit a unique set of chemical
properties. There are about 90 naturally occurring elements on Earth.
(ii) Any single part of an optical train such as a mirror or lens. In the case of lenses or eyepieces they may comprise several
pieces of glass, each one known as an element. The combination of elements is used to correct the faults that would often be
present in a single element lens or eyepiece.
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Elementary Particle
Any of a number of sub-atomic particles.
Ellerman Bombs
Ellerman bombs are bright explosions seen in the Sun’s chromosphere around newly formed active regions. They are named
after the solar astronomer Ferdinand Ellerman who discovered them in September 1915.
Ellipse
An ellipse is an oval shape. Johannes Kepler discovered that the orbits of the planets were elliptical in shape rather than circular.
Elliptical Galaxy
A galaxy whose structure shaped like an ellipse and is smooth and lacks complex structures such as spiral arms.
Elongation
The angular distance of a planetary body from the Sun as seen from Earth. A planet at greatest eastern elongation is seen in the
evening sky and a planet at greatest western elongation will be seen in the morning sky.
Emersion
A term used to describe when an object re-emerges after an occultation or eclipse.
Emission Line
A bright line in a spectrum caused by emission of light. Each chemical element emits and absorbs radiated energy at specific
wavelengths. The collection of emission lines in a spectrum corresponds to the chemical elements contained in a celestial object.
Emission Nebula
A cloud of interstellar gas that glows by the light of emission lines. The source of excitation that causes the gas to emit may be
radiation from a nearby star, or heating by any of a variety of mechanisms.
Energy
The ability to do work. Energy can be in either kinetic form, when it is a measure of the motion of an object, or potential form,
when it is stored but capable of being released into kinetic form.
Energy Levels
The specific, quantized energy levels that an electron may have in an atom.
Enrichment (Isotope)
The process by which the proportion of one isotope of an element is increased relative to the others.
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Entropy
Tendency of systems to become more disordered (and thus more uniform) over time; also a measure of disorder; in
thermodynamics, a measure of the amount of heat energy in a closed system that is not available to do work.
Ephemeris
A table of data arranged by date. Ephemeris tables are typically to list the positions of the Sun, Moon, planets and other solar
system objects.
Epicycle
A small circle whose centre is on the circumference of a larger circle; in Ptolemaic astronomy it was seen as the basis of
revolution of the "seven planets", given a fixed central Earth.
Epoch
(i) An epoch is a division of a geologic period; it is the smallest division of geologic time, lasting several million years.
(ii) The date at which the co-ordinates on a star chart will be correct with respect to precession.
Equant
(i) The centre of a planetary epicycle.
(ii) Having comparable measurements in all directions.
Equation of Time
The correction which must be applied to solar time in order to obtain mean solar time.
Equatorial Co-Ordinates
The astronomical coordinate system in which positions are measured with respect to the celestial equator (in the north-south
direction) and with respect to a fixed direction (in the east-west direction). The coordinates used are declination (north- south,
in units of angle) and right ascension (east-west, in units of time).
Equatorial Mount
A telescope mount with one axis parallel to the earth’s polar axis. This provides easy tracking of celestial objects, and is
preferred by many for astrophotography.
Equinox
Literally meaning "equal night (as day)”. An intersection of the ecliptic and the celestial equator. The two points (vernal,
autumnal) at which the Sun crosses the celestial equator in its yearly path in the sky. The equinoxes occur on or near March 21
and September 22. The equinoxes signal the start of the Spring and Autumn seasons.
Eras (Geologic)
All of Earth's history since the appearance of the first life forms is divided roughly into four eras: Precambrian, from 3.5 billion to
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570 million years ago; Palaeozoic, from 570 to 225 million; Mesozoic, from 225 to 65 million; and Cainozoic, from 65 million to
the present. The last two eras are broken down into the following periods: the Mesozoic into Triassic) Jurassic, and Cretaceous;
the Cainozoic into Tertiary and Quaternary.
Erfle
See “Eyepiece”
Erg
A unit of energy in the metric system, corresponding to the work done by a force of one dyne (the force that is required to
accelerate one gram by one cm/sec2) producing a displacement of one centimetre.
Ergosphere
A region surrounding a rotating black hole (or other system satisfying Kerr's solution) from which work can be extracted.
Escape Velocity
The velocity required for an object to escape the gravitational field of a body such as a planet .In a more technical sense, the
escape velocity is the velocity at which the kinetic energy of the object equals its gravitational potential energy; if the object
moves any faster, its kinetic energy exceeds its potential energy, and the object can escape the gravitational field.
Etalon
An etalon is an optical device used to filter specific wavelengths of radiation by reflecting it many times between two parallel
glass panels.
Ether
(i) A compound containing an oxygen atom bonded to two hydrocarbon groups.
(ii) The sky or heavens; the upper air.
(iii) This was the fifth element in addition to air, earth, fire and water.
(iv) A classical physical element, considered as prevalent in the heavens and inaccessible to humans. In some versions of
alchemy.
(v) A substance (aether) once thought to fill all space that allowed electromagnetic waves to pass through it and interact with
matter, without exerting any resistance to matter or energy (disproved by Einstein in his Theory of Relativity).
Euclidean Space
A space with zero curvature; a space where the sum of the angles of a triangle is 180°.
European Space Agency
A fifteen-member consortium of European countries for the design, development, and deployment of satellites. The Space
Telescope - European Coordinating Facility (ST-ECF) supports the European astronomical community in exploiting the research
opportunities provided by the Earth-orbiting Hubble Space Telescope. The ESA members are Austria, Belgium, Denmark, Finland,
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France, Germany, Ireland, Italy, the Netherlands, Norway, Portugal, Spain, Sweden, Switzerland, and the United Kingdom, with
Canada as a cooperating state.
Event Horizon
The invisible boundary around a black hole past which nothing can escape the gravitational pull - not even light.
Evolutionary Track
is a curve on the Hertzsprung–Russell diagram that a solitary star, of a particular mass and composition, is expected to follow
during the course of its evolution. This curve predicts the combination of temperature and luminosity that a star will have during
part or all of its lifetime.
Evolved Star
A star that is near the end of its life cycle where most of its fuel has been used up. At this point the star begins to loose mass in
the form of stellar wind.
Exit Pupil (Eye-Ring)
The position of the image of the objective lens or primary mirror formed by the eyepiece. It is the smallest disc through which
all the collected light passes and is therefore the best position for the eye's pupil.
Exobiology
The study of life as it might occur elsewhere than on earth. Also, the study of the origin of life, at any location.
Exoplanet
An abbreviation of ‘extra solar planet’, meaning a planet not in the Solar System but orbiting another star.
Exosphere
The upper part of an atmosphere where gas is very tenuous and rarefied. Earth’s exosphere lies about 500km above its surface.
Mercury has no substantial atmosphere, but does have a very thin layer of gas extending around it that is essentially an
exosphere.
Extinction
The apparent dimming of star or planet when low on the horizon due to absorption by the Earth's atmosphere.
Extra-Low Dispersion
ED glass is a type of glass that reduces the dispersion of light into its different wavelengths, allowing for better chromatic
aberration correction in refractor.
Extragalactic
A term that means outside of or beyond our own galaxy.
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Extra-Terrestrial
A term used to describe anything that does not originate on Earth.
Eyepiece
The lens at the viewing end of a telescope. The eyepiece is responsible for enlarging the image captured by the instrument.
Eyepieces are available in different types and powers, yielding differing amounts of magnification and field of view.
Telescope eyepiece types:
Brandon
Erfle
Huygens
Kellner
Koenig
Lanthanum
Masayuma
Monocentric
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Four lenses; medium to high power use with any telescope at focal ratios down to f/4; very sharp at the centre
(somewhat less so at the edges) for lunar, planetary, star cluster, and binary star observing; excellent colour
correction and contrast; very low astigmatism; the sharpest design currently available for eyepiece projection
photography; very minor field curvature and ghosting.
An early 20th-century, 5-element eyepiece design that performs reasonably well at low-power focal lengths.
Although Panoptics and Naglers are more highly regarded, an Erfle can provide a decent wide-field view at a
much lower price.
The two-element Huygenian eyepiece was invented by Christiaan Huygens (pronounced “HOY-kens”) in the
1600s. This design is inferior to more recent designs, so it is now obsolete, except that some Huygenian (“H”)
eyepieces are still supplied with cheap imported telescopes. Eye relief is extremely short and it has a narrow
apparent field of view. Will work reasonably well with focal ratios of perhaps f/15 or more.
A 3-element eyepiece design that can perform reasonably well at low-power (wide-view) focal lengths.
Although this is a very old eyepiece design it is often sold at reasonable prices, under the “modernized” name
Super Modified Achromat (SMA). In essence, the Kellner is an achromatized Ramsden.
The König eyepiece has a concave-convex positive doublet and a convex~flat positive singlet. The strongly
convex surfaces of the doublet and singlet face and (nearly) touch each other. The doublet has its concave
surface facing the light source and the singlet has its almost flat (slightly convex) surface facing the eye. It
was designed in 1915 by German optician Albert König as a simplified Abbe. The design allows for high
magnification with remarkably high eye relief. The field of view is typically 55° - 70°
A specific eyepiece design. Lanthanum LV is a "house brand" of the large Japanese optical manufacturer, Vixen.
The Lanthanum LV eyepiece series is particularly noted for having a long eye relief of 20 mm, even for the
shortest focal lengths. At least some of the Lanthanum LV eyepieces are composed of seven simple lenses,
cemented together into three doublets and a singlet. The front most doublet consists of a built-in Barlow lens.
The rest of the eyepiece comprises two doublets separated by a singlet.
A 5-element eyepiece design that provides a wider FOV than orthoscopic eyepieces, without sacrificing very
much contrast. The highly regarded Takahashi and Celestron Ultima eyepieces use this design.
A Monocentric is an achromatic triplet lens with two pieces of crown glass cemented on both sides of a flint
glass element. The elements are thick, strongly curved, and their surfaces have a common centre giving it the
name "monocentric". This design, is free from ghost reflections and gives a bright contrasty image, a desirable
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feature when it was invented (before anti-reflective coatings). It has a narrow field of view of around 25°
Nagler
Naglers have seven lenses, Nagler 2 and Ultra Wide have eight; up to three times the field area of a Plössl, but
sharper edge to edge; up to ten times sharper at the edges than older wide angle designs such as Erfles; field
is so wide you have to move your head from side to side to see all of it; more like looking out a window into
space rather than looking through an eyepiece; superb colour correction, with little ghosting or field curvature;
some loss of contrast on planets due to many lens elements; short focal lengths use a built-in Barlow to
achieve high power without reducing eye relief; medium to high power use down to f/4; for star clusters,
nebulas, and galaxies with any scope, but particularly good with fast focal ratio reflectors.
Ortho
A 4-element eyepiece design, sometimes referred to as an Abbe design after its inventor, the great German
Abbe
optical designer Ernst Abbe who created the design in 1880 (and subsequently became a partner in the Carl
Zeiss Optical Works). In high-power focal lengths an ortho provides an excellent view for planetary observing,
at a reasonable price.
Panoptic
Six lenses, with an ED (extra low dispersion glass) lens for exceptional contrast and a very wide field; up to five
times the field edge correction of older wide field designs; for low to medium powers down to f/4 with any
telescope, although particularly good with fast focal ratio reflectors when combined with a Paracorr coma
corrector.
Pentax
A manufacturer of very high quality (and expensive) eyepieces whose design works particularly well for
medium-power to medium-high-power views. In that range, personally I absolutely love these eyepieces, but
note that eyepiece preference is a personal thing that varies depending on the idiosyncrasies of any particular
person’s eyesight.
Plössl
A 4-element eyepiece design invented by the Austrian optician Simon Plössl in the 19th century, that provides a
somewhat wider FOV than does an ortho. Unlike orthos, the design of Plössls has been modified and improved
by manufacturers to the point that it now provides excellent views at both long and short focal lengths. The
design provides good value for its price and has thus become the most popular general-eyepiece design among
folks new to observing, or observers on a more limited budget. Nevertheless at any specific short, medium, or
long focal length there are better specific eyepiece designs available (albeit at generally higher prices) so in the
long run experienced observers usually end up not using many Plössls.
Ramsden
The Ramsden eyepiece comprises two plano convex lenses of the same glass and similar focal lengths, placed
less than one eye-lens focal length apart. It is suitable for use with instruments operating using near
monochromatic light sources, e.g. polarimeters.
RKE
An RKE eyepiece has an achromatic field lens and double convex eye lens, a reversed adaptation of the Kellner
eyepiece. This design provides slightly wider field of view than classic Kellner design and makes it design
similar to a widely spaced version of the König. RKE stands for Rank, Kaspereit, Erfle, the three designs from
which the eyepiece was derived.
SMA
Super Modified Achromat - a fancy term for an eyepiece using the Kellner optical design.
Speers-Waler A specific eyepiece design, noted for a very wide apparent field of view, almost 80 degrees, and for excellent
correction at fast focal ratios. "Waler" is an acronym for "Wide Angle, Long Eye Relief".
Takahashi
Proprietary fully-multicoated five lens design with long eye relief; for low to very high power use to f/4; two
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shortest focal lengths use one ED (extra low dispersion glass) lens for exceptional contrast and colour
correction; usable with all scope types; suitable for all types of observing; very good sharpness and contrast;
very little ghosting.
Eye Relief
The distance, within which, the eye can be placed from the eyepiece, to obtain sharp vision.
..F
F-Number (Focal Ratio)
The ratio of the focal length divided by the aperture (in the same units).
Faculae
Faculae are brighter, more prominent patches of the solar photosphere (the ‘surface’ of the Sun). They are sometimes an
indication of a forming sunspot and show up particularly well through white-light solar filters.
Faint Object Camera
An instrument aboard the Hubble Space Telescope that recorded high-resolution images of faint celestial objects in deep space.
Built by the European Space Agency, the camera collected ultraviolet and visible light from celestial objects. The camera served
as Hubble’s “telephoto lens” - recording the most detailed images over a small field of view. The FOC’s resolution allowed Hubble
to single out individual stars in distant star clusters. The instrument was replaced in March 2002 during Servicing Mission 3B.
Faint Object Spectrograph
An instrument aboard the Hubble Space Telescope that acted like a prism to separate light from the cosmos into its component
colours, providing a wavelength “fingerprint” of the object being observed. Such information yields clues about an object’s
temperature, chemical composition, density, and motion. Spectrographic observations also reveal changes in celestial objects as
the universe evolves. The instrument was replaced in February 1997 during the Second Servicing Mission.
Far-Infrared Spectrum
The region of the infrared spectrum that exhibits the longest wavelengths and the lowest frequencies and energies.
Fault
A geological term that refers to a fracture or a break in a hard surface like the Earth’s crust. This area is a zone of weakness and
may be the site of earthquakes or volcanoes. All planets or moons with a hard crust are candidates for faults or breaks on their
surfaces.
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Feed (Antenna Feed, Line Feed)
In a reflecting antenna system, the device that converts a guided (by wire, cable, or wave guide) electromagnetic wave into an
electromagnetic radiation field, and vice versa, when reciprocity theorem holds as it so often does. Commonly, feeds are some
form of horn antenna, but they may be dipole arrays.
Field Curvature
An aberration of optical systems, in which the surface on which images in sharp focus are formed is not flat, but curved.
Field Flattener
Field curvature can be corrected by a simple lens placed at or close to the focal "plane" of the instrument in question. Such a
lens is called a field flattener. If the field curvature is concave toward the incoming beam of light, the field flattener must be a
negative lens. If the field curvature is convex toward the incoming beam of light, the field flattener must be a positive lens. The
size of the field flattener must be equal to or slightly greater than the size of the desired flat field.
Field Galaxy
This is a galaxy that does not belong to a larger cluster of galaxies, but is gravitationally alone.
Field Glasses
An instrument rather resembling a binocular, but which uses a special eyepiece design, instead of prisms, to achieve an upright
image. Field glasses are thereby generally lighter and less expensive than binoculars, but they have an extremely narrow
apparent field of view.
Field of View
The amount of sky seen through a telescope’s eyepiece or optical device (the angular separation between opposite edges of the
visual field).
Field Rotation
If a mount is not well polar aligned, even though it may track an object very well, there will be some residual rotation over time
due to the error in alignment. This sometimes manifests itself in images as elongated stars in the corners of the image.
Field Star
This is a randomly situated star that lies along the line of sight to a group of physically associated stars under study, such as a
star cluster. These field stars can contaminate the results for a study and so they need to be identified.
Field Stop
The field stop is the physical size of aperture available to look through on any given eyepiece.
Filament
A strand of cool gas suspended over the photosphere by magnetic fields, which appears dark as seen against the disk of the Sun.
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Filter
A type of window that absorbs certain colours of light while allowing others to pass through. Astronomers use filters to observe
how celestial objects appear in certain colours of light or to reduce the light of exceptionally bright objects. For example, a pair
of sunglasses acts as a type of filter, reducing the amount of incoming light while still allowing some light to pass through to the
eyes.
Filter Wheels
Rotating wheels in a telescope instrument that allow specific colours of light from a celestial object to pass through and form an
image on the detector. The Wide Field and Planetary Camera 2 aboard the Hubble Space Telescope has 12 filter wheels, each of
which holds four filters.
Filtergram
A photograph taken through a filter that passes only a very narrow band of wavelengths; usually applied to solar photographs.
Finder (Finderscope)
A small, wide-field telescope attached to a larger telescope. The finder is used to help point the larger telescope to the desired
viewing location.
Fine Guidance Sensors
The Fine Guidance Sensors are cameras that help keep the Hubble Space Telescope pointed precisely in the right direction, and
lock onto “guide stars” and measure their positions relative to the object being viewed. Adjustments based on these precise
readings keep Hubble pointed in the right direction. The sensors also are used to perform celestial measurements.
Fireball
An extremely bright meteor. Also known as bolides, fireballs can be several times brighter than the full Moon. Some can even
be accompanied by a sonic boom.
First Law of Thermodynamics
Energy is neither created nor destroyed.
First Point of Aries
The Vernal Equinox point, i.e. that where the centre of the Sun, moving northwards, crosses the equator. It is the reference
direction for the equatorial system of co-ordinates.
Fission, Nuclear
The splitting of an atomic nucleus.
Flare
A rapid eruption of material from the surface of the sun or other star.
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Flat Frames
Flat frames are images taken with a constant, 'flat' light source such as that from a light box or the dusk sky. They show up
things like dust motes on the sensor, Vignetting or uneven illumination in the optical system. Expose them to about 1/3
histogram.
Flat Universe
A geometric model of the universe in which the laws of geometry are like those that would apply on a flat surface such as a table
top.
Flint Glass
The lead glass that was produced in the United States and the United Kingdom prior to the 1860s. This glass is used to make
telescope lenses and prisms. Flint glass bends and disperses, or spreads out, light more than crown glass.
Flocculus
(plural flocculi) A marking on the surface of the sun associated with a solar prominence.
Fluorite
A special kind of optical material, of particular value in making achromats and apochromats.
Fluorite Doublet
An achromat in which one lens is fluorite. Fluorite doublets are better than achromats made with conventional glass, by a factor
of about eight, at bringing all colours of visible light to the same focal position.
Flux
The flow of fluid, particles, or energy through a given area within a certain time. In astronomy, this term is often used to
describe the rate at which light flows. For example, the amount of light (photons) striking a single square centimetre of a
detector in one second is its flux.
Focal Length
The distance from a lens or mirror to the point at which the rays from an object at infinity, come to a focus.
Focal Plane
The plane (usually this is actually the surface of a sphere of large radius) where the image is formed by the main optics of the
telescope. The eyepiece examines this image.
Focal Point
The focal point of a lens or mirror is the point in space where parallel light rays meet after passing through the lens or bouncing
off the mirror. A “perfect” lens or mirror would send all light rays through one focal point, which would result in the clearest
image.
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Focal Ratio
(f/ number) - The Focal Length of a scope divided by its Aperture (in the same units). Low is Fast; good for faint objects. High
is Slow; good for brighter, planetary objects.
Focuser
The focuser is the small piece of mechanical equipment that holds the eyepiece. It is moveable to vary the distance between the
eyepiece and the lens or mirrors to achieve focus. There are many different types of focuser on the market from simple rackand-pinion devices to more elaborate double speed, feather-touch focusers.
Force
In physics, something that can or does cause a change of momentum, measured by the rate of change of momentum with time.
Force Field
A way of describing phenomena that result from action at a distance, that is, even though objects are not touching.
Fork Mount
A mount where the telescope swings in declination or in altitude between two arms. It is suited only to short telescope tubes,
such as Cassegrains, and variations thereof. It requires a wedge to be used equatorially.
Fossil Group
These are the remnants of galaxy clusters that have seen most of their galaxies merge together. They can be identified by their
vast haloes of X-rays. Massive elliptical galaxies will form in these groups, which contain the dark matter of entire galaxy
clusters.
Foucault Pendulum
A pendulum that varies in the direction of its swing as the Earth rotates. Used to demonstrate that it is Earth that rotates and
not the sky.
Fraunhofer Lines
The absorption lines of a solar or other stellar spectrum.
Frequency
The rate (in units of Hertz, or cycles per second) at which electromagnetic waves pass a fixed point. The frequency, usually
designated ƒ, is related to the wavelength λ and the speed of light c by ƒ = c/λ.
Full Well Capacity
This is the number of electrons that a pixel can hold before saturating and possibly causing blooming.
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Fully Coated
A term for an optical system in which every optical surface where light passes from glass into air, or from air into glass, is coated
with low-reflection coatings, probably magnesium fluoride.
Fully Multicoated
A term for an optical system in which every optical surface where light passes from glass into air, or from air into glass, is coated
with high-tech, low-reflection coatings, which transmit more light than magnesium fluoride.
Fusion
The process by which atomic nuclei collide so fast that they stick together, form new atoms, and emit a large amount of energy.
In the centre of most stars, hydrogen fuses into helium. The energy emitted by fusion prevents the star’s enormous mass from
collapsing in on itself and causes the star to glow.
..G
Gain
When a CCD is read the pixel charge may be in micro-volts and an amplifier is needed to bring the signal into the range where it
is usable. The amount that the signal is increased is the gain. An amplifier not only reads the signal, but reads any noise
inherent in the chip which has to be accounted for.
Gain Stability
crudely defined by AG/G = g(t)', the smaller this quantity over the relevant time interval, the less the gain instability or the
greater the gain stability. Gain (amplification) of analog signalling systems always varies somewhat with time; g(t) contains a
variety of "noise" terms of zero mean and various secular terms; the latter dominate unless proper precautions are in force.
Galactic Cluster
An asymmetric type of collection of stars that shared a common origin.
Galactic Co-Ordinates
The system of celestial co-ordinates in which the galactic plane as the reference plane and the galactic centre as the reference
direction. The positions are given in galactic latitude and galactic longitude.
Galactic Disk
A flattened disk of gas and young stars in a galaxy. Some galactic disks have material concentrated in spiral arms (as in a spiral
galaxy) or bars (as in barred spirals).
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Galactic Halo
Spherical regions around spiral galaxies that contain dim stars and globular clusters. The radius of the halo surrounding the
Milky Way extends some 50,000 light-years from the galactic centre.
Galactic Nucleus
The central concentration of matter (stars, gas, dust, and perhaps a black hole) in a galaxy, typically spanning no more than a
few light-years in diameter.
Galactic Plane
The imaginary projection of the Milky Way’s disk on the sky. Most of the galaxy’s stars and interstellar matter reside in this disk.
Objects in the galaxy are often referred to as being above, below, or in the galactic plane.
Galaxy
A large grouping of stars in a formation. Galaxies are found in a variety of sizes and shapes. Our own Milky Way galaxy is spiral
in shape and contains several billion stars. Some galaxies are so distant the their light takes millions of years to reach the Earth.
Galaxy Cluster
A collection of dozens to thousands of galaxies bound together by gravity.
Galaxy Epoch
fifth epoch in the history of the Universe, lasting on the order of 1010 yr., during which matter largely coagulated into galactic
masses.
Galaxy Evolution
The study of the birth of galaxies and how they change and develop over time.
Galaxy Supercluster
A vast collection of galaxy clusters that may contain tens of thousands of galaxies spanning over a hundred million light-years of
space. Galaxy Superclusters are the largest structures in the universe.
Galilean Moons (Satellites)
The name given to Jupiter's four largest moons, Io, Europa, Calisto & Ganymede. They were discovered independently by Galileo
Galilei and Simon Marius.
Galilean Telescope
The kind of telescope built by Galileo featured a singlet objective and a singlet eyepiece. The eyepiece consisted of a so-called
"negative" lens, which is what most people think of as the opposite of a magnifying glass. This kind of lens gives a very narrow
apparent field of view, but it gives an image that is upright and has the left and right sides correctly positioned, whereas most
astronomical telescopes give an image that is upside down, and/or has left and right reversed.
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Gamma-Ray
The form of electromagnetic radiation with the highest energy and the shortest wavelength. Any photon having an energy
greater than about 100,000 electronVolts (eV). In comparison, visible light has an energy of 1.65 to 3.1 eV, and x-rays have an
energy of 124 eV and upward.
Gamma-Ray Burst
Flashes of high-energy electromagnetic radiation from random locations in space. They may be caused by the merger of two
black holes or neutron stars, or the explosion of a supernova. They are the brightest and most energetic explosions known.
Ganymede
One of Jupiter’s largest moons. Ganymede, the largest satellite in our solar system, is about 5300 kilometres (3300 miles) wide
and larger than the planet Mercury.
Gas
The state of matter in which the substance maintains neither shape nor volume.
Gas Giant
A large planet with a small, rocky core and a deep atmosphere composed mostly of hydrogen and helium. Our solar system
contains four gas giants: Jupiter, Saturn, Uranus, and Neptune. This group is also known as Jovian planets.
Gaseous Nebula
A glowing cloud of gas in interstellar space. The cloud of gas may be either an emission nebula, which absorbs ultraviolet light
from nearby stars and re-radiates visible light, or a reflection nebula, which reflects light off of its dust particles.
Gauss
A unit of measurement of a magnetic field. Earth has a surface magnetic field strength of between 0.3 and 0.6 Gauss.
Gegenschein
The diffuse glowing spot, seen on the ecliptic opposite the sun's direction, created by sunlight reflected off of interplanetary dust.
General Relativity
This is Einstein’s theory of gravity, which states that mass and energy curve space-time – the fabric of the Universe. Gravity is
the result of the curvature of space-time, which causes light to bend around an object and planets to orbit stars.
Geocentric
An adjective meaning “cantered on the Earth.” Most early civilizations had a geocentric view of the universe.
Geodesy
Is the study of the size and shape of the Earth. The corresponding adjective is geodetic.
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Geoid
Is the equipotential surface corresponding to mean sea level.
Geometric Albedo
Is the ratio of the brightness of an astronomical body at a phase angle of zero to an idealized flat, fully reflecting, diffusively
scattering (Lambertian) disk with the same cross-section. It is a measure of how much of the incoming illumination is being
scattered back toward an observer and has a value between zero and one.
Geometric Horizon
Where the apparent sea horizon would be if there were no refraction; equivalently, where the cone with vertex at the observer's
eye and tangent to the sea surface would meet the celestial sphere.
Geostationary Orbit
The orbit of a satellite which is both geosynchronous and in the equatorial plane. The satellite will appear to remain in a fixed
position in relation to the observer.
Geosynchronous Orbit
An orbit in which a satellite's orbital velocity is matched to the rotational velocity of the planet. A spacecraft in geosynchronous
orbit appears to hang motionless above one position of a planet's surface.
Geothermal Energy
Energy derived from the heat of Earth's interior.
German Equatorial Mount
A common type of equatorial mount. As opposed to an English equatorial mount, here the OTA is offset from the centre of the
mount and is balanced by means of counterweights on the other side and an interconnecting shaft. The shaft rotates at 90
degrees to the mount’s polar axis (i.e. the shaft points constantly at 0 degrees declination but can move through all hours of
right ascension), and the OTA can rotate about the shaft’s axis so as to point at any angle of declination.
Ghost Crater
These are craters who have been in filled, buried or submerged by geological processes but are sometimes still just visible on the
surface of a planet or moon.
Ghost Images
Ghost images are spurious images caused by unwanted reflections in an optical system. They are sometimes in focus, or nearly
so. Several common eyepiece designs are plagued by ghosts.
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Giant Molecular Cloud
Massive clouds of gas in interstellar space composed primarily of hydrogen molecules. These clouds have enough mass to
produce thousands of stars and are frequently the sites of new star formation.
Giant Planets
Jupiter, Saturn, Uranus, and Neptune.
Giant Star
A type of star brighter than main sequence stars of the same spectral type.
Gibbous
A lunar phase either side of the full moon when the moon's disc as seen from the Earth is larger than a semicircle (or of a
planet).
Giga
109 (as in gigahertz, GHz); one billion (U.S.A.).
Globular Cluster
A tight, spherical grouping of hundreds of thousands of stars. Globular clusters are composed of older stars, and are usually
found around the central regions of a galaxy.
Gnomon
The raised area of a sundial that casts a shadow on a dial so that the time of day can be read.
Golden Handle
When the Moon is about 2 days past first quarter, the mountains Jura Montes on the Moon, start to get sunlight, while the plain
Sinus Iridium is still in deep night. The bright mountains look like a handle stretching from the bright side of the Moon to the
dark side.
Goto (Go-To)
Shorthand for the kind of command interface to a computer-controlled telescope, in which the observer tells the telescope what
object to "go to" next.
Gradient
An image showing a gradient has dark sky at one edge lighter than the dark sky at another edge. This is not wanted. Some
gradients such as those caused by the optical system can be handled by flat frames, others need a bit of ingenuity.
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Gram
A unit of mass equal to the quantity' of mass contained in one cubic centimetre of water.
Grand Unified Theory
A theory stating that that strong and weak nuclear forces and electromagnetic forces are varying aspects of the same
fundamental force.
Granulation
The "grains of rice" appearance of the Sun's surface, which results from convection cells within the Sun.
Graticule
A system of parallel lines or crossed lines at the telescope's focal plane, used in micrometres.
Grating
A surface ruled with closely spaced lines that, through diffraction, breaks up light into its spectrum.
Gravitation
One of the four fundamental forces of nature, the force by which two masses attract each other.
Gravitational Clustering
The process by which a large-scale structure grows as its gravity attracts smaller building blocks. Astronomers believe that all
the large-scale structures (such as galaxies, galaxy clusters, and galaxy superclusters) that we see in the universe today formed
through gravitational clustering.
Gravitational Collapse
Occurs when a massive body collapses under its own weight. For example, interstellar clouds collapse to become stars until the
onset of nuclear fusion stops the collapse.
Gravitational Constant
A value used in the calculation of the gravitational force between objects. In the equation describing the force of gravity, “G”
represents the gravitational constant and is equal to 6.672x10–11 Nm2/kg2.
Gravitational Instability
A condition that occurs when an object’s inward-pulling gravitational forces exceed the outward-pushing pressure forces, thus
causing the object to collapse on itself. For example, when the pressure forces within an interstellar gas cloud cannot resist the
gravitational forces that act to compress the cloud, then the cloud collapses upon itself to form a star.
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Gravitational Lens
A massive celestial object, like a galaxy, that curves space-time around it to such an extent that it bends the path of light from
distant bright objects like quasars. This phenomenon was predicted by Albert Einstein in his general theory of relativity.
Gravitational Redshift
The reddening of light from a very massive object caused by photons escaping and traveling away from the object’s strong
gravitational field. An example of gravitational redshift is light escaping from the surface of a neutron star.
Gravitational Wave
Predicted in Einstein’s general theory of relativity, these waves are believed to be ripples in the fabric of space-time caused by
bodies such as binary systems (binary white dwarfs, neutron stars or black holes). The Laser Interferometer Space Antenna
(LISA) is one of several missions that will hopefully detect gravitational waves.
Gravitationally Bound
Objects held in orbit about each other by their gravitational attraction. For example, satellites in orbit around the Earth are
gravitationally bound to this planet since they can’t escape its gravity. By contrast, the Voyager spacecraft, which explored the
outer solar system, was launched with enough energy to escape Earth’s gravity altogether, and hence it is not gravitationally
bound.
Gravity (Gravitational Force)
The attractive force between all masses in the universe. All objects that have mass possess a gravitational force that attracts all
other masses. The more massive the object, the stronger the gravitational force. The closer objects are to each other, the
stronger the gravitational attraction.
Gravity Assist
An effect through which an orbiting object, such as a spacecraft or a comet, gains or loses speed by virtue of the gravitational
might of a planet or other celestial object that it passes. For example, the Cassini spacecraft in its journey to Saturn used a
gravity assist from Earth to increase its velocity by about 36,000 kilometres per hour (22,300 miles per hour).
Gravity Waves
These are waves produced by the gravitational effects of one body on another. The Moon causes rocks on Earth to physically
bulge up – not as much as water, but measurably. Gravity waves shouldn’t be mistaken for gravitational waves predicted by
general relativity.
Great Circle
The intersection of a plane that passes through the centre of a sphere with the surface of that sphere; the largest possible circle
that can be drawn on the surface of a sphere.
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Great Red Spot
A circulating storm located in Jupiter’s upper atmosphere. The storm, which rotates around the planet in six days, is the width of
two to three Earths. Galileo first observed the spot in the 17th century.
Great Rift
the Great Rift (sometimes called the Dark Side, Dark Rift, or, Dark River) is a series of overlapping, non-luminous, molecular
dust clouds that are located between the Solar System and the Sagittarius Arm of the Milky Way Galaxy at a distance of about
100 parsecs (300 ly or 3×1015 km) from Earth. The clouds are estimated to contain about 1 million solar masses of plasma and
dust. To the naked eye, the Great Rift appears as a dark lane that divides the bright band of the Milky Way lengthwise, through
about one-third of its extent, and is flanked by lanes of numerous stars.
Greatest Eastern Elongation
The greatest angular distance to the east of the Sun reached by Mercury or Venus. When a planet is at its eastern elongation, it
sets after the Sun and is at its best visibility in the evening sky.
Greenhouse Effect
A warming of the Earth's surface and lower layers of the atmosphere caused by interaction of solar radiation with atmospheric
gases (mainly carbon dioxide, methane, and water vapour) and its conversion to heat because it is transparent to incoming
visible radiation but opaque to the infrared radiation that is emitted by the surface of the planet.
Greenhouse Gas
Gases such as carbon dioxide, methane, and water vapour that produce a greenhouse effect.
Gregorian Calendar
The calendar in current use, with normal years that are 365 days long, with leap years every fourth year except for years that
are divisible by 100 but not by 400.
Gregorian Telescope
A Gregorian telescope is a reflecting telescope that uses two specific shapes of concave mirrors to form the image. The primary
mirror is a concave paraboloid, just as in a Newtonian. The secondary mirror is a much smaller concave ellipsoid, positioned a
little way beyond the primary's focal plane. The secondary reflects the beam of light diverging from the focus, back toward the
primary, which typically has a hole in the centre, so that the focal plane of the combined mirrors is easily accessible behind the
primary.
Ground State
The state of an atom in which all electrons are in the lowest possible energy levels.
Group
A vertical column of the periodic table; a family of elements; also, A characteristic part of a molecule.
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Great Red Spot
A large high pressure storm system on Jupiter akin to a cyclone on earth. It is almost three times the diameter of the earth,
produces winds of up to 400km/h and has been raging for at least 300 years. It transits the face of Jupiter every 8 hours.
Guan Sheng Optical
Maker of telescopes in Taiwan, many of which are re-branded as "Bintel", "Southern Cross", "SkyWatcher", etc., particularly the
Dobsonians.
Guide Star
A star that a telescope’s guidance system locks onto to ensure that a celestial object is followed and observed as the telescope
moves, owing either to the Earth’s rotation or the telescope’s orbital trajectory. The Hubble Space Telescope uses two of its
three Fine Guidance Sensors to detect and lock onto guide stars. The telescope’s science operations centre has more than 15
million guide stars in its database - the Guide Star Catalogue.
Guiding
Used in astroimaging. Guiding uses a second imaging device to "lock on" to a stars centroid and send correcting signals to the
mount as this star moves. With good Polar Alignment, a stable mount and good guiding, long exposures are possible with very
little translation of the objects being imaged. Guiding becomes more and more difficult as the scope focal length and FOV
reduces.
Guidescope
A (usually) smaller telescope operated in conjunction with a CCD camera or modified webcam, mounted on top of a main imaging
scope. It’s used to guide the movement of the mount to iron out any imperfections in the tracking.
Gyrochronology
Scientists have found a more accurate way to calculate the ages of stars. Gyrochronology uses the rotation period and colour of
a star to determine its age, with the age of the Sun being used to calibrate the measurements. Unlike another technique for
calculating stellar ages, gyrochronology works well for main sequence stars (stars that are still burning hydrogen) and lone stars
that don’t belong to a cluster.
Gyroscope
A gyroscope is a spinning wheel mounted on a movable frame that assists in stabilizing and pointing a space-based observatory.
Gyroscopes are important because they measure the rate of motion as the observatory moves and help ensure the telescope
retains correct pointing during observations. The gyroscopes provide the general pointing of the telescope while the fine
guidance sensors provide the “fine tuning.” Gyroscopes are used in navigational instruments for aircraft, satellites, and ships.
The Hubble Space Telescope has six gyroscopes for navigation and sighting purposes.
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..H
H-I Region
An interstellar region of neutral hydrogen.
H-II Star Forming Region
A large nebulous region of hydrogen gas that is being excited and ionised by the strong ultraviolet radiation from hot newly
formed stars.
Ha Filter
A particular type of filter used for observing the sun, or DSO Hydrogen line emissions.
Hb Filter
Used to observe very faint objects which wouldn't normally be seen without one e.g. Horsehead Nebula.
H-Alpha Light
Atoms emit radiation at certain wavelengths based on how the electrons move between certain fixed energies. If an electron
jumps to a specific energy level within a hydrogen atom it will emit radiation of a specific wavelength.
Habitable Zone
A region around a star where planets with liquid water may be present. A planet on the near edge of the habitable zone would
have a surface temperature slightly lower than the boiling point of water. A planet on the distant edge of the habitable zone
would have a surface temperature slightly higher than the freezing point of water.
Hadron
A class of subatomic particle that includes mesons and baryons. A hadron is defined as a particle that interacts via the strong
force.
Hadron Epoch
Second epoch in the history of the Universe, lasting on the order of a second; named for the heavy elementary particles
(protons, neutrons, mesons) that were the most abundant form of matter at the time.
Half-Life
The period of time needed for half of the radioactive isotopes in a sample to decay to daughter atoms.
Halo
(a) The extended outer portions far above and below the plane of a galaxy such as the Milky Way. The halo is thought to contain
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a large fraction of the total mass of the galaxy, mostly in the form of dim stars and interstellar gas.
(b) the extensive cloud of gas surrounding the head of a comet.
Hartmann Mask
An alternative focusing tool, like a Bahtinov Mask.
HDF-N
Hubble Deep Field North (HDF-N) is a tiny region of the northern sky near the Big Dipper toward which the Hubble Space
Telescope was pointed for ten straight days in 1995. Because this observation was designed to detect very faint light from the
most distant galaxies Hubble can observe, the field contains few bright celestial objects. Seemingly devoid of light, this small
area provided a “keyhole” view of the universe’s past, reaching across space and time to see infant galaxies. By probing these
remote regions of space, astronomers are gaining more information on galaxy development.
HDF-S
Hubble Deep Field South (HDF-S) is a tiny region of the southern sky near the Southern Cross toward which the Hubble Space
Telescope was pointed for ten straight days in 1998. Because this observation was designed to detect very faint light from the
most distant galaxies Hubble can observe, the field contains few bright celestial objects. Seemingly devoid of light, this small
area provided a “keyhole” view of the universe’s past, reaching across space and time to see infant galaxies. By probing these
remote regions of space, astronomers are gaining more information on galaxy development.
Head
Of a comet, the nucleus and coma together.
Heat
A measure of a quantity of energy; of how much energy a sample contains.
Heat Capacity (of a substance)
The amount of heat needed to change the temperature of the substance by 1°C.
Heat of Vaporization (of a substance)
The amount of heat involved in the evaporation or condensation of 1g of the substance.
Height
Linear distance (usually measured in meters) above sea level; as contrasted with altitude, which is angular distance above the
astronomical horizon. Usually called “elevation” by geographers.
Heliacal Rising
The date when a star first becomes visible in the dawn sky, rising just before the Sun.
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Helical Focuser
A telescope focuser in which adjustment of focus is made by screwing in and out either the eyepiece itself, or a focus tube which
contains it. Many binoculars have a helical focuser for one or both eyepieces.
Heliocentric
Sun-centred; using the sun rather than the earth as the point to which we refer. A heliocentric measurement, for example,
omits the effect of the Doppler shift caused by the earth's orbital motion.
Heliocentric Model
This literally means ‘Sun-centred’ and is the accepted model for the Solar System that puts the Sun at the centre, with the
planets orbiting around it.
Heliopause
The point where the pressure of the local interstellar medium overwhelms that of the solar wind, stopping it from expanding any
further out into space.
Helioseismology
The study of the internal structure of the Sun, which involves examining how pressure waves propagate within it.
Heliosheath
The heliosheath is the region of the heliosphere beyond the termination shock. Here the wind is slowed, compressed and made
turbulent by its interaction with the interstellar medium. Its distance from the Sun is approximately 80 to 100 astronomical units
(AU) at its closest point.
Heliosphere
A ‘bubble’ that surrounds our Solar System created by the influence of the Sun’s solar wind and magnetic field interacting with
interstellar space.
Helium
Atom consisting of two protons and two electrons.
Helium Flash
A rapid burst of nuclear reactions in the degenerate core of a moderate-mass star in the hydrogen shell-burning phase. The
flash occurs when the core temperature reaches a sufficiently high temperature to trigger the triple-alpha reaction.
Hemisphere
Half of a spherical or roughly spherical body; for example, the northern and southern halves of the Earth, above and below the
equator.
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Hertz (Hz)
Measure of frequency with units of sec-1, formerly called cycles per second; an oscillating system that completes a cycle a second
has a frequency of 1Hz.
Hertzsprung- Russell Diagram
A diagram on which stars are represented according to their absolute magnitudes (on the vertical axis) and spectral types (on
the horizontal axis).Because the physical properties of stars are interrelated, stars do not fall randomly on such a diagram, but
instead lie in well-defined regions according to their state of evolution. Very similar diagrams can be constructed that show
luminosity instead of absolute magnitude, and temperature or colour index in place of spectral type.
Hertzsprung Gap
A region above the main sequence in a Hertzsprung-Russell diagram through which stars evolve rapidly and thus in which few
stars are found.
Heterocyclic Compound
A cyclic compound in which one or more atoms in the ring is (are) not carbon.
Heterotrophic Hypothesis
The concept introduced by A.I. Oparin and J.B.S. Bernal that the earliest forms of life were heterotrophs that used nonbiologically produced organic matter as their carbon source.
Heterotrophy
Literally, other-feeding; the condition of an organism that is not able to obtain nutrients by synthesizing non organic materials
from the environment, and that therefore must consume other life forms to obtain the organic products necessary for life e.g.,
animals, fungi, most bacteria.
High-Velocity Star
A star whose velocity relative to the solar system is large. As a rule, high-velocity stars are Population II objects following orbital
paths that are highly inclined to the plane of the galactic disk.
Histogram
A histogram is a graph of how many pixels of each intensity level occur in an image. They are X/Y graphs and show pixel
intensity along the X axis and number of pixels on the Y axis. An un-stretched histogram may look like a flat line with a large
spike at the far left side.
Histogram Stretching
Turning the histogram with a 'flat line with a bump' into a wide-based mountain. Most software allows this to be done linearly
(levels) or non-linearly (curves).
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Homeobox
A highly conserved sequence of 180 nucleotides common to many regulatory genes and coding for the DNA-binding part of the
corresponding regulatory proteins.
Homeostasis
The ability of living organisms to keep constant certain of their physical or chemical properties by self-regulation.
Homeotic Gene
A regulatory gene containing a homeobox sequence.
Homogeneous
Having the quality of being uniform in properties throughout. In astronomy, this term is often applied to the universe as a
whole, which is postulated to be homogeneous.
Homologous Series
A series of compounds in which adjacent members of the series differ by a fixed unit of structure.
Horizon
The horizontal line that appears to separate the Earth from the sky. A confusion astronomically, as the term is used in many
ways: apparent horizon, astronomical horizon, and geometric horizon. To an astronomer, one of these horizons is a collection of
directions in space, and in fact a circle on the celestial sphere.
Horizon Co-Ordinates
The system of celestial co-ordinates in which the observer's horizon is the reference plane and the north point is the reference
direction. The positions are given in altitude and azimuth.
Horizontal Gene Transfer
The transfer of genes from one organism to another, as opposed to vertical gene transfer, from parent to offspring.
Horizontal Branch
A sequence of stars in the H-R diagram of a globular cluster, extending horizontally across the diagram to the left from the redgiant region. These are probably stars undergoing helium burning in their cores, by the triple-alpha reaction.
Horizontal Refraction
The astronomical refraction at the astronomical horizon. Not refraction parallel to the horizon, as non-astronomers might
suppose.
Hormone
connection from the cells that secrete it to the cells on which it acts.
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Host Galaxy
A galaxy in which a cosmic phenomenon, such as a supernova explosion or a gamma-ray burst, has occurred.
Hot Pixels
These are pixels that have 'turned on' on your sensor for whatever reason. They show up in the image as static red, green or
blue 'dots' that are fairly bright. Dark frames usually get rid of them but sometimes one has to go through a final image
removing them with the clone stamp tool.
Hour Angle
Of a celestial object as seen from a particular location, the difference between the local sidereal time and the right ascension
(H.A. = L.S.T. - R.A.).
Hour Circle
(i) A great circle passing through an object and the celestial poles.
(ii) The setting circle on the polar axis of an equatorial mount.
Hubble
A diagram formulated by astronomer Edwin Hubble that is used to classify galaxies. It is the shape of a tuning fork, beginning at
one end with elliptical galaxies and then splitting into spirals and barred spirals.
Hubble Constant
The numerical factor, usually denoted H, that describes the rate of expansion of the universe. It is the proportionality constant in
the Hubble law v ~Hd, which relates the speed of recession of a galaxy (v) to its distance (d) . The present value of H has
recently become fairly well known; estimates range between 47 and 63 km/sec Y Mpc, giving an age of the universe around 14
billion years.
Hubble's Law
Mathematically expresses the idea that the recessional velocities of faraway galaxies are directly proportional to their distance
from us. Hubble’s Law describes the relationship of velocity and distance by the equation V=Hoxd, where V is the object’s
recessional velocity, d is the distance to the object, and Ho is the Hubble constant. Essentially, the more distant two galaxies are
from each other, the faster they are traveling away from each other. American astronomer Edwin Hubble discovered this
relationship in 1929 when he observed that galaxies and clusters of galaxies were generally moving away from each other.
Hubble Colours
Artificially coloured image using Ha S2 and O-III Filters instead of RGB.
Hubble Space Telescope
An orbiting telescope that collects light from celestial objects in visible, near-ultraviolet, and near-infrared wavelengths. The
telescope was launched April 24, 1990 aboard the NASA Space Shuttle Discovery. The 12.5-ton (11,110-kg), tube-shaped
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telescope is 13.1 m (43 ft) long and 4.3 m (14 ft) wide. It orbits the Earth every 96 minutes and is mainly powered by the
sunlight collected by its two solar arrays. The telescope’s primary mirror is 2.4 m (8 ft) wide. The telescope is operated jointly
by the National Aeronautics and Space Administration (NASA) and the European Space Agency (ESA). HST is one of the many
NASA Origins Missions, which include current satellites such as the Far Ultraviolet Space Explorer (FUSE) and future space
observatories such as the James Webb Space Telescope (JWST).
Human Genome Project
An international protect to map the entire genome of Homo sapiens.
Humectant
A moistening agent.
Huygens
See “Eyepiece”
Hydrocarbon
Any of a diverse group of organic compounds composed of hydrogen and carbon.
Hydrogen
Simplest atom, consisting only of one proton and one electron; the most abundant element in the Universe.
Hydrogen-Alpha (Sun)
A spectral line which has a wavelength of 656.3nm. It is this wavelength of light, from the Sun, which is observed with certain
specialist solar telescopes.
Hydrogen-Alpha Filter
A type of optical filter that’s used for observing the Sun. It blocks all wavelengths of light except for a very narrow portion at
656.3nm.
Hydrogen-Beta Filter
This filters out all light except that of the hydrogen-beta line that has a wavelength of 486.5nm. It is used to observe very faint
objects, such as the Horse Head Nebula and California Nebula, which wouldn’t normally be seen without one.
Hypergiant
Unstable massive stars who are constantly loosing mass and who have tremendous luminosities of between 100,000 and a
million times that of our Sun.
Hypervelocity Stars
These stars are thought to be the result of a violent close encounter of a binary star with a supermassive black hole. As one of
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the stars is consumed by the black hole, the other is violently thrown away. It’s sent travelling through the galaxy at
phenomenal speed.
Hypotheses
Guesses that can be tested by experiment.
..I
Ice
A term used to describe water or a number of gases such as methane or ammonia when in a solid state.
Ideal Gas Law
The volume of a gas is proportional to the amount of gas and its Kelvin temperature and inversely proportional to its pressure.
Illuminance
The amount of visible light per unit area. This is the visual quantity analogous to the radiometric quantity “irradiance”.
Image Depth
Generally how well faint elements are clearly resolved. A good measure of an images depth is to consider the faintest star
clearly visible.
Image Processing
We are trying to capture details of objects that are generally not visible to the naked eye. Images straight from a CCD or DSLR
tend to be very dark with all the information within a very narrow part of the histogram. They may also not be well colour
balanced and be lacking in sharpness and contrast. Image Processing is akin to developing a negative in that all the detail
present is coaxed back. Common image processing software include Photoshop, GIMP and PixInsight. Image Processing is a
huge topic in its own right but is an important aspect of AstroImaging.
Image Scale
How many arc seconds each pixel in the image represents. Calculated via the formula [pixel size (in µm) x 206 / Scope Focal
Length].
Image Tube
An electronic device that receives incident radiation and intensifies it or converts it to a wavelength at which photographic plates
are sensitive.
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Imbrian Period
The period in the Moon’s history when the large basins Imbrium, Crisium, Tranquilitatis, Serenitatis, Fecunditatis and Procellarum
were formed, and filled with basalts. The Imbrian period occurred between 3.8 and 3.2 billion years ago.
Immersion
The entry of an object into shadow during an eclipse, or the covering of an object during an occultation.
Impact Crater
A crater formed on the surface of a terrestrial planet or a satellite by the impact of a meteoroid or planetesimal.
Impact Event
A collision between two solar system bodies that releases exceptionally large amounts of energy. Some examples are the 1908
Siberian Tunguska impact by a comet or an asteroid and the asteroid that struck Earth 65 million years ago, which may have led
to the extinction of the dinosaurs and other species of the Cretaceous-Tertiary era.
Impactor
The part of the Deep Impact spacecraft that crashed into comet 9P/Tempel 1. When launched, the impactor and the flyby
spacecraft were attached to each other. The spacecraft launched the impactor a day before the crash. As the impactor punched
through the comet’s crust, the flyby craft recorded the event from a safe distance away.
Inclination
A measure of the tilt of a planet's orbital plane in relation to that of the Earth.
Index of Refraction
Ratio of the speed of light in a vacuum to its speed in a given medium.
Inertia
The tendency of an object to remain in its state of rest or uniform motion; this tendency is directly related to the mass of the
object.
Inferior Conjunction
A conjunction of an inferior planet that occurs when the planet is lined up directly between the Earth and the Sun.
Inferior Planet
A planet that orbits between the Earth and the Sun. Mercury and Venus are the only two inferior planets in our solar system.
Inflation
The theorised rapid expansion (inflation) of the Universe right after the Big Bang.
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Infrared
Electromagnetic radiation at wavelengths that are longer than those the red end of the visible-light spectrum and shorter than
microwaves (roughly between 1 and 100 microns). Very little infrared light reaches the surface of the Earth, although some can
be observed by high-altitude aircraft (such as the Kuiper Observatory) or telescopes on high mountaintops (such as the peak of
Mauna Kea in Hawaii).
Ingress
The period during a transit or an eclipse when the smaller celestial body begins to cross the disc of the larger object. Egress
marks the exit of the transiting or eclipsing body.
Integrated Magnitude
The integrated magnitude is the magnitude of a diffuse object if it were gathered up into a point like source.
Intensity
The amount, degree, or quantity of energy passing through a point per unit time. For example, the intensity of light that Earth
receives from the Sun is far greater than that from any other star because the Sun is the closest star to us.
Interstellar Medium
The matter that lies between the stars of a galaxy. It is composed of 99 per cent gas and 1 per cent dust. Hydrogen makes up
90 per cent of the gas and the remainder is helium and other heavier elements. The dust is mainly made up of carbon and
silicates.
Interference
The property of radiation, explainable by the wave theory, in which waves that are in phase can add (constructive interference)
and waves that are out of phase can subtract (destructive interference); for light, this gives alternate light and dark bands.
Interferometer
A device that uses the property of interference to measure such properties of objects as their positions or structure.
Interferometry
An astronomical technique in which the images from two or more telescopes are superimposed. Interferometry has many uses,
and one of the most intriguing is its ability to cancel out the light from a point source such as a star by superimposing the images
in such a way that the oscillations in light intensity from one image are the reverse of the oscillations from the other image. With
this method, planets orbiting the star can be directly observed, and the composition of their atmospheres can be investigated
using spectral analyses.
Intergalactic
Occurring between galaxies.
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International Space Station
The manned ISS is according to NASA the biggest and most complex scientific project in history. During twilight passed, the
space station is easily seen by everyone as a strikingly bright and silently running star. It crosses the sky in a few minutes
basically from west to east.
Interplanetary
Existing or occurring between planets.
Interplanetary Dust Particle
Small, often microscopic rocky particles of interplanetary debris.
Interplanetary Medium
Gas and dust between the planets.
Interstellar Cloud
A region of relatively high density in the inter-stellar medium. Interstellar clouds have densities ranging between 1 and 1011
particles per cubic centimetre, and in aggregate, contain most of the mass in interstellar space.
Interstellar Extinction
The obscuration of starlight by interstellar dust. Light is scattered off of dust grains, so that a distant star appears dimmer than
it otherwise would. The scattering process is most effective at short (blue) wavelengths, so that stars seen through inter- stellar
dust appear reddened and dimmed.
Interstellar Medium
The sparse gas and dust located between the stars of a galaxy.
Interstellar Reddening
This is an effect produced by the incremental absorption and scattering of electromagnetic energy from interstellar matter, an
effect known as extinction. This effect causes the more distant objects such as stars to appear redder and dimmer than
expected. It is not to be confused with the separate phenomenon of red shift.
Interstellar Space
The dark regions of space located between the stars.
Intervalometer
A camera accessory that measures time intervals, often used to create time-lapse sequences.
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Inverse Compton Scattering
A method of converting photons from lower to higher energy through interaction with electrons that are moving with velocities
close to the speed of light.
Inverse-Square Law
Any law describing a force or other phenomenon that decreases in strength as the square of the distance from some central
reference point. The term inverse-square law is often used by itself to mean the law staling that the intensity of light emitted by
a source such as a star diminishes as the square of the distance from the source.
Inversion
This refers to something being upside down.
Ion
An atom that has gained or lost one or more electrons, giving it an overall negative or positive electrical charge respectively.
Ionic Gas
Also known as ionized gas. Gas whose atoms have lost or gained electrons, causing them to be electrically charged. In
astronomy, this term is most often used to describe the gas around hot stars where the high temperature causes atoms to lose
electrons.
Ionization
This is when an atom is stripped of or receives electrons. It often happens in astronomy where a gaseous nebula is ionized by
stars within it.
Ionizing Radiation
Radiation that produces ions as it passes through matter.
Ionosphere
The zone of the earth's upper atmosphere, between 80- and 500-km altitude, where charged subatomic particles (chiefly protons
and electrons) are trapped by the earth's magnetic field.
Iridium
An element with an atomic number of 77 and is possibly the densest of all elements (Osmium being the other contender). It is
considered a low toxicity substance, but in powder form is a known irritant and a fire hazard. Pure iridium is very brittle and is
nearly impossible to machine, and is the most corrosive resistant metal known.
A thin, worldwide layer of iridium exists in a layer of sediment that was put down at the end of the Cretaceous period (approx 65
million years ago). Since meteors and asteroids contain a higher percentage of iridium than the earth's crust, this iridium
enriched layer is seen as evidence that the earth was struck by a large meteor or asteroid at that time. Dust from the impact
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would have spread around the globe, depositing the iridium. The dust also would have blocked the sun for a time, resulting in
the extinction of many plant and animal species.
Iridium Flare
A bright flash in the night sky caused by sunlight glinting off solar panels of Iridium communications satellites. See
www.heavens-above.com for details of when an Iridium flare will be visible from your location.
Irradiance
The amount of power per unit area in a beam of light. Sometimes also called flux. Units: W m-2
Irregular Galaxy
A galaxy with no spiral structure and no symmetric shape. Irregular galaxies are usually filamentary or very clumpy in shape.
ISO Number
This is the number that indicates the sensitivity (or ‘speed’) of a film or a given DSLR exposure.
Isochron
A parameter used in isotopic dating of geologic materials, experimentally determined from comparison of the isotopic
compositions of two or more components (usually minerals) that share a common age.
Isomerization
The conversion of a compound into one or more of its isomers.
Isomers
Compounds that have the same molecular formula but different structural formulas and properties.
Isotopes
different forms of an element that have the same number of protons in their nuclei, and thus the same atomic number, but that
have different numbers of neutrons and thus different atomic masses. There are two kinds of isotopes, stable and unstable.
Isotopes that are unstable are called radioactive and disintegrate at a constant decay rate. Examples of stable isotopes include
carbon-12 and carbon-13. Carbon-14, and uranium-238 and -235, are examples of unstable isotopes.
Isotopic
This term describes anything relating to isotopes. Isotopes are atoms of the same element that have the same number of
protons, but different numbers of neutrons
Isotopic Date
Age of a rock (or organic substance less than 60,000 years old) determined by measurement of the ratio of a parent isotope to
one of the products of its radioactive decay.
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Isotopic Fractionation
Separation of isotopes of an element, in organisms often mediated by an enzyme.
Isotropic
Having the property of appearing the same in all directions. In astronomy, this term is often postulated to apply to the universe
as a whole.
..J
J2000, Precession, Nutation
The plains of ecliptic and equator shift with time by perturbations from the Sun, Moon and planets. The long-term shift is called
precession; the short periodic variations are called nutation. The given celestial coordinates are referred to the true direction of
the vernal equinox and the true obliquity of the ecliptic to the standard reference time 1 January 2000. For this date many star
charts and coordinate tables are printed.
Jansky (Jy)
Convenient unit of incident spectral flux density used in radio astronomy; 1 Jy = 10-26 W/m2 Hz (named for Karl G. Jansky,
initial discoverer of extraterrestrial radio radiations).
Jeans Instability
Is a physical state in which an interstellar cloud of gas will begin to undergo collapse and form stars. A cloud can become
unstable against collapse when it cools sufficiently or has perturbations of density, allowing gravity to overcome the gas
pressure.
Jets
Narrow, high-energy streams of gas and other particles generally ejected in two opposite directions from some central source.
Jets appear to originate in the vicinity of an extremely dense object, such as a black hole, pulsar, or protostar, with a
surrounding accretion disk. These jets are thought to be perpendicular to the plane of the accretion disk.
Joule (J )
Unit for work in the MKS system of units: 1 J = 107 ergs = 0.239 cal.
Jovian Moon
A natural satellite belonging to the gas giant Jupiter.
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Jovian Planet
Same as giant planet; in our solar system: Jupiter (Jove), Saturn, Uranus and Neptune.
Julian Date
The interval of time in days (and fraction of a day) since Greenwich noon on Jan. 1, 4713 BC. The JD is always half a day off
from Universal Time, because the current definition of JD was introduced when the astronomical day was defined to start at noon
(prior to 1925) instead of midnight. Thus, 1995 Oct. 10.0 UT = JD 2450000.5.
..K
Kardashev Cultures
N. X. Kardashev has distinguished three types of technological societies according to the amount of power they can harness:
Type I can engage the power available on a planet; Type II. the power output of a star; and Type III, the power output of a
galaxy.
Kellner
See “Eyepiece”
Kelvin
A temperature scale used in sciences such as astronomy to measure extremely cold temperatures. The Kelvin temperature scale
is just like the Celsius scale except that the freezing point of water, zero degrees Celsius, is equal to 273 degrees Kelvin.
Absolute zero, the coldest known temperature, is reached at 0 degrees Kelvin or -273.16 degrees Celsius.
Kepler Elements
Another term for orbital elements.
Kepler's First Law
A planet orbits the Sun in an ellipse with the Sun at one focus.
Kepler's Second Law
A ray directed from the Sun to a planet sweeps out equal areas in equal times.
Kepler's Third Law
The square of the period of a planet's orbit is proportional to the cube of that planet's semi major axis; the constant of
proportionality is the same for all planets.
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Kilohertz,
1000 Hertz.
Kilocalorie
A unit of energy, the amount of heat required to raise the temperature of 1kg of water 1°C, 1000 calories.
Kilogram
The SI unit of mass, a quantity about equal to 2.2 pounds.
Kiloparsec
A unit of distance equal to 1,000 parsecs.
Kinetic Energy
The energy of motion. The kinetic energy of a moving object is equal to one-half times its mass times the square of its velocity.
Kinetic Isotopic Fractionation
Separation of isotopes of an element as a result of their speeds of movement, in organisms mediated by an enzyme that
interacts more readily with one of two or more isotopes of an element.
Kinetic-Molecular Theory
A model that uses the motion of molecules to explain the behaviour of the three states of matter.
Kirkwood's Gaps
Narrow gaps in the asteroid belt created by orbital resonance with Jupiter
Koenig
See “Eyepiece”
Kuiper Belt
A large ring of icy, primitive objects beyond the orbit of Neptune between 4.5 and 7.5 billion km from the Sun. Kuiper Belt
objects are believed to be remnants of the original material that formed the solar system. Some astronomers believe Pluto and
Charon are Kuiper Belt objects.
..L
L Wave
A type of seismic wave that travels only over the surface of the earth.
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Lagrange Point
The points in the vicinity of two massive bodies (such as the Earth and the Moon) where each others’ respective gravities
balance. There are five, labelled L1 through L5. The first three lie along the centreline between the centres of mass between the
two masses: L1 is on the inward side of the secondary, L2 is on the outward side of the secondary, and L3 is on the outward side
of the primary. L4 and L5, the so-called Trojan points, lie along the orbit of the secondary around the primary, sixty degrees
ahead and behind of the secondary. L1 through L3 are points of unstable equilibrium; any disturbance will move a test particle
there out of the Lagrange point. L4 and L5 are points of stable equilibrium, provided that the mass of the secondary is less than
about 1/25.96 the mass of the primary. These points are stable because centrifugal pseudo-forces work against gravity to cancel
it out.
Laminar Air Flow
This is where air flows smoothly in layers, without any interference to disrupt the flow. Such conditions enable better quality
seeing. Turbulent air flow reduces the observing quality.
Lanthanum
See “Eyepiece”
Lapse Rate
The rate at which temperature decreases with height in the atmosphere.
Large Magellanic Cloud
An irregular galaxy, the largest of two satellites of the Milky Way.
Lateral Mirage
A much mis-used term. Refraction in the horizontal direction is appreciable only when a boundary layer is stabilized by a wall or
other near-vertical surface, in such cases, the term “mural mirage” is more descriptive.
Latitude
The angular distance of an object north or south of Earth’s equator expressed in degrees. The latitude of the Equator, for
example, is 0°; Chicago is at 42° North; the North Pole is at 90° North; and Lima, Peru, is at 12° South. Latitudes south of the
equator are expressed in degrees South or as negative degrees.
Launch Window
The time opportunity for launching a spacecraft to another body in the Solar System involving minimal energy or the shortest
flight time.
Law of Conservation of Energy
The amount of energy within the universe is constant; energy cannot be created or destroyed, only transformed.
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Lens
A carefully ground or moulded piece of glass, plastic, or other transparent material that causes light to bend and either come
together or spread apart to form an image.
Lens Doublet
A set of two lenses, one concave and one convex, made from different types of glass. Together the lenses correct both spherical
and chromatic aberrations. A single lens alone cannot correct these aberrations.
Lenticular Galaxy
A disk-shaped galaxy that contains no conspicuous structure within the disk. Lenticular galaxies tend to look more like elliptical
galaxies than spiral galaxies.
Lepton Epoch
Lighter elementary particles such as electrons, neutrinos, and muons were the dominant form of matter.
Libration
The Moon rotates in one month in a constant motion around its own axis. In the same interval, it revolves on an elliptical motion
around the Earth. This combination leads to an inconstant exposure of the lunar surface to an observer on Earth: around 59% of
the surface is visible during one month.
Libration Zone
Regions of the Moon occasionally visible due to the Moon’s apparent periodic wobble.
Life Era
Era in the history of the Universe when life emerges as the dominant element.
Light
The common term for electromagnetic radiation, usually referring to that portion of the electromagnetic spectrum that is visible
to the human eye. Other bands of the EM spectrum, however, are also often referred to as different forms of light.
Light Bucket
Slang term for a telescope of large aperture.
Light Curve
A light curve is a graph of the brightness of an astronomical object. It’s usually applied to variable stars, novae or stars that are
transited by extra-solar planets.
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Light Echo
This is the reflection of light usually seen around the gaseous ejections from massive stars or sometimes supernova. Perhaps the
most prominent of all recently observed light echoes was the one seen around the exploding star V838 Monocerotis.
Light Frames
These are the actual pictures. The shutter is open, the telescope is tracking and the sensor is being bathed in ancient photons.
Light-Gathering Power
The ability of a telescope to collect light from an astronomical source. The light-gathering power is directly related to the area of
the primary mirror or lens.
Light-Harvesting Pigment
Organic compounds, such as chlorophyll and bacteriochlorophyll, that absorb light energy in photosynthesis.
Light Pollution
This is the pervasive orange glow which appears throughout much of the night sky in urban and indeed some rural areas. It is
caused by light reflected of bright surfaces and poorly positioned (or badly designed) lighting.
Light-Pollution (Reduction) Filter
A filter specially prepared to reflect wavelengths which are prominent in light pollution, and transmit other wavelengths. Such a
filter improves the contrast of deep-sky objects, when light pollution is present.
Light Speed
Also Lightspeed. The speed of light; In a perfect vacuum, is exactly 299,792,458 metres per second.
Light Year
An astronomical unit of measure equal to the distance light travels in a year, 9.4645661 x1012km. (5. 8786125 x1012 miles).
Limb
The outer edge or border of a planet or other celestial body.
Limb Darkening
The dark region around the edge of the visible disk of the sun or a planet, caused by a decrease in temperature with height in
the atmosphere.
Limestone
A kind of sedimentary rock consisting mainly of calcium carbonate minerals.
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Limiting Magnitude
The magnitude of the dimmest object that can be seen through any optical instrument (including the eye). The limiting
magnitude of an instrument varies with light conditions.
Line of Nodes
The imaginary line passing through the ascending and descending nodes of an orbit. It is the line of intersection of the orbital
plane with the reference plane.
Line Spectrum
The pattern of coloured lines emitted by an element.
Liquid
A state of matter in which the substance assumes the shape of its container, flows readily, and maintains a fairly constant
volume.
Liquid Metallic Hydrogen
Hydrogen in a state of semi rigidity that can exist only under conditions of extremely high pressure, as in the interiors of Jupiter
and Saturn.
Litre
Unit of volume equal to a cube with sides 10 cm.
Lithosphere
The layer in the earth, moon, and terrestrial planets that includes the crust and the outer part of the mantle.
Local Group (Local Supercluster)
A small group of 36 nearby galaxies of which our own Milky Way galaxy is a member.
Locked Rotation
The condition in which a moon has the same period of rotation as its period of revolution around its parent body. This means
that the moon always shows the same face to its parent. Our Moon is in locked rotation around Earth.
Longitude
The angular distance of an object east or west of the Prime Meridian—the line of zero longitude that runs through Greenwich,
England—expressed in degrees. The longitude of Chicago, for example, is 88° West.
Longitude of the Ascending Node
The angular distance, measured around the reference plane, between the FPA and the ascending node of an orbit.
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Longitudinal Chromatic Aberration
An aberration of optical systems, in which the focal length of a system is different for different colours of light.
Looming
The appearance above the horizon of a distant object that would normally be hidden below it. This effect is caused by unusually
large terrestrial refraction, usually due to a thermal inversion. Not Mirages.
Losmandy
United States manufacturer of well-machined, high-quality, German equatorial mountings, accessories, and attachments.
Low-Reflection Coating
Any of several coating materials whose purpose is to reduce unwanted reflections from optical elements which are supposed to
transmit light.
Luminance
(i) This is the name often given to the ‘light frames’ or exposures gathered from a monochrome CCD i.e. without colour.
(ii) The amount of visible light per unit area and per unit solid angle. Often called “surface brightness”.
Luminosity
The total energy emitted by an object per second; that is, the power of the object. For stars, the luminosity is usually measured
in units of ergs per second.
Luminosity Class
One of several classes to which a star can be assigned on the basis of certain luminosity indicators in its spectrum. The classes
range from I for supergiants to V for main-sequence stars (also known as dwarfs).
Lunar Eclipse
A phenomenon that occurs when the Moon passes into the shadow of the Earth. A partial lunar eclipse occurs when the Moon
passes into the penumbra, or partial shadow. In a total lunar eclipse, the Moon passes into the Earth's umbra, or total shadow.
Lunar Limb
The extreme edge of the disc of the Moon.
Lunar Month
The average time between successive new or full moons. A lunar month is equal to 29 days 12 hours 44 minutes. Also called a
synodic month.
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Lunation
The interval of a complete lunar cycle, between one new Moon and the next. A lunation is equal to 29 days, 12 hours, and 44
minutes.
Lyman Alpha Blob
These are vast glowing gas clouds that emit a characteristic wavelength of light known as the Lyman-alpha spectral line in
Hydrogen’s spectral signature.
Lyman Limit
A specific wavelength (91.2 nm) that corresponds to the energy needed to ionize a hydrogen atom (13.6 eV). Galactic space is
opaque at wavelengths shorter than the Lyman limit. Subsequently, light from cosmic objects at wavelengths less than the
Lyman limit is exceedingly difficult to detect.
..M
Ma
Mega Anna, one million (1,000,000 or 1 x 106) years.
Magellanic Clouds
Two small, irregular galaxies found just outside our own Milky Way galaxy. The Magellanic clouds are visible in the skies of the
southern hemisphere.
Magnetars
These are large Neutron Stars with super powerful magnetic fields, upwards of many trillions of times stronger than our Earths.
They are at least 30 times denser than our sun, but 45,000 times smaller.
Magnetic Braking
The slowing of the spin of a young star (such as the early sun) by magnetic forces exerted on the surrounding ionized gas.
Magnetic Dynamo
A rotating internal zone inside the sun or a planet, thought to carry the electrical currents that create the solar or planetary
magnetic field.
Magnetic Field
A condition found in the region around a magnet or an electric current, characterized by the existence of a detectable magnetic
force at every point in the region and by the existence of magnetic poles.
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Magnetic Pole
Either of two limited regions in a magnet at which the magnet's field is most intense.
Magnetosphere
A region, surrounding a star or planet, that is permeated by the magnetic field of that body.
Magnification
A measurable increase in the apparent size of an object. Magnification = (focal length of a telescope) / (focal length of the
eyepiece).
Magnitude
The units used to describe brightness of astronomical objects. The smaller the numerical value, the brighter the object. The
human eye can detect stars to 6th or 7th magnitude on a dark, clear night far from city lights; in suburbs or cities, stars may
only be visible to mag 2 or 3 or 4, due to light pollution. The brightest star, Sirius, shines at visual magnitude -1.5. Jupiter can
get about as bright as visual magnitude -3 and Venus as bright as -4. The full moon is near magnitude -13, and the sun near
mag -26. The magnitude scale is logarithmic, with a difference of one magnitude corresponding to a change of about 2.5 times
in brightness; a change of 5 magnitudes is defined as a change of exactly 100 times in brightness.
Main Belt
A collection of thousands of rocky and metallic bodies revolving more or less together between the orbits of Mars and Jupiter,
roughly 2.0 to 3.5 AU from the Sun. Most asteroids are part of the Main Belt.
Main Group Elements
The elements in the A groups of the periodic table that is customary in the United States and in Groups 1, 2, and 13 to 18 in the
periodic table recommended by IUPAC.
Main Sequence
principal sequence of stars on the graph of luminosity versus effective temperature (H-R diagram), encompassing more than
90% of observable stars. These stars are converting hydrogen to helium by nuclear reactions in their cores The lower mass limit
for the Main Sequence is 0.085 Mo and the upper limit is about 60 Mo.
Main-Sequence Fitting
A distance-determination technique in which an H-R diagram for a cluster of stars is compared with a standard H-R diagram to
establish the absolute magnitude scale for the cluster H-R diagram.
Main Sequence Star
A main sequence star is one that is in a stable part of its life converting hydrogen and helium through nuclear fusion. Most stars
we see in the night sky are in the main sequence stage and our Sun is currently also a main sequence star.
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Main-Sequence Turn-Off
In an H-R diagram for a cluster of stars, the point where the main sequence turns off toward the upper right. The mainsequence turn-off, showing which stars in the cluster have evolved to become red giants, is an indicator of the age of the cluster.
Major Axis
The longest diameter of an ellipse; the line from one side of an ellipse to the other that passes through the foci. Also, the length
of that line.
Major Planet
A name used to describe any planet that is considerably larger and more massive than the Earth, and contains large quantities of
hydrogen and helium. Jupiter and Neptune are examples of major planets.
Maksutov, Maksutov-Cassegrain, Maksutov-Newtonian
These are forms of catadioptric telescope.
Maksutov Corrector
A particular kind of lens used in Maksutov optical systems. Maksutov correctors are thick, and have strongly curved, nearly
concentric, surfaces. They resemble large, thick watch-glasses.
Mantle
The area in-between the Earth’s crust and its core (roughly 3,000km thick) that consists of hot solid rock and magma (molten
rock).
Mare (plural - maria)
A large plain on the lunar surface. Formed by cooling lava, they are generally much smoother than surrounding regions and
appear dark compared with the lunar highlands, and are characterised by large basalt plains enriched with titanium and iron.
Mare is Latin for ‘sea’.
Masayuma
See "Eyepiece"
Mascon
A concentration of mass under the surface of the moon, discovered from its gravitational effect on spacecraft orbiting the moon.
Maser
An acronym for "microwave amplification by stimulated emission of radiation," a device by which certain energy levels are more
populated than normal, resulting in an especially dense emission of radio radiation at a certain frequency when the system drops
to a lower energy level. A laser operating in the microwave region of the electromagnetic spectrum.
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Mass
A measure of the total amount of material in a body, defined either by the inertial properties of the body or by its gravitational
influence on other bodies.
Mass-Energy Equation
Einstein's equation E = mc2, in which E is energy, m is mass, and c is the speed of light.
Mass-Luminosity Relation
A well-defined relation between the mass and luminosity for main sequence stars.
Mass Number A
Nucleon number, the sum of the numbers of protons and of neutrons in the nucleus of an atom.
Mass Spectrometry
Instrumental method of identifying the chemical constituents of a substance by means of the separation of gaseous ions
according to their differing mass and charge.
Massing
The close alignment of three or more planets—or two or more planets and the Moon—as seen from Earth. This occurs when all
the bodies involved in the massing have similar ecliptic longitudes.
Matter
A word used to describe anything that contains mass. (As opposed to pure energy)
Matter Era
collective name for the most recent three epochs in the history of the Universe (atom, galaxy, stellar), covering all of time after
the Radiation Era.
Maunder Minimum
Virtual disappearance of sunspots in the period 1645 to 1715.
Mean Anomaly
The anomaly which would exist if a planet orbited at a uniform speed in a circular orbit.
Mean Solar Time
Time based on an imaginary "mean Sun". See the tutorial on Time.
Mega
106 (as in megahertz, MHz); one million.
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Megahertz
One million Hertz; a million cycles per second.
Megaparsec
Equals one million parsecs (3.26 million light-years) and is the unit of distance commonly used to measure the distance between
galaxies.
Melting Point
The temperature at which a substance changes from the solid to the liquid state.
Meridian
An imaginary line that circles the Earth from north to south that marks the point at which the Sun is at its highest. Anti- and
post-meridian (am and pm), mark the times before and after the Sun crosses the Meridian at midday, respectively.
Meridian Flip
A defined point for the limits of east or west side viewing where in order to prevent damage to the equipment caused by hitting
the pier or tripod the mount is stopped and its direction is reversed, effectively the mount moves so the telescope and weights
are now opposite side to where they were.
Messier Catalogue
The famous catalogue of deep sky objects such as nebulae, galaxies and clusters compiled by Frenchman Charles Messier and his
colleague Pierre Méchain in the 18th Century.
Metagalaxy
Any very large-scale organized collection of galaxies.
Metal
(a) For stellar abundances, any element higher in atomic number than 2, that is, heavier than helium or hydrogen.
(b) For a planet or solid, matter that is a good conductor of heat and electricity.
Metalicity
Elements in a star heavier than helium.
Meteor
This refers to the ionization trail produced by a small particle of rock or dust (Meteroroid) that burns away in the Earth's
atmosphere. Meteors are also referred to as shooting stars.
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Meteor Shower
An event where a large number of meteors enter the Earth's atmosphere from the same direction in space at nearly the same
time. Most meteor showers take place when the Earth passes through the debris left behind by a comet.
Meteor Trail
Sometimes a meteor leaves a trail of material as it passes through the Earth’s atmosphere. This ionised material glows for a
brief moment before fading and is known as a meteor trail, or train, or more recently, just Meteor.
Meteorite
An object, usually a chunk or metal or rock, that survives entry through the atmosphere to reach the Earth's surface.
Meteoroid
A rocky piece of debris in space that is smaller than an asteroid. If a meteoroid enters Earth’s atmosphere it is called a meteor.
Most meteors burn up in the atmosphere, but those that don’t and survive an impact on the ground are called Meteorites.
Meter
The SI unit of length, slightly longer than 39 inches
Methane
The colourless gaseous hydrocarbon CH4. In the cold vacuum of space, methane is a white solid but, when hit by sunlight, it can
become a gas.
Metonic Cycle
The period of 6 939.6 days, or 19 calendar years, after which the Moon's phases recur on the same day of the year. This period
is also equal to 253 lunations.
Micrometeorite
A particle from space that is small enough to be slowed down when it reaches the Earth's atmosphere without being burnt up.
Approximately 50 micrometeorites per square meter hit the top of Earth’s atmosphere each day.
Micrometer
(i) A unit of length, one-millionth (10-6) of a meter µm.
(ii) A device, of which various types exist, that is used in a telescope for measuring small angular distances between objects.
Micron
A micrometer
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Microwave
Electromagnetic wave roughly in the range 0.01-1 m in wavelength (ordinary broadcasting utilizes waves in the 200-600 m
range; the "short waves" used in long-distance communications are rarely shorter than 10m).
Milky Way
The band of light across the sky from the stars and gas in the plane of the Milky Way Galaxy.
Miller-Urey Experiment
An experiment conducted by Stanley Miller and Harold Urey at the University of Chicago in 1953 that simulated the conditions on
the early Earth. It was designed to test whether organic molecules could be created from inorganic molecules.
Minor Axis
The shortest diameter of an ellipse; the line from one side of an ellipse to the other that passes midway between the foci and is
perpendicular to the major axis.
Minor Planet
Another name used to describe a large asteroid. ie. an object in direct orbit around the Sun that is neither a dominant planet nor
originally classified as a comet. A moon cannot be a minor planet, because it is orbiting another body.
Mira Variable
A red giant star that decreases and increases in brightness over a period of at least 100 days. It’s been shown that Mira
variables can change brightness by as much as eight magnitudes.
Mirage
An inverted image caused by atmospheric refraction.
Mo
The Solar mass.
Mock Sun
An effect caused by ice crystals in Earth's atmosphere which refract sunlight and cause the appearance of two diffuse patches of
light 22° either side of the Sun. These patches are termed Parhelia or Sundogs and often appear on the rim of a halo
surrounding the Sun. The patches often have coloured fringes, red on one side, blue on the other.
Molar Mass
The formula weight expressed in grams.
Molar Volume
The volume occupied by 1 mole of a substance under specified conditions.
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Mole
The formula weight in grams of an element or compound; or a quantity of chemical substance that contains 6.02 x 1023 formula
units of the substance.
Molecular Cloud
A cold, dense, interstellar cloud in which molecules, mostly molecular hydrogen (H2), form.
Molecule
A collection of atoms bound together that is the smallest collection that exhibits a certain set of chemical properties.
Momentum
A measure of the tendency that a moving body has to keep moving. The momentum in a given direction (the "linear
momentum") is equal to the mass of the body times its component of velocity in that direction.
Mono
Black and White Image, or Camera.
Monocentric
See "Eyepiece"
Monochromator
An optical device, consisting of one or more slits, that selects a narrow band of wavelengths from a broader spectrum.
Monocular
A spotting telescope, which resembles half a binocular. Monoculars are often hand-held, and are used in much the same way,
and for the same purposes, as binoculars.
Month, Anomalistic
The interval between two successive perigee passages of the Moon, equal to 27.55 days.
Month, Nodical or Draconic
The interval between successive passages of the Moon through one of its nodes, equal to 27.21 days.
Month, Sidereal
The revolution period of the Moon relative to the stars, equal to 27.32 days.
Month, Synodic
The interval between two successive New Moon's (a lunation), equal to 29.53 days.
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Month, Tropical
The time taken for the Moon to return to the same celestial longitude (7 seconds shorter than the sidereal month).
Moon
Any natural satellite of a planet, that shines by the sun's reflected light.
Moon, Harvest
The Full Moon that occurs nearest the time of the autumnal equinox. So-called because this will occur around the time of
harvest.
Moon, Hunter's
The Full Moon that occurs after Harvest Moon. So-called because this will occur around the start of the hunting season.
Moon Illusion
An optical illusion in which the Moon appears larger when it is near to the horizon than when it is high in the sky.
Moonlet
A satellite of a small celestial object, such as a dwarf planet or an asteroid.
Mount
The mechanical element of the imaging system that permits the telescope to be pointed at a target object. Mounts come in two
types:
Alt/Az - where the movement axes are vertical and horizontal
German Equitorial - where the movements are in Declination and Right Ascension.
Mounting Rings
Circular clamps which fasten around an optical tube assembly and are threaded, machined, or otherwise prepared to fasten to a
mounting. Also called tube rings.
Moving Cluster
A physical grouping of stars moving through space that usually shares a common origin. The most famous moving cluster is the
Ursa Major Moving Group.
Moving Group or Stellar Association
This is a loose grouping of stars that are traveling together through space. Although the members were formed together in the
same molecular cloud, they have since moved too far apart to be gravitationally bound as a cluster.
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Multicoated Lens
This describes a lens or prism in which the surfaces where the beam of light goes from air into glass, or from glass into air, has
been coated with a high-tech, many-layered coating that decreases the amount of light that gets reflected.
Multichannel Spectrum Analyser
Short for MCSA/SD, a digital, energy-efficient, real-time, multichannel spectrum analyzer and signal detector. A hardware device
that continuously accepts a significant portion of the electromagnetic spectrum and divides the spectral energy into a set of many
contiguous frequency bins or output channels. Operational functions included are frequency analysis and signal detection,
averaging, threshold testing, etc..
Muon Collider
A type of particle accelerator capable of smashing particles called muons and anti-muons together to study high-energy
collisions.
..N
Nadir
The point on the celestial sphere directly below the observer. Opposite of zenith.
Nagler
See “Eyepiece”
Nanometer
A unit of length, one billionth (10-9) of a meter nm.
Narrow Band
The use of filters to restrict wavebands of light reaching the imaging device. Usually designed to capture the emission line of
common elements Ha (Hydrogen), OIII (Oxygen) and SII (Sulphur). When processing and imaging using Narrow Band data, the
colours are typically "mapped" to the RGB channels where Ha = Red, OIII = Blue and SII = Green. For the Hubble Palette,
R=Sulphur (wavelength is more "red"), Blue=OIII & Green=Ha, but results in a very "green" tint as Ha is the dominant.
Aesthetically, the "Green" is then translated to a "gold" colour & the rest towards a blue/cyan/teal colour.
National Aeronautics And Space Administration
A Federal agency created on July 29, 1958 after President Dwight Eisenhower signed the National Aeronautics and Space Act of
1958. NASA coordinates space exploration efforts as well as traditional aeronautical research functions.
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Natural Philosophy
Philosophical speculation about nature.
Nautical Twilight
When the centre of the Sun is between 6° and 12° below the horizon; the marine horizon becomes invisible.
Near-Earth Asteroid
An asteroid whose orbit brings it close to Earth's orbit. The criterion is when one is plotted to come closer than 20 times the
distance from Earth to the Moon AND is more than 150metrs wide. NASA, will then designate this asteroid as a "Potentially
Hazardous Object".
Near-Earth Object
A comet or asteroid whose orbit brings it close to Earth's orbit.
Near-Infrared
The region of the infrared spectrum that is closest to visible light. Near-infrared light has slightly longer wavelengths and slightly
lower frequencies and energies than visible light.
Nebula
An interstellar gas cloud. Those that emit visible light are called diffuse nebulae, and there are two types: emission nebulae are
self-luminous; reflection nebulae are illuminated by radiation from nearby stars
Nebular Hypothesis
A general theory that describes how stars and their associated solar systems are formed from the condensation of clouds of dust
and gas in space
Neutrino
Spinning, tiny, elementary particle, always travels at the speed of light, with essentially no mass and no electric charge, that
hardly interacts with matter and can pass straight through it. These particles are created in nuclear reactions in the Sun and
supernovae. Very high-energy neutrinos may also be created in active galactic nuclei or gamma-ray bursts.
Neutron
A massive, neutral elementary particle, one of the fundamental constituents of an atom. Neutrons and protons are similar in
mass, about 1830 times more massive than electrons.
Neutron Star
A compressed core of an exploded star made up almost entirely of neutrons. Neutron stars have a strong gravitational field and
some emit pulses of energy along their axis. These are known as pulsars.
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New Moon
The phase of the moon when the side of the moon facing the earth is the side that is not illuminated by sunlight.
Newtonian Telescope
A type of reflecting telescope where the beam reflected by the primary mirror is reflected by a flat secondary mirror so that the
focus falls to the side of the telescope tube.
Newton’s Law of Universal Gravitation
Two bodies attract each other with equal and opposite forces. The magnitude of this force is proportional to the product of the
two masses and is also proportional to the inverse square of the distance between the centres of mass of the two bodies.
Newton’s First Law of Motion
A body continues in its state of constant velocity (which may be zero) unless it is acted upon by an external force.
Newton’s Second Law of Motion
For an unbalanced force acting on a body, the acceleration produced is proportional to the force impressed. The constant of
proportionality is the inertial mass of the body.
Newton’s Third Law of Motion
In a system where no external forces are present, every action force is always opposed by an equal and opposite reaction.
New General Catalogue
The New General Catalogue (NGC) was originally compiled and published by J. L. E. Dreyer in 1888. Two additional
supplements—called the Index Catalogue (IC)—were published in 1895 (IC I) and 1908 (IC II). These catalogues were an early
attempt to create a single list containing all the non-stellar objects known at the time. The original NGC/IC catalogue contains
13,226 entries.
Night Vision
The increased ability to see dim object, such as faint stars, due to a sensitization of the eye's 'rod' receptors. Exposure to bright
light desensitizes the rods and therefore reduces night-vision.
Noctilucent Clouds
These are thin clouds of ice and dust particles that are so high in the Earth’s atmosphere that they are bright during twilight, in
the early summer months (dusk, and just before dawn).
Noble Gas
One of a group of rare but extremely stable gases with low reaction rates (helium, neon, argon, krypton, xenon, radon).
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Node
A point of intersection between two great circles. Eclipses of the Sun and Moon occur when these bodies are simultaneously near
the nodes of their paths in the sky.
Noise
In astronomy this refers to the unwanted signal in CCD images. There are two main causes of noise in CCD cameras used for
astronomical imaging: the intrinsic noise in the image itself and thermal noise. Having the CCD cooled via fans or Peltier cooling
systems can reduce noise.
Non-Metals
The group of elements to the right of the heavy, stepped, diagonal line in the periodic table.
Nonthermal Radiation
Radiation that cannot be characterized by a single number (the temperature). Normally, we derive this number from Planck's
law, so that radiation that does not follow Planck's law is called non-thermal.
North Celestial Pole
A direction determined by the projection of the Earth’s North Pole onto the celestial sphere. It corresponds to a declination of
+90 degrees. The North Star, Polaris, sits roughly at the NCP.
Northern Lights
The aurora of the northern hemisphere. Also called the Aurora Borealis.
Nova
A star that flares up to several times its original brightness for some time before returning to its original state.
Nuclear Fission
The splitting of an atomic nucleus into two large fragments.
Nuclear Fusion
A nuclear process whereby several small nuclei are combined to make a larger one whose mass is slightly smaller than the sum
of the parts. The difference in mass is converted to energy according to Einstein’s famous equation, E=mc2 where he discovered
that the energy contained in such a reaction (E) is equivalent to the difference in mass (m) times the square of the speed of light
(c) which is a very, very large number. This is the source of the Sun’s energy.
Nuclear Reactor
A plant that produces energy by nuclear fission.
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Nuclear Transformation
The process by which an atomic nucleus is transformed into another type of atomic nucleus. For example, by removing an alpha
particle from the nucleus, the element radium is transformed into the element radon.
Nuclear Winter
A period of dark, cold weather that may be caused by the dust and smoke entering the atmosphere from the explosion of nuclear
bombs.
Nucleon Number
The total number of protons and neutrons in an atom; the mass number A.
Nucleons
Protons and neutrons in an atomic nucleus.
Nucleosynthesis
The binding of atoms to produce heavier elements. It takes place in stars (see nuclear fusion) and occurred in the first three
minutes and 45 seconds after the Big Bang when it’s referred to as Big Bang nucleosynthesis.
Nucleus
(i) Of an atom, the core of an atom, which has a positive charge, contains most of the mass, and takes up only a small part of
the volume.
(ii) of a comet, the chunks of matter, taking up a volume no more than a few kilometres across, at the centre of the head of a
comet.
(iii) of a galaxy, the innermost regions of a spiral galaxy; it does not show spiral structure and is visible from the sky as a bulge
in the otherwise flat disk of the galaxy.
Number Density
Is the quantity of some specified particle or object class per unit volume. For atoms, molecules or subatomic particles, the
volume is typically in cm–3 or m–3. With stars, cubic parsecs (pc–3) are often used.
Nutation
A bobbing motion that accompanies the precession of a spinning rigid body
Any of several irregularities in the precession of the equinoxes, caused by varying torque applied to the Earth, by the Sun and
the Moon
..O
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O-III filter
This is a special filter designed to let through a very specific wavelength of light emitted by oxygen in nebulae. It can help filter
out light pollution and improves the view of objects such as planetary nebulae.
O and B Association
A group of O and B giant Young stars close together in space. The members of an O and B association were formed at roughly
the same time.
Objective
The principal lens or mirror of an optical system.
Oblate
Having an equatorial diameter greater than the polar diameter.
Obliquity
The angle by which the spin axis of a planet to the plane of its ecliptic differs from 90°.
Obliquity of the Ecliptic
The angle between the plane of the ecliptic and that of the celestial equator.
Observable Universe
The portion of the entire universe that can be seen from Earth.
Observation
In science, an observation is a fact or occurrence that is noted and recorded. The Hubble Space Telescope is a tool astronomers
use to make observations of celestial objects.
Observatory
A structure designed and equipped for making astronomical observations. Observatories are located on Earth and in space.
Obstruction Ratio
In telescopes which have a diagonal mirror or a secondary mirror so positioned as to obstruct part of the incoming beam of light,
the presence of that obstruction reduces the contrast of the image somewhat. The amount of contrast degradation depends on
the relative size of the obstruction, compared to the clear aperture of the telescope.
Occultation
An event that occurs when one celestial body conceals or obscures another. For example, a solar eclipse is an occultation of the
Sun by the Moon.
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Ocular Adjustment
Increasing or decreasing the distance between the two eyepieces (which can also be called the ocular lenses) in a pair of
binoculars.
Off-Axis
A telescope whose optical elements are not mechanically symmetric about a single optical axis. The common types are
reflectors, in which the primary mirror is tilted, so as to deflect the reflected beam off to the side, so that other optical
components, which might otherwise partially obscure the incoming beam, will not do so.
Okta
In meteorology, an okta is a unit of measurement used to describe cloud cover from 0 = clear to 8 = overcast.
Olbers's Paradox
Time observation that the sky is dark at night contrasted to a simple argument that shows that the sky should be uniformly
bright, due to there being a star in every line of sight in an infinite and uniform universe. The paradox is resolved by the redshift
of the expanding universe.
Oort Cloud
A theoretical shell of comets that is believed to exist at the outermost regions of our solar system. A vast collection of bodies
made up of rock and ice that orbit the Sun at a distance starting in the region beyond the orbit of Pluto and extending out to
nearly 1.5 light-years or 50,000 A.U. Unlike the bodies of the Solar System, which orbit the Sun in roughly the same plane, the
Oort cloud objects form a vast sphere around the Sun. It is estimated that billions of objects exist in this region, and there is
evidence that this is the place where most comets originate.
The Oort cloud was named after the Dutch astronomer who first proposed it.
Opacity
The degree to which light is prevented from passing through an object or a substance. Opacity is the opposite of transparency.
As an object’s opacity increases, the amount of light passing through it decreases. Glass, for example, is transparent and most
clouds are opaque.
Open Cluster
A collection of young stars that formed together. They may or may not be still bound by gravity. Some of the youngest open
clusters are still embedded in the gas and dust from which they formed.
Open Universe
The version of big bang cosmology in which the universe will expand forever. An open universe has infinite volume.
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Opposition
The position of a planet when it is exactly opposite the Sun as seen from Earth. A planet at opposition is at its closest approach
to the Earth and is best suitable for observing.
Optical Activity
The property of some crystals, gases, liquids, and solutions to rotate plane-polarized light to the left or right. It occurs because
the molecules that make up the substance through which the light beam is shone are asymmetric, i.e., they have no plane of
symmetry. Asymmetric molecules are mirror images of each other that cannot be superimposed. This asymmetric property is
also referred to as handedness. Examples are the L- and D-forms of amino acids.
Optical Brightener
A compound that absorbs the invisible ultraviolet component of sunlight and re-emits it as visible light at the blue end of the
spectrum.
Optical Depth
The number of factors of the transcendental number (2.71828. . . ) that radiation is dimmed in passage through a gas.
Optical Double
A pair of stars that appear extremely close together in the sky even though they are at different distances from us and are not
physically linked.
Optical Tube Assembly
The tube that holds your mirrors and/or lenses.
Optics
The science that deals with the properties of light; in this case specifically dealing with the way light changes directions when it is
either refracted and dispersed by a lens or reflected from a mirror.
Orbit
The path of a celestial body as it moves through space.
Orbital
A region of space in an atom occupied by one or two electrons.
Orbital Elements
The six numerical values that completely define the orbit of one body about another of known mass. They are the semi-major
axis (a), the eccentricity (e), the inclination to the reference plane (i), the mean anomaly (M), the argument of the pericentre
(ω), and the longitude of the ascending node (Ω). The elements vary with time as a consequence of perturbations of other
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bodies, so their epoch is important. For comets and asteroids, the perihelion conditions are often of interest, so the date of
perihelion (T) and perihelion distance (q) are usually used instead of M and a. (At T, M=0; q = a(1-e) )
Orographic Clouds
These are clouds that form as air is forced to rise over high obstructions on the surface of a planet. This could be a mountain on
Earth or a volcano on Mars, for example.
Orrery
A dynamic model of the Solar System that shows the relative positions and motion of the planets and moons.
Orthoscopic
See “Eyepiece”
Osculating Orbit
The orbit that a body would follow if the only gravitational force acting on it was that of the primary body, i.e. if its motion was
not perturbed by the presence of other bodies.
Outer Space
Any region of space beyond limits determined with reference to boundaries of a celestial system or body, especially the region of
space immediately beyond Earth's atmosphere.
Outgassing
The slow release of gas from a solid or liquid; especially the release of gases into the atmosphere of a planet.
Ozone
O3 a gaseous molecule, made of three oxygen atoms, which forms a layer in the upper atmosphere that shields the Earth against
excessive ultraviolet radiation.
Ozone Layer
Layer of Earth's atmosphere at about 20 to 30 miles, marked by a high ozone (O3) content.
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Pancam Camera
This is the high-resolution stereo panoramic camera (pancam) that the Mars Exploration Rovers Spirit and Opportunity are
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equipped with. It is used to give a geological overview of a region and to navigate the Martian surface, as well as to choose what
places are potentially interesting for the science instruments to study.
Panoptic
See “Eyepiece”
Parabolic Mirror
A mirror formed in a precise curve to cause the incoming light rays to be focused at the same point.
Paradigm
A well-established example or model, an archetype.
Parallax
(a) When used by itself, the word "parallax" refers to trigonometric parallax, half the angle through which a star appears to be
displaced when the earth moves from one side of the sun to the other, that is, through 2 A.U. The parallax of a star is inversely
proportional to the distance to the star from the sun.
(b) Some of the other ways of measuring distances, usually in those cases referred to with an adjective, as in spectroscopic
parallax.
Parhelic Circle
A luminous halo, each side of the sun parallel to the horizon, caused by refraction of sunlight through ice crystals.
Parsec
A large distance often used in astronomy. A parsec is a parallax second, the distance at which 1 AU subtends an angle of 1
second of arc: equal to 30.857 x 1012 km = 3.2616 light years.
Peak to Valley Wavefront
This is a reading of how smooth a parabolic mirror is by measuring the height difference from the centre (valley) of the mirror to
the edge (peak) of it. It is typically a fraction of the wavelength of optical light.
Perchlorates
These are a type of salt that have been found on the surface of Mars by the Phoenix lander. On Earth certain microbes are
known to use perchlorates as an energy source.
Peculiar Velocity
The velocity of a star with respect to the local standard of rest.
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Penumbra
(a) For an eclipse, the part of the shadow from which the sun or other radiating body is only partially occulted.
(b) of a sunspot, the outer region of the sunspot, not as dark as the central umbra.
Perfect Cosmological Principle
The assumption that on a large scale the universe is homogeneous and isotropic in space and unchanging in time.
Periapse (Periapsis, Pericentre)
The position in an orbital path that is the least distance from the primary body.
Periastron
The near point of the orbit of a body to the star around which it is orbiting.
Perigee
The point where (and when) an object's orbit about the earth in which it is closest to the earth; only applicable to objects orbiting
the earth (not to objects orbiting the sun).
Perihelion
The point in the orbit of a planet or other body where it is closest to the Sun.
Period
A period of time in history seen as a single coherent entity; an epoch, era.
The length of time during which the same characteristics of a periodic phenomenon recur, such as the repetition of a wave or the
rotation of a planet.
Period-Luminosity Law
A relationship that describes how the luminosity or absolute brightness of a Cepheid variable star depends on the period of time
over which that brightness varies.
Periodic Comet
A comet in a closed, elliptical orbit within our solar system. These comets typically have orbital periods of less than 200 years.
Many comets have orbits that keep them in the inner solar system and allow their trajectories to be calculated with great
accuracy and precision. Perhaps the best-known periodic comet is Halley’s comet, whose orbital period is 76 years.
Periodic Table
A systematic arrangement of the elements in columns and rows; elements in a given column have similar properties.
Perturbations
Gravitational influences ("tugging" and "pulling") of one astronomical body on another. Comets are strongly perturbed by the
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gravitational forces of the major planets, particularly by the largest planet, Jupiter. These perturbations must be allowed for in
orbit computations, and they lead to what are known as "osculating elements" (which means that the orbital element numbers
change from day to day and month to month due to continued perturbations by the major planets, so that an epoch is
necessarily stated to denote the particular date that the elements are valid.
Phase
The apparent change in shape of the Moon and inferior planets as seen from Earth as they move in their orbits.
Phase Angle
Is the elongation or angle between an orbiting body and the Sun as viewed from a particular perspective such as the Earth. It
determines the amount of a planet or moon's visible surface that lies in shadow. Inferior planets such as Venus generally have a
low phase angle as seen from Earth, so they are often viewed as a crescent. Superior planets such as Mars and Jupiter usually
have a high phase angle, so little of the shadowed side is visible.
Photometer
A device that measures the intensity of light from a particular source.
Photometry
A measurement of the intensity of electromagnetic radiation emitted by a celestial body.
Photomultiplier
An electronic device that through a series of internal stages multiplies the small current that is given off when light is incident on
the device so that a relatively large current results.
Photon
A packet of energy that can be thought of as a particle of light travelling at the speed of light. A photon of energy E is equivalent
to an electromagnetic wave of wavelength λ=hc/E, where h is Planck's constant and c is the speed of light in a vacuum.
Photosphere
The visible surface layer of the sun and stars; the layer from which continuous radiation escapes and where absorption lines
form.
Photovoltaic Cell
A solar cell; a cell that converts sunlight directly to electrical energy.
Physical Change
A change in physical state or form.
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Physical Properties
The qualities of a substance that can be demonstrated without changing the composition of the substance.
Pickering Scale
A scale of Seeing conditions devised by William Pickering. 1 is very poor to 10 which is perfect.
Pinched Optics
Mechanical stress affects reflective and refractive imaging element properties. If one of these elements comes under local stress
from the support system, this can translate into artefacts in the image.
Pincushion Distortion
An aberration of optical systems, in which magnification increases with distance away from the optical axis. With such a system,
squares are imaged with their sides curved inward, looking sort of like pincushions.
Pixel
A light-sensitive picture element on a charge-coupled device (CCD) or some other kind of digital camera. A pixel is a tiny cell
that, placed together with other pixels, resembles the mesh on a screen door. The Hubble Space Telescope’s Wide Field and
Planetary Camera 2 has four CCDs, each containing 640,000 pixels. Each pixel collects light from a celestial object and converts
it into a number. The numbers (all 2,560,000 of them) are sent to ground-based computers, which convert them into an image.
The greater number of pixels, the sharper the image.
Plage
The part of a solar active region that appears bright when viewed in H0.
Planck Constant
The numerical factor h relating the frequency v of a photon to its energy e in the expression e = hn. The Planck constant has the
value h = 6.62620 x 10-27 erg sec.
Planck Function (also known as the Planck Law)
The mathematical expression describing the continuous thermal spectrum of a glowing object. For a given temperature, the
Planck function specifies the intensity of radiation as a function of either frequency or wavelength.
Planet
A very large body in orbit around a star. Planets can be composed mainly of rock or of dense gases.
Planetarium
A display museum in which images of stars and other astronomical phenomena are projected onto a domed ceiling. A computer
program that displays celestial objects, that may also be used to control a telescope.
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Planetary Nebula
A shell of gas surrounding a small, white star. The gas is usually illuminated by the star, producing a variety of colours and
shapes.
Planetesimal
A small (diameter up to several hundred kilometres) solar-system body of the type that first condensed from the solar nebula.
Planetesimals are thought to have been the principal bodies that combined to form protoplanets.
Planetoid
Another term for a minor planet – the official name for asteroids, as used by the International Astronomical Union (IAU).
Planisphere
A star chart with an overlay that shows you what stars are visible at a specific time, date and latitude.
Plasma
A state of matter similar to a gas but composed of isolated electrons and nuclei rather than discrete whole atoms or molecules.
Plate Tectonics
The theory that the Earth's continental and oceanic crust and outermost portion of the mantle is fractured into large plates that
move relative to each other. Convective currents in the mantle provide the driving force for this motion. The plate motion is
responsible for global mountain building, earthquake activity, and volcanism, all of which are most pronounced along plate
boundaries.
Plossl
See “Eyepiece”
Plutino
Plutinos form the inner part of the Kuiper belt and represent about a quarter of the known Kuiper belt objects. Plutinos are the
largest class of the resonant trans-Neptunian objects (i.e. bodies in orbital resonances with Neptune). For every 2 orbits that a
Plutino makes, Neptune orbits 3 times.
Plutoid
If an object in our Solar System has an orbit that takes it beyond Neptune, is near spherical and hasn’t cleared its orbit of
detritus like small asteroids and meteoroids, it’s now classed as a Plutoid or trans-Neptunian dwarf planet. There are now many
known Plutoids: Eris, Makemake, Haumea, Sedna, and Pluto itself. The new category was invented by the International
Astronomical Union (IAU). It follows the downgrading of Pluto, from planet to dwarf planet in 2006.
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Polar Alignment
The process of getting the axis of an equatorial mounting that is supposed to point parallel to the Earth's axis of rotation, to do
so.
Polar-Alignment Telescope
A small telescope permanently attached to an equatorial mounting, aligned with the mounting axis that is supposed to point
parallel to the Earth's axis of rotation. The polar-alignment telescope probably has markings visible through its eyepiece which
can be aligned with stars near the north or south celestial poles, to aid in performing a polar alignment.
Polar Axis
The axis of an equatorial telescope mounting that is parallel to the earth's axis of rotation.
Polarscope
A small scope that goes into equatorial mounts to help you achieve an accurate polar alignment for your telescope. Accurate
polar alignment is essential to take good quality, long-exposure images, for which the star field needs to be tracked.
Polarization
The arrangement of electromagnetic waves so that all the planes in which the waves are oscillating are parallel to each other.
Polycyclic Aromatic Hydrocarbon
Any of various organic compounds composed of a few to many six-membered rings of carbon atoms linked by an alternating
sequence of single and double bonds and to which hydrogen atoms are attached.
Population I
The class of stars with relatively high abundances of heavy elements. These stars are generally found in the disk and spiral arms
of spiral galaxies, and are relatively young. The term Population I is also commonly applied to other components of galaxies
associated with the star formation, such as the interstellar material.
Population II
The class of stars with relatively low abundances of heavy elements. These stars are generally found in a spheroidal distribution
about the galactic centre and throughout the halo, and are relatively old.
Population III
Hypothetical class of 1st generation stars composed solely of H and He, with mass 100-1000 x solar and luminosity 1-30 million
times greater. As usual in astronomy, the numbering is backward: these are the first stars.
Positron
A subatomic particle with the same mass as the electron, but with a positive electrical charge; the antiparticle of the electron.
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Potential Energy
Energy that is stored, and which may be converted into kinetic energy under certain circumstances. In astronomy, the most
common form of potential energy is gravitational potential energy.
Prebiotic
Relating to the chemical or environmental precursors of the origin of life.
Precession
A rotation of the direction of the axis of rotation. Normally refers to the precession of the equinoxes, a consequence of the effect
of the Sun's gravity on Earth's equatorial bulge. Earth's axis of rotation precesses with a period of about 25,770 years, during
which time the equinoxes make a complete revolution about the celestial equator. Because the Vernal Equinox is the reference
direction for the equatorial co-ordinate system, the co-ordinates of "fixed" objects changes with time and must therefore be
referred to an epoch at which they are correct.
Primary Body
The body that is being orbited. E.g. the Sun is the primary of the orbits of the planets and comets. With respect to multiple
star systems, it is the most massive star.
Primary Cosmic Rays
The cosmic rays arriving at the top of the earth's atmosphere.
Primary Mirror
The principal light-gathering mirror in a reflecting telescope.
Prime Focus
The location at which the main lens or mirror of a telescope focuses an image without being reflected or refocused by another
mirror or other optical element.
Prime Meridian
An imaginary line on the surface of a planet from which longitude is measured. On Earth, the line passes through the Royal
Observatory at Greenwich, England.
Primordial Background Radiation
Radiation detected in millimetre and sub-millimetre wavelength regions that is coming from all directions in space and
interpreted to be the remnant of the big bang. Also known as 3° background radiation, background radiation, or the remnant of
the primeval fireball.
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Primordial Nucleosynthesis
Element building that occurred in the early universe when the nuclei of primordial matter collided and fused with one another.
Most of the helium in the universe was created by this process.
Prism
A prism is usually a triangular-shaped piece of glass used to refract, or bend, light. The shape of the glass causes the light to
disperse, or spread out, as it bends, producing a rainbow of colours from the white light.
Prograde Motion
Orbital or spin motion in the forward or "normal" direction; in the solar system, this is counter-clockwise as viewed looking down
from above the north pole.
Project Cyclops
A 10-week design study sponsored by NASA, Stanford University, and the American Institute for Engineering Education, of
possible means for detecting extra-terrestrial civilizations.
Prolate
Having the diameter along the axis of rotation longer than the equatorial diameter.
Prominence
An explosion of hot gas that erupts from the Sun's surface. Solar prominences are usually associated with sunspot activity and
can cause interference with communications on Earth due to their electromagnetic effects on the atmosphere.
Proper Motion
Motion across the sky with respect to a framework of galaxies or fixed stars, usually measured in seconds of arc per century.
ProtoA prefix from the Greek for "before." When used in conjunction with the name of a celestial body, means the state of the body
just before it is considered to have formed.
Protogalaxy
Matter that is beginning to come together to form a galaxy. It is the precursor of a present-day galaxy and is sometimes called a
“baby galaxy.”
Protoplanet
Astronomical objects, approximately the size of the Moon, formed from the mutual gravitational attraction of Planetesimals; they
are thought to collide with each other and slowly form planets.
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Proton
A positively charged elementary particle of mass equal to that of a neutron and almost two thousand times that of an electron; a
component, with neutrons, of all atomic nuclei : The nucleus of the hydrogen atom : A hydrogen ion, H*, arising either through
the removal of the peripheral electron from a hydrogen atom or, together with a hydroxyl ion, from the dissociation of a water
molecule.
Protoplanetary Disc
The disc of gas and dust surrounding a newly formed star. New planets may coalesce out of the matter in the protoplanetary
disc.
Protoplanetary Nebula
The initial stage in the formation of a ‘planetary nebula’ – a glowing cloud of gas and dust surrounding a star that has ceased
nuclear fusion and blown off its outer layers of gas. The term is also sometimes used in astronomy to describe the disc of matter
around a star in which planets may form.
Protostar
A dense accumulation of dust and gas that is on the verge of collapsing to form a full blown nuclear burning star.
Pulsar
A rapidly rotating neutron star that emits energy along its gravitational axis; an extremely dense collapsed star where the
electrons have been forced into the protons. The object is thus made up mainly of neutrons and a few kilometres in diameter.
Pulsars emit magnetic field constrained beams of radio, visible, x-ray and gamma radiation, which we perceive as pulses as these
objects whirl, sometimes at hundreds of revolutions a second.
Pyrex
A Trade Name for a glass having a low coefficient of expansion due to high silica content.
Pyroclastic
The type of rock formed from fragments or ash resulting from volcanic activity, especially the hot, fast-moving dense clouds that
occur during an eruption.
..Q
Q0
The deceleration parameter, a cosmological parameter that describes the rate at which the expansion of the universe is slowing
up.
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Quadrature
A point in the orbit of a superior planet where it appears at right angles to the Sun as seem from Earth.
Quantum
The amount of energy associated with a photon, equal to hv, where h is the Planck constant, and v is the frequency. The
quantum is the smallest amount of energy that can exist at a given frequency.
Quantum Mechanics
The branch of 20th century physics that describes atoms and radiation; the theory involves bundles of energy known as quanta.
Quantum Efficiency
The percentage of photons hitting the photo reactive surface that will produce an electron-hole pair (the electron in our bucket).
Quantum Theory
This is the study and respective theories on how the Universe is put together on a tiny sub-atomic scale. It’s the dream of many
scientists to combine quantum theory with the theories that explain the large scale Universe in one unified theory.
Quark
One of the subatomic particles from which many modern theoreticians believe such elementary particles as protons and neutrons
are composed. The various kinds of quarks have positive or negative charges of 1/3 or 2/3.
Quark Star
A theoretical type of massive neutron star that is so dense that matter is broken down to its constituent particles. Quarks are
the hypothetical constituent particles of subatomic particles, like neutrons.
Quasar
An unusually bright object found in the remote areas of the universe. Quasars release incredible amounts of energy and are
among the oldest and farthest objects in the known universe. They may be the nuclei of ancient, active galaxies.
Quasi-Stellar Object
Any of a class of extragalactic objects also known as quasars, characterized by emission lines with very large redshifts. The
quasi-stellar objects are thought to lie at great distances, in which case they existed only at earlier times in the history of the
universe; they may be cores of young galaxies.
Quiet Sun
The collection of solar phenomena that do not vary with the solar activity cycle.
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Rack and Pinion Focuser
A focuser in which the tube containing the eyepiece has a straight row of gear teeth (the rack) running along one side of it, which
are engaged by a more conventional-looking gear (the pinion), attached to a knob, in order to adjust the focus.
Radar
The acronym for radio detection and ranging; an active rather than passive radio technique in which radio signals are transmitted
and their reflections received and studied.
Radial Velocity
The velocity of an object along a line (the radius) joining the object and the observer; the component of velocity toward or away
from the observer.
Radian
The unit of angular measure, defined as the ratio of a length of arc intercepted by two radii to the length of the radius. P radians
= 180°.
Radiance
The amount of power per unit area and per unit solid angle in a beam of light. (Think of it as the irradiance per unit solid angle.)
Units: W m-2 sterad-1
Radiant
The radiant is the point in the sky where meteors (associated with a specific meteor shower) appear to come from. The
constellation where the radiant is located determines the name of the meteor shower. So for example, the Orionids have their
radiant in Orion.
Radiation
The process by which electromagnetic energy moves through space as vibrations in electric and magnetic fields. This term also
refers to radiant energy and other forms of electromagnetic radiation, such as gamma rays and X-rays.
Radiation Era
Collective name for the first three epochs of the history of the Universe (chaos, hadron epoch, lepton epoch), lasting overall
about 100 sec and dominated by radiation rather than matter.
Radiation Pressure
Pressure created by light hitting a surface.
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Radiative Process
An event involving the emission or absorption of radiation. For example, a hydrogen atom that absorbs a photon of light
converts the energy of that radiation into electrical potential energy.
Radiative Transport
The transport of energy, inside of a star or in other situations, by radiation.
Radioactive
Having the property of spontaneously changing into another isotope or element.
Radioactive Dating
A technique for estimating the age of material, such as rock, based on the known initial isotopic composition and the known rate
of radioactive decay for unstable isotopes originally present.
Radioactive Decay
Disintegration of an unstable atomic nucleus by spontaneous emission of radiation.
Radioactivity
Spontaneous emission of alpha, beta, or gamma rays by the disintegration of the nuclei of atoms.
Radio Galaxy
Any of a class of galaxies whose luminosity is greatest in radio wavelengths. Radio galaxies are usually large elliptical galaxies,
with synchrotron radiation emitted from one or more pairs of lobes located on opposite sides of the visible galaxy.
Radionuclide
Radioactive nuclear species or nuclide.
Radio Astronomy
The branch of astronomy which utilizes radio waves through the use of radio telescopes to study celestial bodies and
occurrences.
Radio Telescope
An antenna or set of antennas, often together with a focusing reflecting dish, that is used to detect radio radiation from space.
Radio Waves
Electromagnetic radiation with wavelengths longer than about one millimetre.
Ramsden
See “Eyepiece”
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Rare Earth Elements
Series of elements usually taken to include elements with atomic numbers 58 to 71 (lanthanum) and sometimes yttrium and
scandium.
Ray
(a) A light ray, a wave of electromagnetic radiation
(b) on the surface of a moon or planet, a streak of material that is relatively light in shade, presumable representing material
ejected when a crater was formed.
Rayleigh Criterion (Rayleigh Limit)
Lord Rayleigh, a 19th century physicist, showed that a telescope optic would be sensibly indistinguishable from a theoretical
perfect optic if the light (strictly, the wavefront) deviated from the ideal condition by no more than one quarter of its wavelength.
Receiver
The part of the radio telescope that detects long wavelength electromagnetic radiation and converts it to an electrical signal so
that we can sense it.
Recessional Velocity
The velocity at which an object moves away from an observer. The recessional velocity of a distant galaxy is proportional to its
distance from Earth. Therefore, the greater the recessional velocity, the more distant the object.
Recombination
The addition of an electron to an ion, usually resulting in radiation (recombination lines) when the electron subsequently jumps
down to lower energy states.
Recurrent Nova
A star known to flare up in nova outbursts more than once. A recurrent nova appears to be a binary system containing a white
dwarf and a mass-losing star, in which the white dwarf sporadically flares up when material falls onto it from the companion.
Red Dwarf
Red dwarfs make up most of the stars in the Universe. They are small cool stars that usually have less than 10 per cent the
mass of the Sun with surface temperatures of between 2,200 and 3,700°C.
Red Giant
A stage in the evolution of a star when the fuel begins to exhaust and the star expands to about fifty times its normal size. The
temperature cools, which gives the star a reddish appearance.
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Reddening
The phenomenon by which the extinction of blue light by interstellar matter is greater than the extinction of red light so that the
redder part of the continuous spectrum is enhanced.
Redshift
The increase in the wavelength of visible light from a distant celestial body towards the red end of the electromagnetic spectrum.
This is due to the Universe’s expansion, which causes the wavelength of light to increase as it travels through space.
Reducing Agent
A substance that causes reduction and is itself oxidized.
Reducing Atmosphere
Atmosphere comprised of substances that readily provide electrons.
Reduction
The gain of one or more electrons or hydrogen atoms (electrons + protons) by an atom or molecule.
Reflection
Reflection occurs when light changes direction as a result of "bouncing off" a surface like a mirror.
Reflector (Reflecting Telescope)
A telescope that uses a concave mirror to gather light and form an image at a focal plane.
Reflection Nebula
An interstellar cloud containing dust that shines by light reflected from a nearby star.
Reflex Finder (Sight)
Also called Unit Magnification Finder. A finder which operates without magnification, as do many rifle 'scopes. Such a device is
in essence a miniature head-up display: It projects a cross-hair, illuminated bulls-eye, or similar pattern on the sky, where the
observer may view it easily with both eyes open. Reflex finders make it very easy to find objects which are bright enough to be
seen with the naked eye, or which are close enough to naked-eye stars.
Refraction
The bending of electromagnetic radiation as it passes from one medium to another or between parts of a medium that has
varying properties. The index of refraction of a substance is the ratio of the speed of light in a vacuum to that in the substance.
Refractor (Refracting Telescope)
A telescope that uses a transparent objective lens to refract, or bend, light in order to form an image at the focal plane.
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Refractoritis
An amateur astronomer who is fond of refractors to the point of obsession, is said to have refractoritis.
Refractory
The property of being able to exist in solid form under conditions of very high temperature. Refractory elements are
characterized by a high temperature of vaporization; they are the first to condense into solid form when a gas cools, as in the
solar nebula.
Refractory Grain
A grain that has been formed at high temperatures. This type of grain was found in samples of cometary dust by the Stardust
spacecraft, and hints at a process that might have deposited these grains in the far reaches of the Solar System.
Regolith
The mantle of unconsolidated fragmental material that covers a land surface; i.e., soil and fractured rock.
Regression of the Nodes
This is a slow movement of the lunar nodes caused by the gravitational pull of the Sun. The lunar nodes move slowly westward,
taking 18.6 years to complete one revolution.
Relativistic
Having a velocity that is such a large fraction of the speed of light that the special theory of relativity must be applied.
Relativity
Either of the theories of relativity worked out by Albert Einstein. The special theory of relativity (1905) is a theory of relative
motion. The general theory of relativity (1916) is a theory of gravitation.
Research
A study or investigation to find new information.
Resolution
The ability of an optical system to distinguish fine detail. This is the greatest amount of detail that a telescope is capable of
delivering. A telescope’s resolution can be roughly estimated by observing binary stars. If a binary pair can just be made out to
show two separate stars, the telescope’s resolution (or ‘resolving power’) is roughly equivalent to their angular separation in the
sky.
Resolve
The ability of a telescope to distinguish objects that are very close to each other as two separate objects.
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Resolving Power
Another word for Resolution.
Rest Wavelength
The wavelength of a spectral line as measured in a laboratory, when there is no relative motion between source and observer.
Retardation
This is the time difference between Moon rise one day, and Moon rise the following day.
Reticle
A system of lines and/or concentric circles at the focal plane of a telescope, used for positioning or guiding the telescope, or
polar-aligning an equatorial mount. Is usually incorporated into an eyepiece and may be illuminated in order to render the lines
visible against a dark background sky.
Reticule
A system of lines forming a pattern of squares or ‘ticks’ at the focal plane of a telescope, used in micrometres.
Retrograde
The rotation or orbital motion of an object in a clockwise direction when viewed from the north pole of the ecliptic. Any motion
that is in the opposite sense from the great majority of solar system bodies.
Retrograde Motion
The phenomenon where a celestial body appears to slow down, stop, then move in the opposite direction. This motion is caused
when the Earth overtakes the body in its orbit.
Reversing Layer
The layer of the Sun's atmosphere above the photosphere.
Revolution
The orbiting of one body around another.
Right Ascension
The east-west coordinate in the equatorial coordinate system. One element of the astronomical coordinate system on the sky,
which can be though of as longitude on the earth projected onto the sky. Right ascension is usually denoted by the lower-case
Greek letter alpha and is measured eastward in hours, minutes, and seconds of time from the vernal equinox. There are 24
hours of right ascension, though the 24-hour line is always taken as 0 hours. More rarely, one sometimes sees right ascension in
degrees, in which case there are 360 degrees of right ascension to make a complete circuit of the sky.
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Rille
A long, narrow depression on the surface of the Moon – and other planetary bodies – that resembles a channel. They are
thought to be caused by collapsed lava flows just under the surface.
Ring Galaxy
A galaxy that has a ring-like appearance. The ring usually contains luminous blue stars. Ring galaxies are believed to have been
formed by collisions with other galaxies.
Ritchey-Chretien
A telescope closely resembling a classical, two-mirror-only Cassegrain, except that the primary mirror is an hyperboloid. Slightly
more strongly figured than the Cassegrain's paraboloid, and the secondary is a slightly stronger hyperboloid than that of the
Cassegrain. Ritchey-Chretiens are corrected for coma as well as for spherical aberration, thus they can deliver relatively sharp
images across a wide field of view. The Hubble is a Ritchey-Chretien.
Roche Limit
The point near a massive body such as a planet or star, inside of which the tidal forces acting on an orbiting body exceed the
gravitational force holding that body together. The location of the Roche limit depends on the size of the orbiting body.
Roche Lobe
This is a theoretical spherical boundary around (for example) each star in a binary system. Matter is bound by the star’s
gravitational attraction within each lobe. When two Roche lobes mix they may begin to transfer material.
Roentgen
(obsolete) Unit of X-ray or gamma radiation dosage. The quantity of radiation which liberates by ionisation one esu of electricity
per cm3 of air under normal conditions of temperature and pressure."Using 1 esu ≈ 3.33564×10−10 C and the air density of
~1.293 kg/m³ at 0°C and 101kPa , this converts to 2.58×10−4 C/kg.
An exposure of 500 roentgens in five hours is usually lethal for human beings.
Ronchi Grating
A Ronchi grating is a special type of diffraction grating used in optical tests. It’s used to find the quality of a scope’s mirror by
studying the diffraction patterns produced by light bounced off the mirror.
Rosetta Stone
A slab of black basalt stone found in 1799 that bears an inscription in hieroglyphics, demotic characters, and Greek, celebrated
for having given the first clue to the decipherment of Egyptian hieroglyphics; from this, any breakthrough discovery of great
magnitude.
Rotation
The spin of a body about its axis.
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RR Lyrae Variable
A member of a class of pulsating variable stars named after the prototype star, RR Lyrae. These stars are blue-white giants with
pulsation periods of less than one day, and are Population II objects found primarily in globular clusters.
Russell-Vogt theorem
The theorem that the evolution of a star is completely determined by its mass and chemical composition.
..S
S Wave
A type of seismic wave that is a transverse, or shear, wave, and which can travel only through rigid materials.
Saber's Beads
Lunar phenomenon seen on extremely young and old crescents. The striking resemblance to 2nd and 3rd contacts during a total
solar eclipse was first noted by American amateur astronomer Stephen Saber.
Sampling
General wisdom and rules of thumb suggests that the image scale (above) should optimally be 1/2 of the seeing though some
people suggest this should be 1/3. In the UK, at sea level, seeing varies from about 2" per pixel (very good) to 6" per pixel
(poor) but more typically ~3" per pixel. This suggests an optimal image scale of ~1"-1.5" per pixel.
Undersampled: Based on the above, if the image scale is greater than 1/2 the seeing it is considered under sampled i.e. the
imaging system may be holding back the resolution of fine detail in the image.
Oversampled: The converse of the above. In oversampled images the imaging system has a higher resolution than atmospheric
conditions permit. A degree of oversampling is, not necessarily, a bad thing as it allows you to take advantage of any period of
better than average seeing.
Saros Cycle
An 18 year, 11 day repeating pattern of solar and lunar eclipses caused by a combination of the tilt of the lunar orbit with respect
to the ecliptic and the precession of the plane of the moon's orbit.
Satellite
A natural or artificial body in orbit around a planet.
Scattering
The random reflection of photons by particles such as atoms or ions in a gas, or dust particles in interstellar space.
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Schiefspiegler
A particular type of off-axis compound telescope. The original Schiefspiegler has a primary mirror with a relatively long focal
ratio.
Schmidt, Schmidt-Cassegrain, Schmidt Newtonian
Forms of catadioptric telescope.
Schmidt Camera
A type of telescope that uses a spherical mirror and a thin lens to provide photographs of a wide field.
Schmidt Corrector Plate
The key piece of any Schmidt-type optical system, such as a Schmidt camera, a Schmidt-Cassegrain, or any of many others. To
the naked eye, a Schmidt corrector plate looks like a flat piece of glass, but its surface is actually figured in a subtle manner, to
affect the spherical aberration of the system in which it is used.
Schröter Effect
The strange observational effect phenomenon in which Venus’s disc reaches half phase a few days before or after the predicted
date.
Schwarzschild Radius
The radius that, according to Schwarzschild's solutions to Einstein's equations of the general theory of relativity, corresponds to
the event horizon of a black hole.
Science
A branch of knowledge based on the laws of nature.
Scientific Law
A summary of experimental data; often expressed in the form of a mathematical equation.
Scientific Model
A representation that serves to explain a scientific phenomenon.
Scintillation
Acting like a lens, the Earth’s atmosphere diffracts the starlight passing through it causing the effect known as scintillation. It
can be seen when stars appear to twinkle.
Secondary Cosmic Rays
High energy particles that are generated in the earth's atmosphere by primary cosmic rays.
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Secondary Mirror
The second mirror in a reflecting telescope (after the primary mirror), usually either convex, to reflect the image out of a hole in
the bottom of the telescope to the Cassegrain focus or along the telescope mount axis to the coudé focus; or flat, to reflect the
image out of the side of the telescope to the Newtonian focus.
Secondary Obstruction
The secondary obstruction in a reflecting or Catadioptric telescope is a result of the necessary placement of a secondary mirror in
the optical path. A large secondary obstruction will generally lower the brightness and contrast of images through the eyepiece
at high magnifications.
Second Law of Thermodynamics
The degree of randomness in the universe increases in any spontaneous process.
Seeing
The steadiness of the earth's atmosphere as it affects the resolution that can be obtained in astronomical observations. Good
seeing corresponds to a steady atmosphere, and bad seeing corresponds to an unsteady atmosphere.
SeeligerEffect
When a reflective object such as a planet is in solar opposition it will appear brighter in the night sky than when it is not. This
effect is named after the German astronomer Hugo von Seeliger.
Seismic Wave
A wave created in a planetary or satellite interior, usually caused by an earthquake.
Seismology
The study of waves propagating through a body and the resulting deduction of the internal properties of the body. The prefix
"seismo-" comes from the Greek word for earthquake.
Selection Effect
The tendency for a conclusion based on observations to be influenced by the method used to select the objects for observation.
An example was the early belief that all quasars are radio sources, when the principal method used to discover quasars was to
look for radio sources and then to find out whether they had other properties associated with quasars.
Selenography
The study of the surface of the Moon.
Semi-Apochromatic Refractor
A refracting telescope that uses three or more lenses to bring red, green and blue light to focus at nearly the same point.
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Semi-Major Axis
Half the major axis, that is, for an ellipse, half the longest diameter.
Sensitivity
Modern sensors are usable to very low light levels and therefore are considered highly sensitive. Many can operate down to near
zero lux (lumens/sq. metre).
Sensor Noise
This is noise generated by a digital camera’s sensor. It arises because there is a small amount of electrical activity in the sensor
itself. Noise increases with temperature but its effects in images can be reduced by taking many exposures (to increase the
signal to noise ratio).
Setting Circles
Calibrated discs, in essence, protractors, attached at both the axes of an equatorial mount. The polar axis has a circle scale
divided into 24 equal parts each equivalent to one hour right ascension. The declination axis’s circle has degrees of declination,
from 90 degrees north to 90 degrees south. With the mount correctly aligned to the celestial pole, an object may be located by
setting its co-ordinates on the circles.
Seyfert Galaxy
A type of spiral galaxy that has a bright nucleus and whose spectrum shows emission lines. Historically, N galaxies and Seyfert
galaxies were defined by different astronomers on the basis of different information, and the difference between them is not
always clear cut.
Shadow Transit
A shadow transit is when the shadow of a moon passes over the surface of another planet. The most obvious of these to
amateur astronomers are the shadow transits of Jupiter’s moons across its disc. They can be observed with a medium to large
telescope.
Shear Wave
A wave that consists of transverse motions; that is, motions perpendicular to the direction of wave travel.
Shock Wave
Discontinuity in the flow of a fluid (including a gas or plasma) marked by an abrupt increase in pressure, temperature, and flow
velocity at the shock front.
Shooting Star
A meteor, especially a streak of light in the night sky, caused by a meteoroid burning up as it enters the Earth’s atmosphere.
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Side by Side Guiding
Where the two scopes (imaging and guiding) are mounted side by side. Has the advantage of reducing the lever arm of part of
the weight of the payload thereby reducing the counterweight needed to achieve balance but the combination needs balancing
around all axes which can be quite difficult to achieve.
Sidereal
Of, relating to, or concerned with the stars. Sidereal rotation is that measured with respect to the stars rather than with respect
to the Sun or the primary of a satellite. The length of a sidereal day is 23 hr, 56 min, 4.09 sec of mean solar time.
Sidereal Day
The time it takes Earth to rotate once relative to the stars or 23 hours 56 minutes and 4 seconds, which is 4 minutes less than a
solar day, since during one sidereal day the Sun also moves 1° east along the ecliptic, and Earth has to rotate 4 additional
minutes to complete one rotation relative to the Sun in one 24-hour solar day.
Sidereal Drive
A mechanism used to make a telescope follow stars across the sky as the Earth rotates. Modern sidereal drives for small
telescopes are generally electrically powered and electronically regulated.
Sidereal Month
The average period of revolution of the Moon around the Earth in reference to a fixed star, equal to 27 days, 7 hours, 43 minutes
in units of mean solar time.
Sidereal Period
The period of revolution of a planet around the Sun or a satellite around its primary.
Sidereal Time
The hour angle of the First Point of Aries. It is time measured with respect to the stars
Sidereal Year
The time it takes Earth to complete one orbit of the Sun relative to the stars or 365 days 6 hours 9 minutes and 10 seconds. It
is also the time it takes the Sun to travel once around the sky relative to the stars.
Siderophile Element
Element with a weak affinity for oxygen and sulphur and readily soluble in molten iron (including iron, nickel, cobalt, platinum,
gold, tin, and tantalum). Those measured digits that are known with certainty plus one uncertain digit.
Signal-to-Noise Ratio
Those measured digits that are known with certainty plus one uncertain digit.
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Significant Figures
Those measured digits that are known with certainty plus one uncertain digit.
Singlet
One simple lens used all by itself, as opposed to doublet and triplet.
Singularity
A point in space where quantities become exactly zero or become infinitely large; a singularity is present in a black hole.
Sinking
The disappearance below the horizon of distant objects normally visible; the reverse of looming.
Skyglow Limit
Every image will show some skyglow which is just the brightness of the background (or foreground if it's due to light pollution).
The skyglow limit is the length of exposure where the skyglow swamps any actual data.
Small Magellanic Cloud
An irregular galaxy, the smallest of two satellites of the Milky Way.
Snel’s Law
This gives the quantitative change of direction of a ray of light in passing from one medium to another. The product n sin z is the
same on both sides of a plane interface between two media, where n is the local refractive index, and z is the local angle the ray
makes with the normal to the interface. In the curved atmosphere, Snel's law becomes nR sin z = constant, where R is distance
from the local centre of curvature;
Sol
The name given to our sun.
Solar Arrays
Rigid, wing-like arrays of solar panels that convert sunlight directly into electricity to operate scientific instruments, computers,
and radio transmitters. Some of the energy generated is stored in onboard batteries so the item can operate while in shadow.
Solar Cell
A device used for converting sunlight into electricity; a photoelectric cell.
Solar Constant
Rate at which radiant solar energy is received normally per unit area at the outer layer of Earth's atmosphere; its value is about
1.94 gram calories/cm2/min.
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Solar Cycle
The 11-year period over which the activity of the Sun increases and decreases; phenomena such as sunspots and solar flares are
most common during a time of peak activity, called a solar maximum, but may be absent entirely when activity is at a low ebb, a
solar minimum.
Solar Day
The synodic rotation period of the earth with respect to the sun; that is, the length of time from one local noon, when the sun is
on the meridian, to the next local noon.
Solar Eclipse
A phenomenon that occurs when the Earth passes into the shadow of the Moon. A total solar eclipse occurs when the Moon is
close enough to completely block the Sun's light. An annular solar eclipse occurs when the Moon is farther away and is not able
to completely block the light. This results in a ring of light around the Moon.
Solar Flare
A bright eruption of hot gas in the Sun's photosphere. Solar prominences are usually only detectable by specialized instruments
but can be visible during a total solar eclipse.
Solar Mass
Mass of the Sun, 2x1030 kg, used commonly as a unit to measure the masses of stars.
Solar Maximum
The midpoint in the solar cycle where the amount of sunspot activity and the output of cosmic particles and solar radiation is
highest.
Solar Minimum
The beginning and the end of a sunspot cycle when only a few sunspots are usually observed, and the output of particles and
radiation is normal.
Solar Motion
The deviation of the sun's velocity from perfect circular motion about the centre of the galaxy; that is, the sun's peculiar velocity.
Solar Nebula
Cloud of gas and dust out of which a star condenses. The primordial gas and dust cloud from which the sun and planets
condensed.
Solar System
The Sun and its surrounding matter, including asteroids, comets, planets and moons, held together by the Sun’s gravitational
influence.
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Solar Telescope
A special reflecting telescope designed to study our closest star, the Sun. Solar telescopes differ from normal telescopes in that
they are stationary and use small tracking mirrors to direct sunlight into the primary mirror. This is necessary because the Sun
appears to move across the sky due to Earth’s rotation.
Solar Time
A system of time-keeping with respect to the sun such that the sun is overhead of a given location at noon.
Solar (Stellar) Wind
A flow of charged particles that travels from the Sun out into the solar system.
Solid Angle
A three-dimensional angle.
Solstice
The time of the year when the Sun appears furthest north or south of the celestial equator. The solstices mark the beginning of
the Summer and Winter seasons.
South Celestial Pole
A direction determined by the projection of the Earth's South Pole onto the celestial sphere. The SCP is exactly 180 degrees
from the North Celestial Pole and corresponds to a declination of -90 degrees.
Southern Hemisphere
Half of a spherical or roughly spherical body; for example, the Southern Hemisphere of Earth is the half below the equator.
Space Velocity
The velocity of a star with respect to the sun.
Spacetime
The name often given to the combined ‘fabric’ of the Universe where both three dimensional space and time are linked in their
four dimensions.
Spatial Resolution
In astronomy, the ability of a telescope to differentiate between two objects in the sky that are separated by a small angular
distance. The closer two objects can be while still allowing the telescope to see them as two distinct objects, the higher the
spatial resolution of the telescope.
Special Relativity
The physical theory of space and time developed by Albert Einstein, based on the postulates that all the laws of physics are
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equally valid in all frames of reference moving at a uniform velocity, and that the speed of light from a uniformly moving source
is always the same, regardless of how fast or slow the source or its observer is moving. The theory has as it consequences the
relativistic mass increase of rapidly moving objects, time dilatation, and the principle of mass-energy equivalence which most
people know by the equation E=mc2.
Specific Heat
The amount of heat required to raise the temperature of 1g of the substance by 1°C.
Specific Gravity
The density of a substance taking water to equal 1. For example, Saturn has a specify gravity, or density, of 0.71. Therefore an
equal volume of water would weigh more than the planet - if you had a big enough bowl of water, Saturn would float in it! On
the other hand, Earth has a specific gravity of 5.5, thus Earth would weigh 5.5 times as much as an equal volume of water.
Speckle Interferometry
method of using short-exposure photographs to recover information down to the diffraction limit of large optical telescopes.
Spectral Analysis
study of the distribution by wavelength or frequency of the radiation emitted by an object of interest.
Spectral Binary
A binary system recognized as a binary because its spectrum contains lines of two stars of different spectral types.
Spectral Lines
A rainbow shows the small slice of the entire electromagnetic spectrum that humans can see. When astronomers use
instruments to break down the light from a distant star or galaxy they also produce spectra. These contain signatures of the
chemicals and molecules present in an object
Spectral Type/Class
Classification used to sort stars by photospheric temperature and intrinsic brightness. The seven spectral classes O-B-A-F-G-KM, listed in order of decreasing temperature, include 99% of all known stars. Each spectral type is divided into a variable
number of subtypes.
Spectrogram
A photograph of a spectrum.
Spectrograph
An instrument that spreads electromagnetic radiation into its component frequencies and wavelengths for detailed study. A
spectrograph is similar to a prism, which spreads white light into a continuous rainbow.
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Spectroheliograph
An instrument used in solar telescopes to photograph the Sun in a single wavelength of light. Different wavelengths reveal
different features of the Sun’s surface.
Spectrometer
The instrument connected to a telescope that separates the light signals into different frequencies, producing a spectrum.
Spectroscope
An instrument allowing an observer to view the spectrum of a source of light.
Spectroscopic Binary
Binary star that can be distinguished from a single star only through analysis of the Doppler shift of the spectral lines of one or
both stars as they revolve about their common centre of mass.
Spectroscopic Parallax
The distance to a star derived from comparison of its apparent magnitude with its absolute magnitude deduced from study of its
position on an H-R diagram determined by observation of its spectrum (spectral type and luminosity class).
Spectroscopy
A method of determining the chemical composition of a substance by the electromagnetic radiation it absorbs or emits. Through
spectroscopy, scientists can discover the chemical components of distant celestial bodies.
Spectrum
(a) Grass-like patterns of gas seen in the atmosphere of the Sun.
(b) An arrangement of electromagnetic radiation according to wavelength.
Speed Of Light
Speed of light: The speed at which electromagnetic radiation propagates in a vacuum. It is has been measured at 299,792,458
meters per second (186,212 miles per second). Einstein’s Theory of Relativity implies that nothing can go faster than the speed
of light.
Speers-Waler
See “Eyepiece”
Spherical Aberration
This is an error in some lenses (and mirrors) that causes light rays coming from the edge of the lens to be brought into focus at
a different point to those rays that are passing through the lens’s centre. It causes stars to appear bloated and not as pinpoints
of light.
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Spherochromatism
An aberration of optical systems, in which spherical aberration is different in different colours of light. That is, if you performed a
knife-edge or star test of a system with spherochromatism, you would get different results in different colours of light.
Spicule
A jet of matter ejected from the photosphere of the sun.
Spider
The arms or struts, if any, which support a diagonal mirror or a secondary mirror in position.
Spin-Orbit Coupling
A simple relationship between the orbital and spin periods of a satellite or planet, caused by tidal forces that have slowed the
rate of rotation of the orbiting body. Synchronous rotation is the simplest and most common form of spin-orbit coupling.
Spiral Density Wave
A spiral wave pattern in a rotating, thin disk, such as the rings of Saturn or the plane of a spiral galaxy like the Milky Way
Spiral Galaxy
A galaxy that contains a prominent central bulge and luminous arms of gas , dust, and young stars that wind out from the
central nucleus in a spiral formation. Our galaxy, the Milky Way, is a spiral galaxy.
Sporadic Meteor
A meteor that is not associated with a shower.
Sprites
Gamma-ray flashes produced in Earth’s atmosphere by severe lightning storms and upper atmospheric events.
Stacking
Stacking involves taking many short exposures and adding them together to make one long duration exposure. Good software
will register (align) all of the images while it stacks them.
Standard Candle
An object whose properties allow us to measure large distances through space. The absolute brightness of a standard candle can
be determined without a measurement of its apparent brightness. Comparing the absolute brightness of a standard candle to its
apparent brightness therefore allows us to measure its distance. For example, the distinct variations of Cepheid variable stars in
other galaxies tell us their absolute brightness. By accurately measuring the apparent brightness of these stars, astronomers
can precisely determine the distance to the galaxy in which they reside.
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Star
A giant ball of hot gas that creates and emits its own radiation through nuclear fusion.
Star Cluster
A large grouping of stars, from a few dozen to a few hundred thousand, that are bound together by their mutual gravitational
attraction.
Star Diagonal
A mirror on some refracting telescopes that allows more comfortable viewing than you get by looking along the telescope tube.
The star diagonal bends the light by 90 degrees.
Star Magnitude
The Star Magnitude or brightness is a rating on a scale that gives the brightest stars a low magnitude, and the faintest stars the
highest number. Each step on the scale is 2.5 times difference in brightness; five steps on the scale indicate a difference of 100
time’s comparative brightness.
Starburst Galaxy
A galaxy undergoing an extremely high rate of star formation. Starburst galaxies contain massive, deeply embedded stars that
are among the youngest stars observed.
Starspot
This is the equivalent of a sunspot on another star. It is a region where magnetic field lines of the star are breaking through the
surface slowing the transport of heat and cooling the surface.
Star Test
An optical test of a telescope performed using a star as the light source.
Static
Random noise in a radio receiver. It can also be heard in telephone lines and cell phones.
Steady-State Theory
The theory of cosmology in which the universe is thought to have had no beginning and is postulated not to change with time.
Stefan-Boltzmann Law
The radiation law that states that the energy emitted by a black body varies with the fourth power of the temperature.
Stellar Atmosphere or Envelope
Is the outermost region of a star. Although it forms only a small portion of the star's mass, for some evolved stars the stellar
envelope can form a significant fraction of the radius.
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Stellar Black Hole
A black hole formed from the death of a massive star during a supernova explosion. A stellar black hole, much like a
supermassive black hole, feeds off of nearby material - in this case, the dead star. As it gains mass, its gravitational field
increases.
Stellar Epoch
Sixth epoch in the history of the Universe, lasting perhaps 1010 yr. from the galactic era to the present, dominated by the
formation of stars.
Stellar Evolution
The process of change that occurs during a star’s lifetime from its birth to its death.
Stellar Nursery
A region in space where stars are forming from a cloud of gas and dust.
Stellar Parallax
The apparent annual shifting of position of a nearby star with respect to more distant background stars. The term stellar parallax
is often assumed to mean the parallax angle, which is one- half of the total angular motion a star undergoes.
Stellar Wind
The ejection of gas from the surface of a star. Many different types of stars, including our Sun, have stellar winds. The stellar
wind of our Sun is also known as the Solar wind. A star's stellar wind is strongest near the end of its life when it has consumed
most of its fuel.
Stones
A stony type of meteorite, including the chondrites.
Stooping
Vertical compression of the refracted image of a distant object; the opposite of towering.
Stradian
The unit of solid angular measure, defined as the ratio of the surface area of that section of a sphere intercepted by a solid angle
to the square of the radius. A full sphere subtends 4 P steradians from its centre.
Stratosphere
One of the upper layers of the atmosphere of a planet, above the weather. The earth's stratosphere ranges from about 20 to 50
km in altitude.
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Stroboscopic Imaging
A type of imaging technique that allows studies to be made of oscillating or pulsating celestial object such as a pulsar.
Strong (Nuclear) Force
The nuclear force, the strongest of the four fundamental forces of nature.
Sub or Sub-Exposure
An individual exposure. The duration of the sub-frame will be dictated by the stability of the imaging system to maintain
accurate tracking and the background brightness of the sky (Sky Limited Exposure).
Sublimation
The process of passing from gas to solid state (or vice versa) without becoming a liquid.
Subtend
The angle that an object appears to take up in your field of view; actually, the angle between lines drawn from opposite sides of
the object to your eye. For example, the full moon subtends ½°.
Subtypes
Designated by Arabic numerals. Further, stars are sorted by intrinsic brightness into luminosity classes designated by the first
five Roman numerals. In turn, these are subdivided into a small number of subclasses designated by the first few letters of the
lower case English alphabet; for example, the Sun is a G2 V star (also sometimes denoted as a dwarf G2 star) and Betelgeuse (a
Orionis) is classified as M2 lab (i.e., intermediate between la and lb).
Summer Solstice
The moment when the Sun reaches its greatest distance north of the celestial equator, on or about June 21. In the northern
hemisphere this marks the first day of summer; in the southern hemisphere it marks the first day of winter.
Sun
The star at the centre of our solar system. The energy from the sun supports almost all life on Earth. An average star in terms
of size and mass, the Sun is a yellow dwarf of spectral type G2. It is about 5 billion years old, contains 2x1030 kilograms of
material, has a diameter more than 100 times that of Earth, and a mean distance from the Earth of approximately 149.6 million
kilometres (1 AU). Currently, the Sun (and therefore the Earth and the Solar System) may be found close to the inner rim of the
Galaxy's Orion Arm, in the Local Fluff, inside the Local Bubble, in the Gould Belt, at a distance of 8.33 ± 0.35 kiloparsecs (27,200
± 1,100 ly) from the Galactic Centre.
Sun Pillar
A vertical column of light that appears above or below the Sun at sunset or sunrise.
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Sunspot
Areas of the Sun's surface that are cooler than surrounding areas. The usually appear black on visible light photographs of the
Sun. Sunspots are usually associated disturbances in the Sun's electromagnetic field.
Sunspot Cycle
The roughly 11-year cycle of variation of the number of sunspots visible on the sun.
Super-Earths
These are rocky worlds found by extra-solar planet searches in recent years. They’re more massive than the Earth, but generally
no larger than about 10 Earth masses.
Supercluster
A cluster, of clusters of galaxies.
Supergiant
The stage in a star's evolution where the core contracts and the star swells to about five hundred times its original size. The
star's temperature drops, giving it a red colour. Supergiants fall in the extreme upper right of the H-R diagram.
Supergranulation
The pattern of large cells seen in the sun's chromosphere, when viewed in the light of the strong emission line of ionized
hydrogen.
Superior Conjunction
A conjunction that occurs when a superior planet passes behind the Sun and is on the opposite side of the Sun from the Earth.
Superior Planet
A planet that exists outside the orbit of the Earth. All of the planets in our solar system are superior except for Mercury and
Venus. These two planets are inferior planets.
Supermassive Black Hole
These objects are a million to a billion times more massive than black holes and are found, it is thought, in the centre of every
galaxy.
Supernova
A supernova is a cataclysmic explosion caused when a star exhausts its fuel and ends its life. Supernovae are the most powerful
forces in the universe. All of the heavy elements were created in supernova explosions.
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Supernova Remnant
An expanding shell of gas ejected at high speeds by a supernova explosion. Supernova remnants are often visible as diffuse
gaseous nebulae usually with a shell-like structure. Many resemble "bubbles" in space.
Supernumeraries
These are the faint rainbow bands seen on the inner edge of a rainbow. They appear when rain droplets in the rainbow are
comparatively small and of a similar diameter to each other.
Super Wide Angle
A well corrected eyepiece with large apparent fields of view (60 - 70 degrees).
Surface Brightness
This is a measure of how bright something is (like a galaxy or a nebula) for each unit of area (a square arc-second).
Symmetrical Eyepiece
A specific eyepiece design. The optics of a symmetrical eyepiece comprise two cemented doublets oriented face-to-face. Many
designs sold as Plossl eyepieces are actually symmetrical, but true Plossls are not symmetrical.
Synchotron Self-Absorption
Re-absorption of radiation from accelerated electrons by other nearby electrons; this is a possible source of low frequency
turnovers observed in the radio spectra of compact sources.
Synchotron Emission
Radiation from electrons constantly accelerated in a magnetic field at a rate great enough for relativistic effects to be important.
Predicted long ago, this radiation was first encountered in the particle accelerator called the synchrotron. Much of the radiation
observed by radio astronomers originates in this fashion.
Synchotron Rotation
A situation in which the rotational and orbital periods of an orbiting body are equal, so that the same side is always facing the
companion object.
Synergistic Effect
An effect much greater than the sum of the expected effects.
Synodic
Measured with respect to an alignment of astronomical bodies other than or in addition to the sun or the stars (usually the moon
or a planet). For example, a synodic month depends on the positions of the sun, earth, and moon.
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Synodic Month
The time it takes the Moon to complete one cycle of phases—from new moon to new Moon—an average of 29.53059 days.
Synodic Period
The orbital or rotational period of an object as seen by an observer on the earth. For the moon or a planet, the synodic period is
the interval between repetitions of the same phase or configuration. The synodic period of Mars depends on the relative
positions of the earth and Mars as they orbit the sun.
Systems Analysis
Analysis of the response to inputs of a set of interconnected units whose individual characteristics are known.
Syzygy
An alignment of three celestial bodies. Sometimes applied more specifically to an alignment of the sun, earth, and moon.
..T
T-Tauri Stars
Luminous variable stars associated with interstellar clouds and found in very young clusters; they are believed to be still in the
process of gravitational contraction from their protostellar phase and have not yet arrived at the Main Sequence and begun to
burn hydrogen.
T Association
A grouping of several T Tauri stars, presumably formed out of the same cloud of interstellar dust and gas.
Tachocline
A thin boundary that separates the Sun’s outermost layer, called the convective zone, from the denser internal regions. These
layers rotate at different speeds to create the Sun’s magnetic activity.
Tail
A tail is made up of dust and gas from a comet’s coma. A tail forms when the solar wind separates dust and gas from the coma,
pushing it outward and away from the Sun in either a slightly curved path (for dust) or a straight path (for gas).
Takahashi
See “Eyepiece”
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Technology
The sum total of processes by which humans modify the materials of nature to better satisfy their needs and wants.
Tektites
Small glassy objects found scattered around the southern part of the southern hemisphere of the earth.
Telecompressor
The opposite of a telextender or a Barlow lens. A telecompressor multiplies the focal length of a telescope by a factor which is
less than one, thus reducing the focal length.
Telescope
An instrument used to collect large amounts of light from far away objects and increase their visibility to the naked eye.
Telescopes can also enlarge objects that are relatively close to the Earth or on the Earth. A Telescope can also include any
instrument designed to observe distant objects by their emissions of invisible radiation such as x-rays or radio waves.
Telextender
Another name for a Barlow lens.
Telluric Stars
These have nearly featureless continuum spectra that can be used to correct for the effect of telluric contamination of the Earth's
atmosphere on the spectra of other stars. For example, water vapour in the atmosphere creates significant telluric absorption
bands at wavelengths above 6800 Å. These features need to be corrected for in order to reach a more accurate spectrum.
TelRad
A particularly popular brand of unit-magnification finder.
Temperature
A measure of heat intensity, or how energetic the particles of a sample are.
Termination Shock
This is where the solar wind confronts the interstellar wind and is slowed down considerably. The sudden deceleration of the gas
causes it to become denser and to heat up.
Terminator
The boundary between the light side and the dark side of a planet or other body.
Terra
A rough upland or mountainous region of the moon with a relatively high albedo.
The planet Earth.
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Terrestrial
A term used to describe anything originating on the planet Earth.
Terrestrial Planet
A name given to a planet composed mainly of rock and iron, similar to that of Earth, Mars, Venus, Mercury.
Terrestrial Refraction
The displacement of terrestrial objects from their geometric directions by atmospheric refraction. You may not notice it, but it is
a major headache for geodesists and surveyors.
Theories
Detailed explanations of the behaviour of matter based on experiments; may be revised if new data warrant.
Thermal Inversion
On the average, the temperature in the lower atmosphere decreases with increasing height. (The average gradient is about 6.5
K per kilometer.) A region in which the warmer air lies above the colder air is said to have an “inverted” temperature gradient,
and is called a “thermal inversion,” or “inversion layer”.
Thermal Radiation
Radiation released by virtue of an object’s heat, namely, the transfer of heat energy into the Radiative energy of electromagnetic
waves. Examples of thermal radiation are sunlight, the orange glow of an electric range, and the light from in incandescent light
bulb.
Thermonuclear Reactions
Radiation whose distribution of intensity over wavelength can be characterized by a single number (the temperature). Blackbody radiation, which follows Planck's law, is an example of thermal radiation.
Thin Disk Population
This refers to the layer of the Milky Way galaxy where the spiral arms are found and where most of the star formation takes
place. It is about 300–400 parsecs (980–1,300 light-years) deep and cantered on the galactic plane. Stars belonging to this
population generally follow orbits that lie close to this plane. This is in contrast to members of the thick disk population and halo
stars.
Tidal Braking or Acceleration
Is the transfer of momentum between an astronomical body and an orbiting satellite as the result of tidal forces. This can cause
changes in the rotation periods for both bodies as well as modification of their mutual orbit. A satellite in a prograde orbit will
gradually recede from the primary body, while slowing the rotation rate of both bodies.
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Tidal Force
The differential gravitational pull exerted on any extended body within the gravitational field of another body.
Tidal Locking
Is a net result of continued tidal braking wherein the satellite orbits with the same face always pointed toward its primary. The
Moon is tidally locked with the Earth.
Time Dilation
The increase in the time between two events as measured by an observer who is outside of the reference frame in which the
events take place. The effect occurs in both Special and General Relativity and is quite pronounced for speeds approaching the
speed of light and in regions of high gravity.
Topocentric
Referred to a position on the surface of the Earth (cf geocentric, which is referred to the centre of the Earth.)
Total Eclipse
Any eclipse in which the eclipsed body is totally blocked from view or totally immersed in shadow.
Total (Visual) Magnitude
Total, integrated magnitude of a comet's head (meaning coma + nuclear condensation). This can be estimated visually, as the
comet's "total visual magnitude". The variable m1, usually found in comet ephemerides, is used to denote the total (often
predicted) magnitude.
Towering
Abnormal vertical stretching of the image of a distant object; the opposite of stooping. Towering and stooping are refraction
phenomena that distort but do not invert images; hence, they are not mirages.
Trans Neptunian Objects
An object in the Solar System that orbits the Sun at a greater average distance (semi-major axis) than Neptune. The Kuiper
Belt, Scattered Disk Objects, and Oort cloud, are three conventional divisions of this volume of space, though treatments vary
and a few objects do not fit easily into any division.
The Kuiper Belt contains objects with an average distance to the Sun of 30 to about 55 AU, usually having close-to-circular
orbits with a small inclination from the ecliptic. Kuiper belt objects are further classified into the following two groups:
‘Resonant Objects’ which are locked in an orbital resonance with Neptune. Objects with a 1:2 resonance are called
Twotinos, and objects with a 2:3 resonance are called Plutinos, after their most prominent member, Pluto.
‘Classical Kuiper Belt Objects’ (also called Cubewanos) have no such resonance, moving on almost circular orbits,
unperturbed by Neptune and sit in a band between the inner Plutinos and the outer Twotinos.
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The Scattered Disk contains objects farther from the Sun, usually with very irregular orbits (i.e. very elliptical and having a
strong inclination from the ecliptic).
Transient Lunar Phenomena
These are colour changes and brightening’s on the Moon’s surface, reported by some lunar observers. Today TLPs also include
the flashes produced when meteoroids impact the Moon, which have been recorded many times by CCD cameras.
Transit
(i) The passage of a celestial body across an observer's meridian
(ii) The passage of a celestial body across the disk of a larger one.
Transit Method
A way of searching for exoplanets by looking for the tell-tale dip in brightness of a star as a planet passes in front of it.
Transition Elements
Metallic elements situated in the centre portion of the periodic table in the B groups.
Transit Telescope
A telescope designed to point straight overhead and accurately measure the times at which stars cross the meridian
Transparency
Transparency is a measure of the clarity of the sky.
Triple-Alpha Process
A chain of fusion processes by which three helium nuclei (alpha particles) combine to form a carbon nucleus.
Triple Conjunction
The close alignment of a planet and a star at three distinct times, caused by the retrograde motion of the planet. The planet
passes the star once in its forward motion, once more in its retrograde motion, and a third time when it resumes its forward
motion.
Triplet
Three simple lenses used in combination, placed close together or in contact.
Tritium
A rare radioactive isotope of hydrogen with two neutrons and one proton in the nucleus (a mass of 3 atomic mass units).
Trojan
An object orbiting in the Lagrange points of another (larger) object. This name derives from a generalization of the names of
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some of the largest asteroids in Jupiter's Lagrange points. Saturn's moons Helene, Calypso and Telesto are also sometimes
called Trojans.
Trojan Asteroids
A group of asteroids that precede or follow Jupiter in its orbit by 60°.
Tropic of Cancer
The line of latitude on the Earth’s surface that is 23 1/2 degrees north of the Equator. It marks the northernmost latitude in the
northern hemisphere at which the Sun can appear directly overhead.
Tropic of Capricorn
The line of latitude on the Earth’s surface that is 23 1/2 degrees south of the Equator. It marks the southernmost latitude in the
southern hemisphere t which the Sun can appear directly overhead.
Tropical Year
The length of time between two successive vernal equinoxes.
Troposphere
The lowest layer of the Earth's atmosphere, lying at an average height of up to 11km (6.5 miles). Above the troposphere lies the
stratosphere, above that the ionosphere, above that the exosphere. In the troposphere the temperature drops steadily except
for localised layers of temperature inversion.
True Field of View
The angular size (in degrees) of the actual area of the sky that you can view through a particular telescope with a particular
eyepiece. (The TFOV depends on both, and is calculated by dividing the AFOV by the Magnification.)
Truss-Tube Dobsonian
A Dobson-mounted telescope in which the telescope tube is a truss assembly, perhaps looking like part of a bridge, or a bird
cage. Often designed for quick and easy assembly and disassembly in the field, and the disassembled parts are not very bulky.
Thus a quite large telescope may be transported in modest space.
Tube Rings
Also Mounting Rings. Rings that attach to a telescope’s mounting plate (see dovetail plate) and hold the tube assembly in place.
Turbulence
Unstable and disorderly motion, as when a smooth, flowing stream becomes a churning rapid.
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Twilight
The period of decreasing sky brightness after sunset, or of increasing sky brightness before sunrise. There are three definitions
of twilight: Civil Twilight, Nautical Twilight, and Astronomical Twilight. Twilight lasts longer in higher latitudes.
Type Ia Supernova
A supernova explosion that occurs when a white dwarf accretes enough matter from a companion star to exceed the
Chandrasekhar limit.
..U
Ultra-Low Expansion
ULE, Corning Glass's successor to Pyrex as a material out of which telescope mirrors are made. U.L.E. expands or contracts
very little when the temperature changes, and thus a mirror made out of it holds its shape as the temperature varies without
distorting the image.
Ultraviolet
The region of the spectrum between about 100 and 4000 angstroms; also used in the restricted sense of ultraviolet radiation that
reaches the ground, namely, that between about 3000 and 4000 angstroms.
Ultraviolet Radiation
Electromagnetic radiation that has a shorter wavelength and higher energy than visible light.
Ultra Wide Angle
A specific eyepiece design. Ultra Wide Angle has a very large apparent field of view, of 80+ degrees.
Umbra
The area traced on a planet during an eclipse where the eclipsed light source is completely blocked. Observers in the umbral
shadow of a solar eclipse, for instance, see a total eclipse.
Unit-Magnification Finder
A finder which operates without magnification. Such a device is in essence a miniature head-up display: It projects a cross-hair,
illuminated bulls-eye, or similar pattern on the sky, where the observer may view it easily with both eyes open. Unitmagnification finders make it very easy to find objects which are bright enough to be seen with the naked eye, or which are close
enough to naked-eye stars. Also called a Reflex Finder.
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Universal Time
A measure of time used by astronomers; UT conforms (within a close approximation) to the mean daily (apparent) motion of the
sun. UT is determined from observations of the diurnal (daily) motions of the stars for an observer on the earth. Coordinated
Universal Time (UTC) is that used for broadcast time signals (available via shortwave radio, for example), and it is within a
second of UT. Also known as Greenwich Mean Time.
Universe
everything that came into existence at the moment of the Big Bang, and everything that evolved from that initial mass of
energy, or everything we can in principle, observe.
Uranography
Celestial cartography; the mapping of celestial bodies.
Uranology
(obsolete) The study of the heavens and heavenly bodies, also books about the subject.
Uranometry
A chart or catalogue of fixed stars, especially of stars visible to the naked eye.
UV light
ultraviolet (UV) light.
..V
Van Allen Belt
Zones in the earth's magnetosphere where charged particles are confined by the earth's magnetic field. There are two main
belts, one centred at an altitude of roughly 1.5 times the earth' radius, and the other between 4.5 and 6.0 times the earth's
radius.
Vaporization
The process in which a substance changes from the liquid to the gaseous (vapour) state.
Variable Star
A star whose brightness fluctuates over time. This may be hours, days weeks or many years. The reasons for the fluctuations
are varied, such as another star eclipsing it or shells of gas and dust being ejected.
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Velocity
The measure of the speed and direction of an object; the distance travelled by an object per unit of time.
Velocity Curve
A plot showing the orbital velocity of stars in a spiral galaxy versus distance from the galactic centre.
Velocity Dispersion
A measure of the average velocity of stars in a group or cluster with random internal motions. In globular clusters and elliptical
galaxies, the velocity dispersion can be used to infer the central mass.
Vernal Equinox
The moment when the Sun crosses the celestial equator travelling in a northward direction, on or about March 21. In the
northern hemisphere, it marks the first day of spring. The term is also applied to the Sun’s position in the sky at that moment.
It is one of two points where the ecliptic and the celestial equator intersect, the other being the autumnal equinox.
Very Large Array
One of the world’s premier radio observatories, consisting of 27 antennas arranged in a huge “Y” pattern. The VLA spans up to
22 miles (36 km) across, which is roughly one and a half times the size of Washington, D.C. Each antenna is 81 feet (25 meters)
in diameter. Located in Socorro, New Mexico, the telescopes work in tandem to produce a sharper image than any single
telescope could record.
Very-Long-Baseline Interferometry
The technique of using simultaneous measurements made with radio telescopes at widely separated locations to obtain
extremely high resolution.
Vignetting
An optical effect whereby the corners or edges of a telescope’s field of view are darkened. There are several ways Vignetting can
appear, but it can be reduced by taking flat fields (using software to remove its appearance) and in some cases by stopping
down an aperture.
Visible Light
The part of the electromagnetic spectrum that human eyes can detect; also known as the visible spectrum. The colours of the
rainbow make up visible light. Blue light has more energy than red light.
Virgo Cluster
A gigantic cluster of over 2000 galaxies that is located mainly within the constellation of Virgo. This cluster is located about 60
million light years from Earth.
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Visual Binary
Binary star system whose components can be identified with an optical telescope.
Visual Magnitude
A scale used by astronomers to measure the brightness of a star or other celestial object. Visual magnitude measures only the
visible light from the object. On this scale, bright objects have a lower number than dim objects.
Volatile
The property of being easily vaporized. Volatile elements stay in gaseous form except at very low temperatures; they did not
condense into solid form during the formation of the solar system.
Volatile Organic Compounds
Class of organics that is easily vaporizable at low temperatures and pressures.
Volcano
A break or vent in the crust of a planet or moon that can spew extremely hot ash, scorching gases, and molten rock. The term
volcano also refers to the mountain formed by volcanic material.
..W
Waning
When the phase (illumination) of a planet or moon’s disc is decreasing in size. For example: ‘A waning crescent Moon can be
seen tonight’.
Waning Crescent
The phase of the Moon between third quarter and new moon. Waning means declining or fading.
Waning Gibbous
The phase of the Moon between full moon and last quarter.
Wave
A vibration in some media that transfers energy from one place to another. Sound waves are vibrations passing in air. Light
waves are vibrations in electromagnetic fields.
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Waveguide
Special transmission medium resembling a pipe and often having a rectangular cross section, inside of which radio waves may be
propagated.
Wavelength
The distance between consecutive crests of a wave. This serves as a unit of measure of electromagnetic radiation.
Waxing Crescent
The phase of the Moon between new moon and first quarter. Waxing means increasing.
Waxing Gibbous
The phase of the Moon between first quarter and full moon.
Waxing Moon
Phases of the lunar cycle when the face of the Moon appears to be getting larger each day. In the waning parts of the lunar
cycle, the face of the Moon appears to be getting smaller.
Weak Force
One of the four fundamental forces of nature, weaker than the strong force and the electromagnetic force. It is important only in
the decay of certain elementary particles.
Wedge
The part that fits between the tripod or pillar and the fork of a fork-mounted telescope, which enables the fork to be equatorially
aligned.
Weight
The force determined by the gravitational pull on a mass.
WGS84 (Geographical Coordinates)
Geographical coordinates are given by the angles longitude (Lon), latitude (Lat), and altitude in meters (Alt). A place north of
the equator at marked by N or +, places south of the equator by S or -. The longitude from the meridian of Greenwich is
counted positive towards east (E). Places west from Greenwich are marked W or by -. The geographical coordinates refer to an
ellipsoid, which fits the true shape of the Earth (geoid). The geoid corresponds to calm sea surface. The keyword "Geographic:"
uses the local ellipsoid as reference system. WGS84 mark coordinates referring to the WGS84 ellipsoid. The difference in
altitude to the geoid sums up to 100 meters and is called geoid undulation. This is corrected for when tagged "MSL" (mean sea
level), such that the origin of the height system is at sea level.
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Whistler
Electromagnetic ultra-low frequency radiation observed in planetary magnetospheres; energized by lightning and other
discharges.
White Dwarf
A very small, white star formed when an average sized star uses up its fuel supply and collapses. This process often produces a
planetary nebula, with the white dwarf star at its centre. It has a mass less than the Chandrasekhar limit, which is about 3 x
1030 kg.
White Hole
A theoretically possible but physically highly unlikely singularity from which matter and energy are able to escape; the antithesis
of a black hole.
Wien’s Law
An experimentally discovered law applicable to thermal continuum radiation, which states that the wavelength of maximum
emission intensity is inversely proportional to the absolute temperature.
Winter Solstice
The moment when the Sun reaches its greatest distance south of the celestial equator, on or about December 22. In the
Northern Hemisphere this marks the first day of winter; in the Southern Hemisphere it marks the first day of summer.
Wm-2 (or W/m2 )
Watts per square meter of incident signal flux per whatever resolution bandwidth is in use. It is the total signal flux if the
receiving bandwidth equals or is greater than the bandwidth of the signal.
Wm-2 Hz-1 (or W/m2 Hz)
Spectral flux density (see Jansky).
Wolf-Rayet Star
A type of O star whose spectrum shows very broad emission lines.
Worm Drive
Probably the most common drive on equatorial mounts. It consists of a spirally cut cylinder (the "worm") which rotates
longitudinally such that its thread engage with the specially shaped teeth on the circumference of a disc (the "worm wheel"),
which in turn drives the shaft of the mount.
W Virginis Star
A type of II Cepheid, one of the fainter class of Cepheid variable stars characteristic of Cepheids in globular clusters.
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..X
X-Power
Identifies the magnifying power of a lens or mirror. For example, a 50-power telescope makes the image 50 times larger than it
is when viewed without the telescope.
X Radiation
Electromagnetic radiation in the range of approximately 0.05-100 A.
X-Ray
Electromagnetic radiation of a very short wavelength and very high-energy. X-rays have shorter wavelengths than ultraviolet
light but longer wavelengths than cosmic rays. A photon of electromagnetic radiation in the wavelength interval between about
1 Angstrom and 100 Angstroms.
X-Ray Astronomy
The field of astronomy that studies celestial objects by the x-rays the emit.
X-Ray Sources
Celestial objects that give off X-rays. These exotic objects are producing very energetic radiation and include black holes,
neutron stars (pulsars), supernovae remnants, and the centres of galaxies.
X-Ray Telescope
A special telescope used to detect X-rays - high-energy electromagnetic radiation. The high energy of X-rays means they will go
through rather than bounce off a “normal” telescope mirror. Instead, the mirrors are arranged so the X-rays skip across them
much like a stone skips across the surface of a lake.
X-Ray Star
A bright celestial object that gives off x-rays as a major portion of its radiation.
..Y
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Yarkovsky Effect
This is an effect that can change the movement or rotation of tiny rocky particles and small asteroids in space, caused by the
absorption and reemission of photons of light. The result is that asteroids can be slightly perturbed in their orbits by this effect.
Year
The period of revolution of a planet around its central star; more particularly, the earth's period of revolution around the sun.
Year, anomalistic
The period for successive perihelion passages of the Earth, a little less than 5 minutes longer than the sidereal year.
Year, calendar
The mean length of the year according to the Gregorian calendar, 365 days, 5 hours, 49 minutes and 12 seconds.
Year, sidereal
The period taken by the Earth to complete one orbit of the Sun, 365.26 days.
Year, tropical
The period taken for successive passages of the Sun across the Vernal Equinox, 365.24 days.
Yellow Dwarf
An ordinary star such as the Sun at a stable point in its evolution.
Yoke Mounting
A form of equatorial mounting in which the polar axis consists of a yoke in which the telescope tube is mounted.
..Z
Zeeman Effect
The broadening or splitting of spectral lines caused by the presence of a (strong) magnetic field in the gas where the lines are
formed.
Zenith
The point on the celestial sphere that is directly above the observer. Holding a balloon overhead places the balloon at your
zenith. Although celestial objects appear to rise and set as they move across the sky, they rarely reach the zenith point.
Zenith Distance
The angular distance of an object from the zenith. The zenith distance of an object is the complement of its altitude.
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Zenith Hour Rate (Zenithal)
This is a number for the current or estimated activity of a meteor stream: the number is the hypothetical number of meteors in
the sky for an observer with the radiant in the zenith and a limiting magnitude of 6.5
Zero-Age Main Sequence
The main sequence in the H-R diagram formed by stars that have just begun their hydrogen-burning life- times, and have not
yet converted any significant fraction of their core mass into helium. The zero-age main sequence forms the lower left boundary
of the broader band representing the general main sequence.
Zircon
A silicate mineral, ZrSiO4, diamond like in appearance, that contains silica silicon dioxide, and the element zirconium. Some
zircons in meteorites contain material from the period before the Solar System formed.
Zodiac
An imaginary belt circling the celestial sphere along the ecliptic, broad enough to encompass the paths of all the planets visible to
the naked eye
Zodiacal Light
A faint cone of light that can sometimes be seen above the horizon after sunset or before sunrise. Zodiacal light is caused by
sunlight reflecting off small particles of material in the plane of the solar system.
Zone
A definite area on the mirror, measured from the centre.
Zoom
An eyepiece design whereby you can vary the focal length by twisting the barrel of the eyepiece.
Zulu
Another name for Universal Time, or GMT
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Astronomy Acronyms & Abbreviations
Acronym
Δf
μ
μC-SiC
μm
α-Si:H
β-BaB2O4
+D
-D
1D
2.5G
2A2OA
2D or 2-D
2D-BPM
2G
2P-OBIC
3D BDS
3D OCT
3D or 3-D
3DTV
3D-BPM
3D-FISH
3D-MID
3D-STORM
3G
3PLSM
3R
3T
4G
4GLS
4HG
5-ALA
5G
5HG
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Description
Offset frequency (Delta frequency)
micromicrocrystalline silicon carbide
micrometre; micron
hydrogenated amorphous silicon
beta barium borate
Positive-Dispersion
Negative-Dispersion
One Dimensional
Enhanced Second Generation
2-aminoadipate: 2-oxoglutarate aminotransferase
Two-Dimensional
Two-Dimensional Beam Propagation Method
Second Generation
Two-Photon Optical Beam-Induced Current
Three-Dimensional Beam Delivery System
Three-Dimensional Optical Coherence Tomography
Three-Dimensional
Three-Dimensional Television
Three-Dimensional Beam Propagation Method
Three-Dimensional Fluorescence In Situ Hybridization
Three-Dimensional Moulded Interconnected Device
Three-Dimensional Stochastic Optical Reconstruction Microscopy
Third Generation
Three-Photon-Excitation Laser-Scanning Fluorescence Microscopy
Re-amplify, Re-shape, And Re-time
Three-Transistor
Fourth Generation
Fourth-Generation Light Source
Fourth Harmonic Generation
5-aminolevulinic acid
Fifth Generation
Fifth Harmonic Generation
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AC
ADU
AGN
Alt
AO or A/O
AP
Apo
APOD
AR
ASCOM
ATM
AU
Az
Bins
BRB
c
CA
CCD
CETI
CXO
CHZ
CLS
CMBR
CME
CMOS
CNO
Dec
DN
Dob
DSI
DSLR
DSO
DSS
DTC
ED
EFN
EFR
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Astrochat (.com Forums)
Analog Digital Unit
Active Galaxy Nucleus
Altitude
Adaptive Optics
Astro Photography
Apochromatic Refractor
Astronomy Picture Of The Day
Anti-Reflection (Coated Surface)
Astronomy Common Object Model
Amateur Telescope Maker
Astronomical Unit
Azimuth
Binoculars
Be Right Back
Constant for the Speed of Light
Chromatic Aberration
Charge Coupled Device
Communication With Extra-Terrestrial Intelligence
Chandra X-ray Observatory.
Continuously Habitable Zone
City Light Suppression (type of filter)
Cosmic Microwave Background Radiation
Coronal Massive Ejection
Complementary Metal–Oxide–Semiconductor
Carbon, Nitrogen, Oxygen
Declination
Diffuse nebulae
Dobsonian Telescope
Deep Sky Imager (Meade Product)
Digital Single Lens Reflex (Type Of Camera)
Deep Sky Object
Deep Sky Stacker (Image Processing Software)
Digital Telescope Computer (eg Argo Navis)
Extra Low Dispersion
Extremely Faint nebulae
Emerging Flux Region (On Our Sun)
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EP
ER
EQ
EQDIR
EQMOD
ESA
ESO
ET
EXIF
FAQ
FFT
FIR
FITS
FL
FLI
FLO
FMC
FN
FOC
FOS
FOV
FPN
FSQ
FTA
FTL
FTP
FUV
FW
FWHM
G
GC
GCVS
GEM
GEO
GEOS
GIF
GLOB
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Eye Piece
Eye Relief
Equatorial
Equatorial Direct (Software From Shoestring Astronomy)
GEM Control Software
European Space Agency
European Southern Observatory
Ephemeris Time
Exchangeable Image File (Format)
Frequently Asked Questions
Fast Fourier Transform
Far Infra-Red
Flexible Image Transport System
Focal Length
Finger Lakes Instrumentation Co.
First Light Optics (retail Outlet)
Fully Multi Coated
Faint Nebulae
Faint Object Camera
Faint Object Spectrograph
Field Of View
Faint Planetary nebulae
Flat Field Super Quadruplet (Takahashi 4 Element Refractors)
Field Transition Arch (On Our Sun)
Faster Than Light
File Transfer Protocol
Far Ultra Violet
Filter Wheel
Full Width, Half Maximum
Gravitational Constant
Globular Cluster / Gaseous Comets
General Catalogue Of Variable Stars
German Equatorial Mount (Aka EQ Mount)
Geosynchronous Earth Orbit
Geosynchronous Earth Orbit Satellite
Graphics Interchange Format
Globular Cluster
152
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GLP
GMT
GPS
GR
GRB
GRS
GSO
GUT
H-R
H0
HA (or Ha)
Hb
HC
HD
HDF
HDRI
HF
HR
HST
HTH
Hz
IAU
IDP
IC
ICNR
IR
IIS
IMO
IMHO
ISM
ISS
JPG or JPEG
JPL
KBO
Km
LAN
Lappy
4 October 2012
Green Laser Pointer
Greenwich Mean Time
Global Positioning System
General Relativity
Gamma Ray Burst
Great Red Spot (On Jupiter)
Guan Sheng Optical
Grand Unified Theory
Hertzsprung-Russell
The Hubble Constant
Hydrogen Alpha (First Line Of The Balmer Series - 6563 Angstroms)
Hydrogen Beta (Light Spectrum Emission Line)
Hand Controller
Henry Draper (DSO Catalogue)
Hubble Deep Field
High Dynamic Range Imaging (Or Just HDR)
High Frequency
High Resolution / Hertzsprung-Russell
Hubble Space Telescope
Hope That Helps (Forum Abbreviation)
Hertz (Frequency In Cycles Per Second) Habitable Zone ?
International Astronomical Union
Interplanetary Dust Particle
Index Catalogue (List Of Objects)
In Camera Noise Reduction
Infra-Red (Light)
IceInSpace (Forum)
In My Opinion
In My Humble Opinion (Usually Not Being Very Humble At The Time)
InterStellar Medium
International Space Station
Joint Photographic Experts Group (Very Common Picture File Format)
Jet Propulsion Laboratory
Kuiper Belt Object
Kilometre
Local Area Network (Of Computers)
A Laptop Computer
153
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LB
LBT
LEO
LGM
LIDAR
LMC
LMFAO
LMST
LOL
LP
LPI
LPR
LRGB
LRGB-C
LSB
LST
LV
LVW
M
MA
Mak
MAS
MASAR
MASER
MCT
MIL
MN or MNT
MPC
MPCC
MUSA
mW
NASA
NB
NCP
NEO
NELM
Newt
4 October 2012
Light Bridge (Meade DOB Design)
Large Binocular Telescope
Low Earth Orbit
Little Green Men
Laser Imaging Detection and Ranging
Large Magellanic Cloud
Laugh My Fcuking Arse Off (Forum Abbreviation)
Local Mean Sidereal Time
Laugh Out Loud (Forum Abbreviation) / Lots Of Love
Light Pollution
Lunar Planetary Imager (Meade Product)
Light Pollution Reduction
Luminance Red Green Blue
Luminance Red Green Blue Clear
Low Surface Brightness (ie, Dim!)
Local Sidereal Time
Lanthanum Vixen
Lanthanum Vixen Widefield
Messier (DSO Catalogue) Object Prefix
Modified Achromat / Modern Astronomy (Retail Outlet)
Maksutov-Cassegrain Telescope
My Astro Shop
Microwave Attenuation by Stimulated Absorption of Radiation
Microwave Amplification by Stimulated Emission of Radiation
Maksutov-Cassegrain Telescope
Mother In Law (A Super Massive Blackhole)
Maksutov-Newtonian (Type Of Telescope)
Mega Parsec
Multi-Purpose Coma Corrector.
Multichannel Spectrum Analyzer
Milliwatt, = 0.001 Watt
National Aeronautics And Space Administration (U S A)
Narrow Band
North Celestial Pole
Near Earth Object (Orbit)
Naked Eye Limiting Magnitude
Newtonian Telescope (Reflector)
154
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NGC
NIB
NLV
NOAA
NPB
NPL
OAG
OC
OSC
O-III
OT
OTA
PAE
PAH
PE
PEC
PC
PGC
PHD
PI
PITA
PN
PM
PME
PMX
PN
POTH
PS
PST
QE
QSO
RA
RACI
RADAR
RAM
RAW
RC
4 October 2012
New General Catalogue (Of DSO’s)
New In Box (But Opened)
New Lanthanum Vixen (Eyepiece)
National Oceanic And Atmospheric Administration
Narrow Pass Band
New Plossl (Vixen Eyepiece)
Off Axis Guide
Open Cluster
One-Shot Colour (Camera)
Oxygen III (Or 3) (Doubly Ionized Oxygen Emission Line)
Off-Topic (Forum Post)
Optical Tube Assembly
Pointing Accuracy Enhancement (Feature Of Goto Mount)
Polycyclic Aromatic Hydrocarbon
Periodic Error
Periodic Error Correction / Printed Electronic (Component or Circuit)
Parsec
Principal Galaxies Catalogue
Push Here Dummy (Astrophotography Software)
Pixinsight
Pain In The Arse
Planetary Nebula
Private Message (Between Two Forum Members)
Paramount ME (Mount By Software Bisque)
Paramount MX (Mount By Software Bisque)
Planetary Nebulae
Plain Old Telescope Handset
Photoshop
Personal Solar Telescope (Coronado Product)
Quantum Efficiency (CCD Camera Type)
Quasi-Stellar Object
Right Ascension
Right Angle Correct Image
Radio Detection and Ranging
Random Access Memory
Not An Acronym, It Just Means Raw Imaging Data
Ritchey-Chretien
155
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RCOS
RCT
RCW
RDF
RFT
RGB
RGBb
RGO
RK
RN
ROFL
RVO
S2 or S-II
SA
SAO
SB
SBIG
SC
SCP
SCT
SD
SDM
SEB
SETI
SG
SGL
SII
SLR
SMA
SMC
SN or SNT
SNP
SNR
SP
SPED
SPM
SR
4 October 2012
RC Optical Systems Co.
Ritchey-Chretien Telescope
Rodgers Campbell Whiteoak Catalogue (Of DSO’s)
Red Dot Finder
Rich Field Telescope
Red Green Blue
Red Green Blue Light Blue
Royal Greenwich Observatory
Rank Kellner (Type Of Eyepiece - Cheap)
Reflection Nebula
Roll Over Fcuking Laughing (Forum Abbreviation)
Rother Valley Optics (Retail Outlet)
Type Of Filter
Spherical Aberration
Smithsonian Astrophysical Observatory
Surface Brightness
Santa Barbara Instrument Group
Star Clusters
South Celestial Pole
Schmidt-Cassegrain Telescope
Super-Low Dispersion
Size Does Matter (Australian Telescope Maker Peter Read's Motto)
South Equatorial Belt (On Jupiter)
Search For Extra Terrestrial Intelligence
Spiral Galaxies
Stargazerslounge (.com Forum)
Singly Ionized Sulphur Emission Line
Single Lens Reflex (Camera)
Super Modified Achromat
Small Magellanic Cloud
Schmidt-Newtonian (Type Of Telescope)
Starry Night Pro (Astronomy Software)
Super Nova Remnant / Signal To Noise Ratio
Super Plossl
Saxon Premium ED
Sir Patrick Moore
Super Ramsden Or Symmetric Ramsden
156
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SQM
SSC
STP
STV
SV
SWA
SWMBO
SXV
TAK
TBD
TBH
TEC
TFH
TFOV
TH
TLA
TLAO
TMB
TNO
TOA
TSA
UFO
UHC
UHF
UO
USB
UT
UTA
UTC
UV
UVBY
UWA
VHF
VLA
VLBI
VLT
WDS
4 October 2012
Sky Quality Meter (Brightness Reading)
Smithsonian Stellar Catalogue (Not A Preferred Term)
Standard Temperature (0 °C) And Pressure (1 Atm)
SBIG Television (Camera)
Super View
Super Wide Angle
She Who Must Be Obeyed (Forum Abbreviation)
Starlight Xpress Video (Type Of Camera)
Takahashi (Manufacturer)
To Be Decided / Too Bloody Difficult
To Be Honest (Forum Abbreviation)
Telescope Engineering Company
Too Fcuking Hard
True Field Of View
Telescope House (.com)
Three Letter Acronym (Like This)
Turn Left At Orion (Book)
Thomas M. Back (Of TMB Optical Co)
Trans-Neptunian Object
Triplet Orthoscopic Apochromat (Takahashi Design)
Triplet Super Apochromat (Takahashi Design)
Unidentified Flying Object
Ultra High Contrast
Ultra High Frequency
University Optics
Universal Serial Bus (Computer Data Transport)
Universal Time
Upper Tube Assembly (Of Truss Dobs)
Co-ordinated Universal Time
Ultra Violet (Light)
Photometry system using 3 filters in the UV, Blue, and Green-Yellow
Ultra Wide Angle
Very High Frequency
Very Large Array
Very-Long-Baseline Interferometry
Very Large Telescope
Washington Double-Star Catalogue
157
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WO
Z
ZAMS
ZLM
ZHR
4 October 2012
William Optics
Zulu (GMT / UT)
Zero-Age Main Sequence
Zenith Limiting Magnitude
Zenith Hour(ly) Rate
158
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Spectral Star Classes
The Harvard classification system is a one-dimensional classification scheme. The classes are appropriate only for stars on the
main sequence portion of their lives, and so are not appropriate for red giants etc. The spectral classes O through M are
subdivided by Arabic numerals (0–9). For example, A0 denotes the hottest stars in the A class, and A9 denotes the coolest ones.
The Sun is classified as a G2.
Class
Surface
temperature Conventional colour
(kelvin)
Apparent colour
Mass
(solar
masses)
Radius
(solar radii)
Hydrogen
lines
Fraction of all
main-sequence stars
O
≥ 33,000
blue
blue
≥ 16
≥ 6.6
Weak
~0.00003%
B
10,000–33,000
white to blue white
blue white
2.1–16
1.8–6.6
Medium
0.13%
A
7,500–10,000
white
white to blue white
1.4–2.1
1.4–1.8
Strong
0.6%
F
6,000–7,500
yellowish white
white
1.04–1.4
1.15–1.4
Medium
3%
G
5,200–6,000
yellow
yellowish white
0.8–1.04
0.96–1.15
Weak
7.6%
K
3,700–5,200
orange
yellow orange
0.45–0.8
0.7–0.96
Very weak
12.1%
M
2,000–3,700
red
orange red
≤ 0.45
≤ 0.7
Very weak
76.45%
L
1,300–2,000
purple-red
red
Unknown
Unknown
Extremely weak
≥ 100.00%
T
700-1,300
brown
purple-red
Unknown
Unknown
Extremely weak
≥ 100.00%
Y
<700
dark brown
brown
Unknown
Unknown
Extremely weak
≥ 100.00%
4 October 2012
159
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Stellar Evolutionary Paths
4 October 2012
160
© Paul Thomas
The Solar System
Sun (Sol)
☉
The star at the centre of our solar system. An average star in terms of size and mass, the Sun is a yellow dwarf of spectral type
G2. It is about 5 billion years old, contains 2x1030 kilograms of material, and has a diameter more than 100 times that of Earth.
Mercury
☿
The closest planet to the Sun. The temperature range on Mercury’s surface is the most extreme in the solar system, ranging
from about 400° C (750° F) during the day to about –200° C (–300° F) at night. Mercury, which looks like Earth’s moon, has
virtually no atmosphere, no moons, and no water.
Venus
♀
An inner, terrestrial (rocky) planet that is slightly smaller than Earth. Located between the orbits of Mercury and Earth, Venus
has a very thick atmosphere that is covered by a layer of clouds that produce a “greenhouse effect” on the planet. Venus’s
surface temperature is roughly 480° C (900° F), making it the hottest planet in the solar system. Venus has no moons.
Earth
♁ ⊕
The third planet from the Sun and one of four terrestrial planets in the inner solar system. Earth, the only planet where water
exists in large quantities, has an atmosphere capable of supporting myriad life forms. The planet is 150 million kilometres (93
million miles) away from the Sun. Earth has one satellite — the Moon.
Moon
☽
A large body orbiting a planet. On Earth’s only moon, scientists have not detected life, water, or oxygen on this heavily cratered
body. The Moon orbits our planet in about 28 days.
Mars
♂
The fourth planet in the solar system and the last member of the hard, rocky planets (the inner or terrestrial planets) that orbit
close to the Sun. The planet has a thin atmosphere, volcanoes, and numerous valleys. Mars has two moons: Deimos and
Phobos.
Asteroid Belt
A belt of small pieces of rocks and metals that orbit the sun. Possibly from the remains of a planet that was destroyed, or
material left over from a planet that was never formed. The numbers change all the time as new ones are made from bits
broken off during collisions, and some are lost, as their orbit is changed due to the collisions, and head off into space.
See the section (Asteroids from the main belt) for a breakdown of the largest of them.
4 October 2012
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Jupiter
♃
The fifth planet from the Sun and the largest planet in our solar system, twice as massive as all the other planets combined.
Jupiter is a gaseous planet with a very faint ring system. Four large moons and numerous smaller moons orbit the planet.
Jupiter is more than five times the Earth’s distance from the Sun. It completes an orbit around the Sun in about 12 Earth years.
Saturn
♄
The sixth planet in the solar system, noted for its obvious ring structure. Saturn is almost ten times the Earth’s distance from
the Sun. The planet completes a circuit around the Sun in about 30 Earth years. Saturn is the second largest and the least
dense planet in our solar system. The planet has more than 21 moons, including Titan, the second largest known moon in our
solar system.
Uranus
♅
The third largest planet in the solar system and the seventh from the Sun. Uranus is 19 times the Earth’s distance from the Sun
and completes a circuit around the Sun in about 84 Earth years. This gaseous, giant outer planet has a visible ring system and
over 20 moons, the largest of which is Titania. Uranus is tipped on its side, with a rotation axis in nearly the same plane as its
orbit.
Neptune
♆
The eighth planet and the most distant giant gaseous planet in our solar system. The planet is 30 times the Earth’s distance
from the Sun, and each orbit takes 165 Earth years. Neptune is the fourth largest planet and has at least eight moons, the
largest of which is Triton. Neptune has a ring system, just like all the giant gaseous outer planets.
Pluto
♇ n
A dwarf planet whose small size and composition of ice and rock resembles the comets in the Kuiper Belt, a region beyond
Neptune’s orbit where Pluto resides. Pluto’s orbit is more elliptical than those of the eight solar system planets. Pluto was
considered the ninth planet until August 2006, when the International Astronomical Union reclassified it as a dwarf planet. It is
now referred to as a Plutoid. It has five satellites.
Kuiper Belt
Is a region of the Solar System beyond the planets extending from the orbit of Neptune (at 30 AU) to approximately 50 AU from
the Sun. It is similar to the asteroid belt, although it is far larger - 20 times as wide and 20 to 200 times as massive. Like the
asteroid belt, it consists mainly of small bodies, or remnants from the Solar System's formation. While most asteroids are
composed primarily of rock and metal, Kuiper belt objects are composed largely of frozen volatiles (termed "ices"), such as
methane, ammonia and water. Currently, it is estimated that there are over 100,000 objects over 100km in diameter.
See the section (Asteroids from the Kuiper belt) for a breakdown of the largest of them.
4 October 2012
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Planetary Information
Mercury
Venus
Earth
Mars
Jupiter
Saturn
Uranus
Neptune
Pluto
(Our Moon)
Radius (km)
2,439
6,052
6,378
3,396
71,492
60,268
25,559
24,764
1,153
1,738.00
Mass (Earth=1)
0.0558
0.8150
1.0000
0.1074
317.8300
95.1590
14.5000
17.1470
0.0020
0.0123
Density (g/cm )
5.43
5.24
5.52
3.94
1.33
0.69
1.30
1.64
2.03
3.3464
Escape Velocity
(km/s)
4.30
10.40
11.20
5.00
59.50
35.60
21.22
23.60
1.10
2.38
Gravity (m/s )
3.70
8.87
9.78
3.71
24.79
10.44
8.69
11.15
0.66
1.622
Rotation (hrs)
1047.6
- 5832.5
23.93
24.62
9.93
10.66
- 17.24
16.11
- 153.29
655.73
Year (Earth=1)
0.24085
0.62
1
1.88
11.86
29.46
84.32
164.8
248.09
Orbit (days)
87.969
224.698
365.256
686.971
4,332.590
10,759.220
30,799.095
60,190.030
90,613.305
27.321
Min Distance
from Sun (km)
46,001,200
107,477,000
147,098,290
206,669,000
740,573,600 1,353,572,956 2,748,938,461
4,452,940,833 4,437,000,000
362,570
Max Distance
from Sun (km)
69,816,900
108,939,000
152,098,232
249,209,300
816,520,800 1,513,325,783 3,004,419,704
4,553,946,490 7,311,000,000
405,410
Axial Tilt (deg)
2.11
177.30
23.45
25.19
3.13
26.73
97.77
28.32
119.59
Inclination to
ecliptic (deg)
7.00
3.39
1.85
1.30
2.49
0.77
1.77
17.14
Albedo
0.142
0.670
0.170
0.520
0.470
0.510
0.410
0.500
3
2
0.367
6.68
0.136
Surface Temp
Min/Max (°K)
80/700
735
184/331
130/308
165
134
49/76
72
33/55
70/390
Natural Satellites
0
0
1
2
61 ?
31 ?
25 ?
13 ?
1
-
Atmosphere
CO2
H, He, CH4,
NH3
H, He, CH4,
NH3
H, He, CH4
H, He, CH4
CH4 ?
Ar, He, Na
4 October 2012
N, CO2, H2O N, O, CO2, H2O N, CO2, H2O
163
© Paul Thomas
Asteroids from the Main Belt
Name
Dia (km)
Date Discovered
Class
1 Ceres
2 Pallas
952
544
1 January 1801
28 March 1802
G
B
4 Vesta
525
29 March 1807
V
10
704
52
511
87
65
15
3
31
624
88
324
451
532
48
375
107
45
7
29
423
19
13
4 October 2012
Symbol
Hygiea
Interamnia
Europa
Davida
Sylvia
Cybele
Eunomia
Juno
Euphrosyne
Hektor
Thisbe
Bamberga
Patientia
Herculina
Doris
Ursula
Camilla
Eugenia
Iris
Amphitrite
Diotima
Fortuna
Egeria
431
326
301
289
286
273
268
258
256
241
232
229
225
222
222
216
215
213
213
212
209
208
206
164
12 April
2 October
4 February
30 May
16 May
8 March
29 July
1 September
1 September
10 February
15 June
25 February
4 December
20 April
19 September
18 September
17 November
27 June
13 August
1 March
7 December
22 August
2 November
1849
1910
1858
1903
1866
1861
1851
1804
1854
1854
1866
1892
1899
1904
1857
1893
1868
1857
1847
1854
1896
1852
1850
C
F
C
C
X
C
S
S
C
D
B
C
S
C
C
F
S
S
C
G
G
© Paul Thomas
24
94
702
121
372
128
6
16
120
41
9
11
Themis
Aurora
Alauda
Hermione
Palma
Nemesis
Hebe
Psyche
Lachesis
Daphne
Metis
Parthenope
198
197
195
190
189
188
186
186
174
174
174
153
14 Irene
152
18
8
28
5
12
37
35
26
17
141
128
120
119
113
108
103
95
90
Melpomene
Flora
Bellona
Astraea
Victoria
Fides
Leukothea
Proserpina
Thetis
5 April
6 September
16 July
12 May
19 August
25 November
1 July
17 March
10 April
22 May
25 April
11 May
1853
1867
1910
1872
1893
1872
1847
1852
1872
1856
1848
1850
C
S
M
C
C
S
S
19 May 1851
S
24 June
18 October
1 March
8 December
13 September
5 October
19 April
5 May
17 April
1852
1847
1854
1845
1850
1855
1855
1853
1852
C
C
C
S
S
S
S
S
S
C
S
S
Does not include any natural satellites of the above objects, which in themselves, are candidates for this list.
NB.
Where there is more than one symbol after the object, the first one is usually the oldest version, and the last one, is
the latest, more common, or usually accepted one.
4 October 2012
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Asteroids from the Kuiper Belt
Name
Neptune Triton
136199
Pluto
136108
136472
225088
90377
84522
50000
90482
208966
307261
55565
55637
28978
120347
19521
20000
Eris
Pluto
Haumea
Makemake
2007 OR 10
Sedna
2002 TC 302
Quaoar
Orcus
2003 AZ 84
2002 MS 4
2002 AW 197
2002 UX 25
Ixion
Salacia
Chaos
Varuna
Symbol
Dia (km)
Captured Moon
2700
¥ ¦ ² ³ È
2330
2330
1500
1440
1420
1000
1150
980
850
730
730
730
680
650
613
528
500
o n
À Á
¼ ½
r [ ] ´
( ` µ
d I
c E F G \ ·
= º
# _
Date Discovered
10 October 1846
5 January
18 February
28 December
31 March
17 July
14 November
9 October
5 June
17 February
13 January
18 June
10 January
30 October
22 May
22 September
19 November
28 November
2005
1930
2004
2005
2007
2003
2002
2002
2004
2003
2002
2002
2002
2001
2004
1998
2000
Class
Moon
SDO
Plutino
Plutino
Cubewano
SDO
Detached Object
SDO
Cubewano
Plutino
Plutino
Cubewano
Cubewano
Cubewano
Plutino
Cubewano
Cubewano
Cubewano
Does not include any natural satellites of the above objects, which in themselves, could be candidates for this list.
NB.
Where there is more than one symbol after the object, the first one is usually the oldest version, and the last one, is
the latest, more common, or usually accepted one.
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Modern Constellations
Constellation
Sign Pronunciation
Abbrv Meaning
Origin
Andromeda
an-DROM-eh-da
And
The Chained Maiden
ancient (Ptolemy)
Antlia
ANT-lee-uh
Ant
The Air Pump
1763, Lacaille
Apus
APE-us
Aps
The Bird of Paradise
1603, Uranometria, Keyser & de Houtman
ack-KWAIR-ee-us
Aqr
The Waterbearer
ancient (Ptolemy)
Aquila
ack-WILL-lah
Aql
The Eagle
ancient (Ptolemy)
Ara
AY-rah
Ara
The Altar
ancient (Ptolemy)
AIR-ease
Ari
The Ram
ancient (Ptolemy)
Auriga
or-EYE-gah
Aur
The Charioteer
ancient (Ptolemy)
Bootes
bow-OH-tease
Boo
The Herdsman
ancient (Ptolemy)
Caelum
SEE-lum
Cae
The Chisel
1763, Lacaille
Camelopardalis
ca-MEL-oh-PAR-dal-iss
Cam
The Giraffe
1613, Plancius[5]
KAN-surr
Cnc
The Crab
ancient (Ptolemy)
Canes Venatici
KAN-es veh-NAT-ih-see
CVn
The Hunting Dogs
1690, Firmamentum Sobiescianum, Hevelius
Canis Major
KANE-es MAY-jer
Cma
The Great Dog
ancient (Ptolemy)
Canis Minor
KANE-es MY-ner
Cmi
The Little Dog
ancient (Ptolemy)
CAP-rih-CORN-us
Cap
The Sea Goat
ancient (Ptolemy)
Carina
car-EE-na
Car
The Keel
1763, Lacaille, split from Argo Navis
Cassiopeia
KASS-ee-oh-PEE-ah
Cas
The Queen
ancient (Ptolemy)
Centaurus
sen-TOR-us
Cen
The Centaur
ancient (Ptolemy)
Cepheus
SEE-fee-us
Cep
The King
ancient (Ptolemy)
Cetus
SEE-tus
Cet
The Sea Monster
ancient (Ptolemy)
Chameleon
kah-ME-lee-un
Cha
The Chameleon
1603, Uranometria, Keyser & de Houtman
Circinus
SIR-sin-us
Cir
The Drawing Compass
1763, Lacaille
Columba
ko-LUM-ba
Col
The Dove
1592, Plancius, split from Canis Major
Coma Berenices
CO-ma bare-uh-NYE-sees
Com
Berenice’s Hair
1603, Uranometria, split from Leo
Corona Australis
coe-ROW--nah ow-STRAHL-iss
CrA
The Southern Crown
ancient (Ptolemy)
Corona Borealis
coe-ROW--nah BOR-ee-AL-iss
CrB
The Northern Crown
ancient (Ptolemy)
Corvus
CORE-vuss
Crv
The Raven
ancient (Ptolemy)
Aquarius
Aries
Cancer
Capricornus
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Crater
CRAY-ter
Crt
The Cup
ancient (Ptolemy)
Crux
Kruks
Cru
The Southern Cross
1603, Uranometria, split from Centaurus
Cygnus
SIG-nus
Cyg
The Swan
ancient (Ptolemy)
Delphinus
del-FYE-nus
Del
The Dolphin
ancient (Ptolemy)
Dorado
doh-RAY-doh
Dor
The Goldfish
1603, Uranometria, Keyser & de Houtman
Draco
DRAY-ko
Dra
The Dragon
ancient (Ptolemy)
Equuleus
eh-KWOO-lee-us
Equ
The Little Horse
ancient (Ptolemy)
Eridanus
eh-RID-uh-nuss
Eri
The Celestial River
ancient (Ptolemy)
Fornax
FOR-naks
For
The Furnace
1763, Lacaille
GEM-in-eye
Gem
The Twins
ancient (Ptolemy)
Grus
GROOS
Gru
The Crane
1603, Uranometria, Keyser & de Houtman
Hercules
HER-kyou-leez
Her
The Strongman
ancient (Ptolemy)
Horologium
hor-uh-LOW-gee-um
Hor
The Clock
1763, Lacaille
Hydra
HY-druh
Hya
The Sea Serpent
ancient (Ptolemy)
Hydrus
HY-drus
Hyi
The Water Snake
1603, Uranometria, Keyser & de Houtman
Indus
IN-dus
Ind
The Indian
1603, Uranometria, Keyser & de Houtman
Lacerta
la-SIR-ta
Lac
The Lizard
1690, Firmamentum Sobiescianum, Hevelius
LEE-oh
Leo
The Lion
ancient (Ptolemy)
Leo Minor
LEE-oh MY-ner
Lmi
The Little Lion
1690, Firmamentum Sobiescianum, Hevelius
Lepus
LEE-puss
Lep
The Hare
ancient (Ptolemy)
LEE-bra
Lib
The Scales
ancient (Ptolemy)
Lupus (ex Therion)
LOUP-us
Lup
The Wolf
ancient (Ptolemy)
Lynx
links
Lyn
The Lynx
1690, Firmamentum Sobiescianum, Hevelius
Lyra
LIE-rah
Lyr
The Lyre
ancient (Ptolemy)
Mensa
MEN-sa
Men
The Table Mountain
1763, Lacaille
Microscopium
MY-krow-SKOH-pee-em
Mic
The Microscope
1763, Lacaille
Monoceros
mon-OSS-er-us
Mon
The Unicorn
1613, Plancius
Musca
MUSS-ka
Mus
The Fly
1603, Uranometria, Keyser & de Houtman
Norma
NOR-ma
Nor
The Level
1763, Lacaille
Octans
OCK-tens
Oct
The Octant
1763, Lacaille
Ophiuchus
Oaf-ih-YOU-kus
Oph
The Serpent Bearer
ancient (Ptolemy)
Orion
oh-RYE-un
Ori
The Hunter
ancient (Ptolemy)
Gemini
Leo
Libra
4 October 2012
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Pavo
PAY-vo
Pav
The Peacock
1603, Uranometria, Keyser & de Houtman
Pegasus
PEG-uh-suss
Peg
The Winged Horse
ancient (Ptolemy)
Perseus
PURR-see-u
Per
The Hero
ancient (Ptolemy)
Phoenix
FEE-nix
Phe
The Phoenix
1603, Uranometria, Keyser & de Houtman
Pictor
PICK-tor
Pic
The Painter
1763, Lacaille
PIE-sees
Psc
The Fish
ancient (Ptolemy)
Piscis Austrinus
PIE-sees oss-TREE-nus
PsA
The Southern Fish
ancient (Ptolemy)
Puppis
PUP-iss
Pup
The Poop Deck
1763, Lacaille, split from Argo Navis
Pyxis
PICK-sis
Pyx
The Compass
1763, Lacaille
Reticulum
reh-TICK-yuh-lum
Ret
The Net
1763, Lacaille
Sagitta
suh-JIT-uh
Sge
The Arrow
ancient (Ptolemy)
sa-jih-TARE-ee-us
SKOR-pee-uss
Sgr
Sco
The Archer
The Scorpion
ancient (Ptolemy)
ancient (Ptolemy)
Sculptor
SKULP-tor
Scl
The Sculptor
1763, Lacaille
Scutum
SCOOT-um
Sct
The Shield
1690, Firmamentum Sobiescianum, Hevelius
Serpens Caput
SIR-pens CAP-ut
Ser
The Snake’s Head
ancient (Ptolemy)
Serpens Cauda
SIR-pens KAW-dah
Ser
The Snake’s Tail
ancient (Ptolemy)
Sextans
SEX-tens
Sex
The Sextant
1690, Firmamentum Sobiescianum, Hevelius
TOR-us
Tau
The Bull
ancient (Ptolemy)
Telescopium
tell-es-SCOPE-ee-um
Tel
The Telescope
1763, Lacaille
Triangulum
tri-ANG-yuh-lum
Tri
The Triangle
ancient (Ptolemy)
Triangulum Australe
tri-ANG-yuh-lum aus-TRAY-lee
TrA
The Southern Triangle
1603 Uranometria, Keyser & de Houtman
Tucana
too-KAY-nah
Tuc
The Toucan
1603 Uranometria, Keyser & de Houtman
Ursa Major
URR-sah MAY-jer
UMa
The Great Bear
ancient (Ptolemy)
Ursa Minor
URR-sah MY-ner
UMi
The Little Bear
ancient (Ptolemy)
Vela
VEE-la
Vel
The Sail
1763, Lacaille, split from Argo Navis
VER-go
Vir
The Maiden
ancient (Ptolemy)
Volans
VO-lans
Vol
The Flying Fish
1603, Uranometria, Keyser & de Houtman
Vulpecula
vul-PECK-yoo-la
Vul
The Fox
1690, Firmamentum Sobiescianum, Hevelius
Pisces
Sagittarius
Scorpius
Taurus
Virgo
4 October 2012
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The Greek Alphabet
4 October 2012
Lower
Case
Name
Upper
Case
α
β
γ
δ
ε
ζ
η
ϑ
ι
κ
λ
μ
ν
ξ
ο
π
ρ
σ
τ
υ
ϕ
χ
ψ
ω
alpha
beta
gamma
delta
epsilon
zeta
eta
theta
iota
kappa
lambda
mu
nu
xi
omicron
pi
rho
sigma
tau
upsilon
phi
chi
psi
omega
Α
Β
Γ
Δ
Ε
Ζ
Η
Θ
Ι
Κ
Λ
Μ
Ν
Ξ
Ο
Π
Ρ
Σ
Τ
Υ
Φ
Χ
Ψ
Ω
170
© Paul Thomas
Meteor Showers
Name
Dates
Antihelion Source
Quadrantids
Daytime Capri.-Sagitt.
Alpha Centaurids
Theta Centaurids
Gamma Normids
Kappa Serpentids
Virginids
Lyrids
Pi Puppids
Eta Aquariids
Omega Cetids
Eta Lyrids
Daytime May Arietids
Daytime Eps. Arietids
Alpha Scorpiids
South Omega Scorpiids
North Omega Scorpiids
Arietids
Daytime Zeta Perseids
June Lyrids[1]
South June Aquilids
Pi Cetids
June Bootids
Daytime Beta Taurids
Tau Aquariids
July Phoenicids
North Delta Aquariids
Piscis Austrinids
10 Dec - 10 Sep
28 Dec – 12 Jan
13 Jan - 29 Feb
28 Jan – 21 Feb
23 Jan – 12 Mar
25 Feb – 22 Mar
1 Apr - 7 Apr
22 Mar – 26 Apr
16 Apr – 25 Apr
15 Apr – 28 Apr
19 Apr - 28 May
5 May - 9 Jun
3 May - 14 May
4 May - 6 Jun
4 May - 6 Jun
21 Apr – 26 May
23 May – 15 Jun
23 May - 15 Jun
22 May – 2 Jul
20 May - 5 Jul
11 Jun – 21 Jun
9 Jun - 2 Jul
16 Jun – 4 Jul
22 Jun – 2 Jul
5 Jun - 17 Jul
27 Jun – 6 Jul
24 Jun – 18 Jul
15 Jul - 25 Aug
15 Jul – 10 Aug
4 October 2012
Speed
(km/s)
multiple varies varies 30
4 Jan
15
+49
41
2 Feb
21
-23
29
8 Feb
14
-59
56
10 Feb
15
-44
66
14 Mar
16
-50
56
5 Apr
15
+18
46
17 Apr
12
+10
20
22 Apr
18
+34
49
23 Apr
7
-45
18
5 May
23
-1
66
7 May
1
-7
38
8 May
19
+44
43
15 May
3
+18
28
15 May
3
+21
23
15 May
16
-29
33
31 May
17
-22
26
31 May
17
-15
23
7 Jun
3
+24
37
9 Jun
4
+28
27
16 Jun
19
+44
20
16 Jun
19
-5
39
26 Jun
2
-12
68
27 Jun
15
+48
18
28 Jun
6
+24
31
28 Jun
23
-12
66
12 Jul
2
-48
48
26 Jul
23
0
42
27 Jul
23
-30
35
Peak
RA
Dec
ZHR
4
120
7
6
3
6
4
5
18
var
65
8
3
4
4
3
5
5
54
20
3
3
4
var
10
7
4
4
5
171
Parent Object
Comet C/1490Y1 & C/1385U1
Notes
Faint.
Blue & yellow, med speed. Radiant in Bootes.
Bright.
Bright.
Comet Thatcher
Swift, bright meteors.
Comet 26P/Grigg-Skjellerup Bright. Every 5 years.
Comet 1P/Halley
Bright.
Faint.
Daylight showers. Try about 5am.
Daylight showers. Try about 5am.
Low activity from several radiants.
Comet 96P/Machholz
Comet 7P/Pons-Winnecke
Bright. Unpredictable. Periodic.
Comet 96P/Machholz
Faint.
© Paul Thomas
Southern Delta Aquariids
Beta Cassiopeids
Alpha Capricornids
Eta Eridanids
Perseids
Kappa Cygnids
Gamma Doradids
Alpha Aurigids
September Epsilon Perseids
Daytime Sextantids
Draconids
Southern Taurids
Delta Aurigids
Epsilon Geminids
Orionids
Leo Minorids
Northern Taurids
Nov. Iota Aurigids
Leonids
Alpha Monocerotids
Phoenicids
Puppid-Velids
Monocerotids
Sigma Hydrids
Geminids
Comae Berenicids
Dec. Leonis Minorids
Ursids
12 Jul – 23 Aug
3 Jul - 19 Aug
3 Jul – 5 Aug
3 Aug – 14 Aug
17 Jul – 24 Aug
3 Aug – 25 Aug
27 Aug – 3 Sep
28 Aug – 5 Sep
5 Sep – 21 Sep
26 Sep - 3 Oct
6 Oct – 10 Oct
10 Sep – 20 Nov
10 Oct – 18 Oct
14 Oct – 27 Oct
2 Oct – 7 Nov
19 Oct – 27 Oct
20 Oct – 10 Dec
1 Nov – 23 Nov
6 Nov – 30 Nov
15 Nov – 25 Nov
28 Nov – 9 Dec
1 Dec – 15 Dec
27 Nov – 17 Dec
3 Dec – 15 Dec
7 Dec – 17 Dec
12 Dec – 23 Dec
5 Dec - 4 Feb
17 Dec – 26 Dec
29 Jul
29 Jul
29 Jul
9 Aug
12 Aug
17 Aug
28 Aug
31 Aug
9 Sep
1 Oct
8 Oct
10 Oct
11 Oct
18 Oct
21 Oct
24 Oct
12 Nov
15 Nov
17 Nov
21 Nov
6 Dec
multiple
8 Dec
11 Dec
13 Dec
15 Dec
19 Dec
23 Dec
23
24
20
3
3
19
4
6
3
10
17
2
6
7
6
11
4
5
10
8
1
8
7
8
7
12
11
14
-16
59
-10
-13
+58
+59
-50
+39
+40
-2
+54
+9
+44
+27
+16
+37
+22
+33
+22
+1
-53
-45
+8
+2
+33
+18
+30
+76
41
52
23
65
59
25
42
66
64
33
20
27
64
70
66
62
29
36
71
65
18
40
42
58
35
65
64
33
16
10
5
6
100
3
5
6
5
20
var
5
2
3
25
2
5
8
15
var
var
10
2
3
120
3
5
10
Faint.
Comet 169P/NEAT
Slow, bright, yellow meteors. Fireballs seen.
Comet 109P/Swift-Tuttle
M Planet 2008 ED69
Bright.
Faint.
Comet C/1911N1 (Kiess)
Bright.
Faint.
Comet 21P/Giacobini-Zinner Medium. Very feeble shower. Periodic
Bright.
Faint.
Faint.
Comet 1P/Halley
Bright. Fast meteors with fine, persistent trains.
Faint.
M Planet 2004 TG10
Bright.
Comet 55P/Tempel-Tuttle Bright.
Unknown
Bright. Periodic
Comet D/1819W1 (Blanpain) Medium. Periodic
Medium.
Faint
Faint.
M Planet 3200 Phaethon
Rich shower of medium bright/speed meteors.
Faint.
Faint.
Comet 8P/Tuttle
Faint. Weak shower.
Dates will change slightly each year. Up to date info on the web !
Showers in Orange = Views from the Southern Hemisphere.
4 October 2012
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Significant People in Astronomy
Brahe, Tycho (1546-1601)
A Danish astronomer (also known as Tyge Ottesen) whose accurate astronomical observations of Mars in the last quarter of the
16th century formed the basis for Johannes Kepler’s laws of planetary motion. Brahe lost his nose in a dual at Rostock in 1566
with Manderup Parsberg (a fellow student and nobleman) over a dispute as to who was the better mathematician, and for the
rest of his life is said to have worn a metal replacement. In 1601, rather than commit the social faux pas of leaving a banquet
before it concluded, he stayed put until his bladder ruptured. He died several days later, not from his urinary mishap, however,
but rather from the high levels of mercury in his blood, a result of the medicine he took after falling ill.
Copernicus, Nicolaus (1473-1543)
A Polish astronomer who advanced the theory that Earth and the other planets revolve around the Sun—the heliocentric theory.
This was highly controversial at the time, since the prevailing Ptolemaic model held that the Earth was at the centre of the
universe, and all objects, including the Sun, circled it. The Ptolemaic model had been widely accepted in Europe for 1000 years
when Copernicus proposed his alternative. Although, it should be noted, that the heliocentric idea was first put forth by
Aristarcus of Samos in the third century B.C., a fact known to Copernicus but long ignored by others prior to him.
Einstein, Albert (1879-1955)
The German-American physicist who developed the Special and General Theories of Relativity that—along with quantum
mechanics—are the foundation of modern physics.
Galilei, Galileo (1564-1642)
An Italian scientist renowned for his contributions to physics, astronomy, and scientific philosophy. He is regarded as the chief
founder of modern science. He developed the telescope, with which he found craters on the Moon and discovered the largest
moons of Jupiter. Galileo was condemned by the Catholic Church for his view of the cosmos, which was based on the heliocentric
theory of Copernicus.
Halley, Edmond (1656–1742)
Halley was born in Haggerston, Shoreditch, England. His father, Edmond Halley Sr., came from a Derbyshire family and was a
wealthy soap-maker in London. He studied at St Paul's School, and then, from 1673, at The Queen's College, Oxford. While an
undergraduate, Halley published papers on the Solar System and sunspots. Halley stated his belief that the comet sightings of
1456, 1531, 1607, and 1682 related to the same comet, which he predicted would return in 1758. He did not live to witness the
4 October 2012
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© Paul Thomas
comet's return, but when it did, the comet became generally known as Halley's Comet. Halley held many positions of authority,
and finally succeeded John Flamsteed in 1720 as Astronomer Royal, a position Halley held until his death.
Herschel, William (1738-1822)
Sir William Herschel was a renowned astronomer who first detected the infrared region of the electromagnetic spectrum in 1800.
Hertz, Heinrich (1857-1894)
A German physics professor who performed the first experiments that generated and received electromagnetic waves, in
particular radio waves. In his honour, the unit with which we measure the frequency of these waves is called a hertz.
Hubble, Edwin P. (1889-1953)
An American astronomer whose observations proved that galaxies are what he called “island universes” outside our galaxy and
not nebulae within our galaxy. His greatest discovery, called Hubble’s Law, was the linear relationship between a galaxy’s
distance and the speed with which it is moving. The Hubble Space Telescope is named in his honour.
Huygens, Christiaan (1629-1695)
A Dutch physicist who was the leading proponent of the wave theory of light. He also made important contributions to
mechanics, stating that in a collision between bodies, neither loses nor gains “motion” (his term for momentum). In astronomy,
he discovered Titan (Saturn’s largest moon) and was the first to correctly identify the observed elongation of Saturn as the
presence of Saturn’s rings.
Kepler, Johannes (1571-1630)
A German astronomer and mathematician. Considered a founder of modern astronomy, he formulated three famous laws of
planetary motion that comprised a quantitative formulation of Copernicus’s theory that the planets revolve around the Sun.
Messier, Charles (1730-1817)
The 18th-century French astronomer who compiled a list of 110 fuzzy, diffuse objects that appeared at fixed positions in the sky.
Being a comet-hunter, Messier compiled this list of objects which he knew were not comets. His list is now well known to
professional and amateur astronomers as containing the brightest and most striking nebulae, star clusters, and galaxies in the
sky.
Moore, Patrick (1923 - )
Sir Patrick Alfred Caldwell-Moore, CBE, FRS, FRAS is a British amateur astronomer who has attained prominent status in
astronomy as a writer, researcher, radio commentator and television presenter. He is a former president of the British
Astronomical Association, co-founder and former president of the Society for Popular Astronomy (SPA), author of over 70 books
on astronomy, and presenter of the world's longest-running television series with the same original presenter, The Sky at Night
4 October 2012
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© Paul Thomas
on the BBC. As an amateur astronomer, he became known as a specialist on observing the Moon and creating the Caldwell
catalogue. In 1982, asteroid 2602 Moore, was named in his honour.
Newton, Isaac (1642-1727)
The English cleric and scientist who discovered the classical laws of motion and gravity. The bit with the apple is probably
apocryphal.
Ptolemy (circa 100 to circa 170 AD)
Also known as Claudius Ptolemaeus. Ptolemy believed the planets and the Sun orbited the Earth in the order Mercury, Venus,
the Sun, Mars, Jupiter, Saturn. This system became known as the Ptolemaic system and predicted the positions of the planets
accurately enough for naked-eye observations (although it also made some ridiculous predictions, such as the distance to the
moon should vary by a factor of two over its orbit). He authored a book called Mathematical Syntaxis (widely known as the
Almagest). The Almagest included a star catalogue containing 48 constellations, using names we still use today.
4 October 2012
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SI Units and Specifics
Prefix
Googolplex
Googol
--- Universe
Yotta
Zetta
Exa
--- LightYear
Peta
Tera
Giga
Mega
kilo
hecto
deca
deci
centi
milli
micro
nano
pico
femto
atto
zepto
yocto
--- Plank Length
googolth
Symbol 10n
Y
Z
E
ly
P
T
G
M
k
h
da
d
c
m
μ
n
p
f
a
z
y
lp
-
googol
10
10100
9.3x1026
1024
1021
1018
9.46x1015
1015
1012
109
106
103
102
101
100
10−1
10−2
10−3
10−6
10−9
10−12
10−15
10−18
10−21
10−24
1.6162x1035 -100
10
Decimal
Short scale
10 100
10
1 with 100 zeros
930,000,000,000,000,000,000,000,000
1,000,000,000,000,000,000,000,000
1,000,000,000,000,000,000,000
1,000,000,000,000,000,000
9,460,000,000,000,000
1,000,000,000,000,000
1,000,000,000,000
1,000,000,000
1,000,000
1,000
100
10
1
0.1
0.01
0.001
0.000,001
0.000,000,001
0.000,000,000,001
0.000,000,000,000,001
0.000,000,000,000,000,001
0.000,000,000,000,000,000,001
0.000,000,000,000,000,000,000,001
0.000,000,000,000,000,000,000,000,000,000,000,016
0. with 99 zeros then a 1
Binary prefixes have an i after the Symbol (ie. Mi, Gi, Ti, etc.)
4 October 2012
Googolplex
Ten Duotrigintillion Ten Sexdecilliard
The probable size of the universe.
Septillion
Quadrillion
Sextillion
Trilliard
Quintillion
Trillion
One light year.
Quadrillion
Billiard
Trillion
Billion
Billion
Milliard
Million
Thousand
Hundred
Ten
One
Tenth
Hundredth
Thousandth
Millionth
Billionth
Milliardth
Trillionth
Billionth
Quadrillionth
Billiardth
Quintillionth
Trillionth
Sextillionth
Trilliardth
Septillionth
Quadrillionth
Nothing physically shorter than this.
USA, UK Modern
176
Long scale
UK Archaic, EU,
India, RotW
© Paul Thomas
Unofficial Units and Specifics
Prefix
luma
minga
nena
ocha
pepta
quexa or ultra
rinta or quinsa
sorta or cata
treda or astra
uda or vela
vunda or vendeko or besa
weka or weko or sansa
xona or xenno or hella
xonto or xenna or tiso
wekto or weka or vindo
vunkto or vendeka
unto
trekto
sotro
rimto
quekto
pekro
otro
nekton
mikto
lunto
Symbol 10n
Decimal
Short Scale
10
1060
1057
1054
1051
1048
1045
1042
1039
1036
1033
1030
1027
1, 000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000
1, 000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000
1, 000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000
1, 000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000
1, 000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000
1, 000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000
1, 000,000,000,000,000,000,000,000,000,000,000,000,000,000,000
1, 000,000,000,000,000,000,000,000,000,000,000,000,000,000
1, 000,000,000,000,000,000,000,000,000,000,000,000,000
1, 000,000,000,000,000,000,000,000,000,000,000,000
1, 000,000,000,000,000,000,000,000,000,000,000
1,000,000,000,000,000,000,000,000,000,000
1,000,000,000,000,000,000,000,000,000
quindecillion
quattuordecillion
tredecillion
duodecillion
undecillion
decillion
nonillion
octillion
10-27
10-30
10-33
10-36
10-39
10-42
10-45
10-48
10-51
10-54
10-57
10-60
10-63
0.000,000,000,000,000,000,000,000,001
0.000,000,000,000,000,000,000,000,000,001
0.000,000,000,000,000,000,000,000,000,000,001
0.000,000,000,000,000,000,000,000,000,000,000,001
0.000,000,000,000,000,000,000,000,000,000,000,000,001
0.000,000,000,000,000,000,000,000,000,000,000,000,000,001
0.000,000,000,000,000,000,000,000,000,000,000,000,000,000,001
0.000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,001
0.000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,001
0.000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,001
0.000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,001
0.000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,001
0.000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,001
63
U or ut
Q or qu
C or ca
A or at
V or ve
Bor be
B or s
Xor n
x or t
w or v
v
octillionth
nonillionth
decillionth
undecillionth
duodecillionth
tredecillionth
quattuordecillionth
quindecillionth
Note:
Prefixes in normal text are in use and have been proposed to the Consultative Committee on Units. The prefixes in italics, are in
use, but have not been proposed.
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Formulas
Telescope Formulae
Aperture
D = F/f
Where:
D is the aperture of the objective
F is the focal length of the objective
f is the f-number (f/) of the objective
Magnification: By Fields
M = Alpha/Theta
Where:
M is the magnification
Alpha is the apparent field
Theta is the true field
Apparent Field: the closest separation eye can see is 4', more practically 8-25', 1-2' for good eyes. The Zeta Ursae Majoris
double (Mizar/Alcor) is 11.75'; Epsilon Lyrae is 3'.
True Field (in °) = 0.25 * time * cos of the declination (in ') = 15 * time * cos of the declination where time is the time to
cross the ocular field in minutes. A star therefore moves westward at the following rates:
15° /h (1.25°/5 min) at 0° declination
13° /h (1.08°/5 min) at 30° declination
7.5°/h (0.63°/5 min) at 60° declination.
Magnification: By Focal Lengths
M = F/f
Where:
M is the magnification
F is the focal length of the objective
f is the focal length of the ocular
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At prime focus (ground glass), magnification is 1x for each 25 mm of F
Magnification: By Diameter And Exit Pupil
M = D/d
Where:
M is the magnification
D is the diameter of the objective
d is the exit pupil (5-6 mm is best; 7 mm may not produce a sharp outer image)
The scotopic (dark-adapted) aperture of the human pupil is typically 6 (theoretically 7, 5 if over age 50) mm. Since the human
pupil has a focal length of 17 mm, it is f/2.4 and yields 0.17 per mm of aperture. 2.5 mm is the photopic (light-adapted)
diameter of the eye.
Exit Pupil
d = f/f-number
(by substituting F/f for M) where:
d is the exit pupil
f is the focal length of the ocular
f-number is the f-number (f/) of the objective
By substituting d=7 (the scotopic aperture of the human pupil) and multiplying it by the f-number, the longest useful focal length
of the ocular is given.
Low-Power Law For Limiting Magnification
M = D/6 = 17*D
(by substituting 6 mm for d and taking the reciprocal) where:
M is the minimum magnification without wasting light for a dark-adapted eye (17x per mm of aperture)
D is the diameter of the objective in mm
High-Power Law For Limiting Magnification
M = D/0.63 = 158*D
(by substituting 0.63 mm, the minimum diameter to which the average pupil can contract, for d and taking the reciprocal)
where:
M is the maximum theoretical magnification (158x per mm of aperture); the maximum practical magnification is +50%).
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Limiting Visual Magnitude (Light-Gathering Power)
m = 6.5-5 log Delta+5 log D
= 2.7+5 log D
(assuming transparent dark-sky conditions and magnification >= 1D in mm) where:
m is the approximate limiting visual magnitude
Delta is the pupillary diameter in mm (accepted as 7.5)
D is the diameter of the objective in mm
Relative Light Efficiency (Twilight Factor)
Relative Brightness Value = d2 = (D/M)2
where the larger the relative brightness value, the better the instrument (e.g., binoculars) is for viewing in twilight or for
astronomical use at dusk (low light conditions only):
d is the diameter of the exit pupil
D is the diameter of the objective
M is the magnification
Angular Radius Of Airy (Diffraction) Disc
r = (1.12*Lambda*206265)/D
= 127.1/D
(the second formula is based on Lambda = 0.00055 for yellow) where:
r is the angular radius (one-half the angular diameter) of the Airy disc (irreducible min. size of a star disc in ")
Lambda is the wavelength of the light in mm
206265 is the number of " in a radian
D is the diameter of the objective in mm
Linear Radius Of Airy (Diffraction) Disc
r = 0.043*Lambda*f
Where:
r is the linear radius (one-half the linear diameter) of the Airy disc in mm
Lambda is the wavelength of light in mm (yellow 0.00055)
f is the f-number (f/) of the objective
Depth Of Field (Photovisual)
c = (Distance*Lambda)/D2
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Where:
c is the depth of field in mm
Distance is that to the objective from the object viewed or imaged (in mm)
Lambda is the wavelength of light in mm (yellow light 0.00055 for peak of human visual sensitivity)
D is the diameter of the objective in mm
Dawes Limit (Smallest Resolvable Angle, Resolving Power)
Theta = 115.8/D
Where:
Theta is the smallest resolvable angle in "
D is the diameter of the objective in mm
Atmospheric conditions seldom permit Theta < 0.5". The Dawes Limit is one-half the angular diameter of the Airy (diffraction)
disc, so that the edge of one disc does not extend beyond the centre of the other). The working value is two times the Dawes
Limit (diameter of the Airy disc), so that the edges of the two stars are just touching.
Magnification Needed To Split A Double Star
M = 480/d
Where:
M is the magnification required
480 is number of seconds of arc for an apparent field of 8 minutes of arc
d is the angular separation of the double star
About the closest star separation that the eye can distinguish is 4 minutes of arc (240 seconds of arc). Twice this distance, or an
8-minute (480- second) apparent field angle, is a more practical value for comfortable viewing. In cases where the comes is
more than five magnitudes fainter than the primary, you will need a wider separation: 20 or 25 minutes of arc, nearly the width
of the moon seen with the naked eye.
Resolution Of Lunar Features
Resolution = (2*Dawes Limit*3476)/1800)
Dawes Limit * 38.8
Where:
Resolution is the smallest resolvable lunar feature in km
2*Dawes Limit is the Airy disc (a more practical working value is twice this)
1800 is the angular size of the moon in "
3476 is the diameter of the moon in km
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Light Grasp
Light Grasp = (D/d)2*pi
= 7*D2
Where:
Light Grasp is times that received by the retina
D is the diameter of the objective in mm
d is the diameter of the eye's pupillary aperture in mm (accepted value 7.5)
pi is the transmission factor (approximately equal to 62.5% for the average telescope, up to approximately 180 mm)
To compare the relative light grasp of two main lenses used at the same magnification, compare the squares of their diameters
*------------------------------------------*
Resolving Power (in arcseconds)
Pr = 120/Do
Magnification
M = fo/fe or
M = Do/Dep
Focal Ratio
fr = Fo/Do
Scopes Field of View
FOVs = FOVe/M
Diameter of Exit Pupil
Dep = Do/M or
Dep = fe/fr
Focal Length Of Scope
fo = Do * fr
Focal Length Of Eyepiece
fe = Dep * fr
Gain In Visible Star Magnitudes
Gmag = 5 log(Do/7)
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Where:
Dep = Diameter of the exit pupil
Do = Diameter of the Objective
fe = Focal length of the eyepiece
fo = Focal length of the objective
fr = f-Ratio
FOVe = Field of view of the eyepiece
FOVs = Field of view of the scope
Gmag = Gain in visible star magnitudes
M = Magnification
Pr = Resolving power
*------------------------------------------*
Astrophotography Formulae
F-Number: Prime Focus (Erect Image)
f/ = F/D
Where:
f/ is the f-number of the system (objective)
F is the focal length of the objective
D is the diameter of the objective
F-Number: Afocal, Eyepiece-Camera Lens (Reversed Image)
f/ = F minutes/D
= (M*Fc)/D
= ((F/Fe)*Fc)/D
= (F/D)*(Fc/Fe)
= (M/D)*Fc
Where:
f/ is the f-number of the system
F' is the effective focal length of the system
Fe is the focal length of the ocular (divided by Barlow magnification)
D is the diameter of the objective
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M is the magnification
Fc is the focal length of the camera
F is the focal length of the objective
Fc/Fe is the projection magnification
M/D is the power per mm
The diameter of the first image equals the film diagonal (44 mm for 35 mm film) divided by the magnification
F-Number: Eyepiece Projection, Positive Lens (Reversed Image)
f/ = F minutes/D
= (F/D)*(B/A)
= (F/D)*(((M+1)*Fe)/A)
= (F/D)*((B/Fe)-1)
Where:
f/ is the f-number of the system
F' is the effective focal length of the system
D is the diameter of the objective
F is the focal length of the objective (times any Barlow magnification)
B is the secondary image ("throw"), the distance of the ocular centre from the focal plane of the film, equal to ((M+1)*Fe)/A
A is the primary image, the distance of the ocular centre from the focal point of the telescope objective
M is the projection magnification, equal to (B/Fe)-1
Fe is the focal length of the ocular
F-Number: Negative Lens Projection (Erect Image)
f/ = F minutes/D = (F/D) * (B/A)
Where:
f/ is the f-number of the system
F' is the effective focal length of the system
D is the diameter of the objective
B is the distance of the Barlow centre from the focal plane of the film
A is the distance of the Barlow centre from the focal point of the telescope objective
B/A is the projection magnification (Barlow mag.)
Exposure Comparison For Extended Objects
Exposure Compensation = (f/S)2/(f/E)2 = ((f/S)/(f/E))2
(the ratio of intensities of illumination is squared according to the inverse square law)
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Where:
Exposure Compensation is the exposure compensation to be made to the example system
f/S is the f-number (f/) of the subject system
f/E is the f-number (f/) of the example system
Exposure Comparison For Point Sources
Exposure Compensation = De2/Ds2 = (De/Ds)2
Where:
Exposure Compensation is the exposure compensation to be made to the example system
De is the objective diameter of the example system
Ds is the objective diameter of the subject system
Light-Recording Power Of A System
Power = r2/f2
(the light-recording power is directly proportional to the square of the radius of the objective and inversely proportional to the
square of the f-number)
Where:
Power is the light-recording power of the system
r is the radius of the objective
f is the f-number (f/) of the system
Example: a 200-mm f/8 system compared with a 100-mm f/5 system
(1002)/82 compared with (502)/52
156.25 compared with 100, or 1.56 times more light-recording power
Efficiency Of Lens For Photographing An Average Meteor
Efficiency = F/f2
Where:
Efficiency is the efficiency of the lens for photographing an average meteor (in a meteor shower)
F is the focal length of the lens
f is the f-number (f/) of the lens
Print's Effective Focal Length
Print EFL = Camera FL * Print Enlargement
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Where:
Print EFL is the print's effective focal length
Camera FL. is the camera's focal length
Print Enlargement is the amount of enlargement of the print (3x is the standard for 35-mm film)
Guidescope Magnification
Guidescope M ~ f/12.5
Where:
Guidescope M is the magnification needed
f is the photographic focal length in mm
Experience indicates that the minimum guiding magnification needed is about f divided by 12.5, precisely what a 12.5 mm
guiding ocular used in an off- axis guider for prime-focus photography yields. (Since visual magnification is the ratio of the
objective to ocular focal length, the combination of prime-focus camera and off-axis guider with a 12.5-mm ocular gives a
guiding magnification of f/12.5. f/7.5 (as with a typical focal reducer that reduces the effective focal length by a factor of 0.6) is
a significant improvement. f/5 or higher magnification is for top-quality guiding.
Guidescope M = Guidescope EFL / Print EFL
Where:
Guidescope M is the guidescope's magnification (should be >= 1, preferably 5-8)
Guidescope EFL is the guidescope's effective focal length, the guidescope's focal length times any Barlow magnification (should
be >= to the focal length of the primary and the guidescope's magnification, 0.2x per mm of focal length of the objective, 0.1x
per mm of the camera lens
Print EFL is the print's effective focal length
Guiding Tolerance
Guiding Tolerance = 0.076 * Guidescope M
Where:
Guiding Tolerance is in mm
0.076 is one " at a 254-mm reading distance from the print (a crosshair is usually 0.05 mm)
Maximum Allowable Tracking (Slop) Error
S ~ 8250/(F*E)
Where:
S is the error ("slop") in "
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F is the focal length in mm
E is the amount of enlargement of the print (3x is the standard for 35-mm film)
The slop is derived from the formula Theta = k*(h/F), with k = 206256 (the number of seconds in a radian) and h = 0.04 mm of
image-drift tolerance (an empirical value from astrophotographs).
Conversion Of Plate Scale To Effective Focal Length
EFL = mm per degree * 57.3 = 206265/" per mm
Where:
EFL is the effective focal length in mm
57.3 is the number of degrees in a radian
206256 is the number of " in a radian
Resolving Power Of A Photographic System
Resolving Power = 4191"/F
Where:
Resolving Power is the resolving power of a photographic system with Kodak 103a or colour film
F is the focal length of the system in mm
Maximum Resolution For A Perfect Lens
Maximum Resolution = 1600/f
Where:
Maximum Resolution is the resolution for a perfect lens
f is the f-number (f/) of the lens
Most films, even fast ones, resolve only 60 lines/mm; the human eye resolves 6 lines/mm (less gives a "woolly" appearance).
80 lines/mm for a 50-mm lens is rated excellent (equal to 1 min of arc); a 200-mm lens is rated excellent with 40 lines/mm.
2415 films yields 320 line pairs (160 lines)/mm (equal to 1 second of arc); Tri-X yields 80 lines/mm.
Minimum Resolution Necessary For Film
Minimum Resolution = Maximum Resolution * Print Enlargement
Where:
Minimum Resolution is the min resolution necessary
Maximum Resolution is the max resolution for a perfect lens
Print Enlargement is the amount of enlargement of the print (3x is the standard for 35-mm film)
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Size Of Image (Angular)
h = (Theta*F)/k
Theta = k*(h/F)
F = (k*h)/Theta
Where:
h is the linear height in mm of the image at prime focus of an objective or a telephoto lens
Theta is the object's angular height (angle of view) in units corresponding to k
F is the effective focal length (focal length times Barlow magnification) in mm
k is a constant with a value of 57.3 for Theta in degrees, 3438 in minutes of arc, 206265 for seconds of arc (the number of the
respective units in a radian)
The first formula yields image size of the sun and moon as approximately 1% of the effective focal length (Theta/k = 0.5/57.3 =
0.009).
The second formula can be used to find the angle of view (Theta) for a given film frame size (h) and lens focal length (F).
Example: the 24 mm height, 36 mm width, and 43 mm diagonal of 35-mm film yields an angle of view of 27 deg, 41 deg, and 49
deg for a 50-mm lens.
The third formula can be used to find the effective focal length (F) required for a given film frame size (h) and angle of view
(Theta).
Size Of Image (Linear)
i = (h/D)*F
h = (D*i)/F
D = (h*F)/i
F = (D*i)/h
Where:
i is the linear image size in mm of the image at prime focus of an objective or telephoto lens (for terrestrial objects, equal to 24
mm divided by the amount of enlargement of the print [3x is the standard for 35-mm film] for the smallest dimension of 35-mm
film])
h is the linear height of the object in units corresponding to D
D is the distance of the object in units corresponding to h
F is the effective focal length (focal length times Barlow magnification) in mm
The last formula gives the focal length necessary to photograph a recognizable celestial (Linear Width in km) or terrestrial (Linear
Width in m).
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Size Of Image (Celestial)
h = (Theta*F)/K
Theta = K*(h/F)
F = (K*h)/Theta
Where:
h is the linear height in mm of the image at prime focus of an objective or a telephoto lens
Theta is the object 's angular height (angle of view) in units corresponding to K
F is the effective focal length (focal length times Barlow magnification) in mm
K is a constant with a value of 57.3 for Theta in degrees, 3438 in minutes of arc, 206265 for seconds of arc (the number of the
respective units in a radian)
The first formula yields image size of the sun and moon as approximately 1% of the effective focal length (Theta/K = 0.5/57.3 =
0.009).
The second formula can be used to find the angle of view (Theta) for a given film frame size (h) and lens focal length (F).
Example: the 24 mm height, 36 mm width, and 43 mm diagonal of 35 mm film yields an angle of view of 27 degrees, 41
degrees, and 49 degrees for a 50-mm lens.
The third formula can determine effective focal length (F) required for a given film frame size (h) and angle of view (Theta).
Size Of Image (Terrestrial)
h = (Linear Width / Distance) * F
Linear Width = (Distance * h) / F
Distance = (Linear Width * F) / h
F = (Distance * h) / Linear Width
Where:
h is the linear height in mm of the image at prime focus of an objective or telephoto lens
Linear Width is the linear width of the object in m
Distance is the distance of the object in m
F is the effective focal length (focal length times Barlow magnification) in mm
Length Of A Star Trail On Film
Length = F*T*0.0044
Where:
Length is the length in mm of the star trail on film
F is the focal length of the lens in mm
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T is the exposure time in minutes
0.0044 derives from (2*Pi)/N for minutes (N = 1440 minutes per day)
Exposure Time For Star Trail On 35-Mm Film
T = 5455/F
Where:
T is the exposure time in minutes for a length of 24 mm (the smallest dimension of 35-mm film)
F is the focal length of the lens in mm
Maximum Exposure Time Without Star Trail
T = (1397/F)
Where:
T is the maximum exposure time in seconds without a star trail 1397 derives from 1' at reading distance (254 mm), the smallest
angular quantity that can be perceived by the human eye without optical aid ("limiting resolution") and is equal to < 0.1 mm.
This quantity also applies to the moon. 2-3x yields only a slight elongation. Use 20x for a clock drive.
F is the focal length of the lens in mm
The earth rotates 5' in 20 s, which yields a barely detectable star trail with an unguided 50-mm lens. 2-3' (8-12 s) is necessary
for an undetectable trail, 1' (4 s) for an expert exposure. Divide these values by the proportional increase in focal length over a
50-mm lens. For example, for 3' (12 s), a 150-mm lens would be 1/3 (1' and 4 s) and a 1000-mm lens would be 1/20 (0.15'
and 0.6 s). Note that to compensate for these values, the constant in the formula would be 1000 for a barely-detectable trail,
600 for an undetectable trail, and 200 for an expert
exposure.
N.B. The above formulae assume a declination of 0o. For other declinations, multiply lengths and divide exposure times by the
following cosines of the respective declination angles: 0.98 (10°), 0.93 (20°), 0.86 (30°), 0.75 (40°), 0.64 (50°), 0.50 (60°),
0.34 (70°), 0.18 (80°), 0.10 (85°).
Exposure Duration For Extended Objects
E = f2/(S*B)
Where:
e is the exposure duration in seconds for an image size of >= 0.1 mm
f is the f-number (f/) of the lens
S is the film's ISO speed
B is the brightness factor of the object (Venus 1000, Moon 125, Mars 30, Jupiter 5.7)
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Thus, a 2-minute exposure at f/1.4 is equivalent to a 32-minute exposure at f/5.6 (4 stops squared times 2 minutes), ignoring
the effects of reciprocity failure in the film, which would mean that the 32-minute exposure would have to be even longer.
Surface Brightness Of An Extended Object ("B" Value)
B = 100.4(9.5-M)/D2
Where:
B is the surface brightness of the (round) extended object
M is the magnitude of the object (total brightness of the object), linearized in the formula
D is the angular diameter of the object in seconds of arc (D2 is the surface area of the object)
Exposure Duration For Point Sources
e = (100.4(M+13))/S*a2
Where:
e is the exposure duration in seconds for an image size of >= 0.1 mm
M is the magnitude of the object
S if the film's ISO speed
a is the aperture of the objective
Focal Length Necessary To Photograph A Recognizable Object
F = (Distance / Linear Field) * Image Size
Where:
F is the focal length in mm necessary to photograph a recognizable object
Distance is the distance of the object in m
Linear Field is the linear field of the object in m
Image Size is the image size in mm (equal to 24 mm divided by the amount of enlargement of the print [3x is the standard for
35mm film] for the smallest dimension of 35mm film)
Miscellaneous Formulae
Hour Angle
H = Theta - Delta
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Where:
H is the hour angle
Theta is sidereal time
Delta is right ascension
The Hour Angle is negative east of and positive west of the meridian (as right ascension increases eastward).
Bode's Law
(4 + 3(2n))/10 in AU at aphelion
Where:
n is the serial order of the planets from the sun (Mercury's 2n =1, Venus's n = 0, Earth's n = 1, asteroid belt = 3)
Apparent Angular Size Of An Object
Theta = (h/D)*k
Where:
Theta is the object's apparent angular size in units corresponding to k
h is the linear height of the object in units corresponding to D
D is the distance of the object in units corresponding to h
Theta is the object's angular height (angle of view) in units corresponding to k
k is a constant with a value of 57.3 for Theta in degrees, 3438 in minutes of arc, 206265 for seconds of arc (the number of the
respective units in a radian)
A degree is the apparent size of an object whose distance is 57.3 times its diameter. The formula holds for celestial or terrestrial
objects. e.g., for the width of a quarter at arm's length: (57.3*25 mm)/700 mm = 2°.
Under ideal conditions, the human eye can resolve anything subtending more than a 1' angle, i.e., see an object as an extended
object or see a double star as two stars rather than a single point of light, provided that the two components are of nearly equal
brightness. A more practical value would be 4'; 8' is an even more practical value for comfortable viewing. The best earthbound
telescopes are usually limited by atmospheric effects to objects 1" or larger (0.25" with excellent seeing) in apparent size (before
magnification). In theory, a telescope could see everything with a magnification of 60x (1" magnified to 1').
Angular Size
Theta = (55*h)/d
Where:
Theta is the angular size of the object in degrees
h is the linear size of the object in m
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d is the distance from the eye in m
e.g., for the width of a quarter at arm 's length: (55*0.254)/0.711 = 2 degrees
Twilight Performance
Twilight Performance =
(D * M)
Where the greater the Twilight Performance is, the better suited the instrument (e.g., binoculars) will be for viewing in twilight or
for astronomical use after dusk (low light conditions only). This is only valid when comparing optics of similar quality; consider
for example how a 5.0 litre Ferrari engine will perform differently than a Ford 5.0 litre engine:
= Square Root
D is the diameter of the objective
M is the magnification
Relative Light Efficiency
Relative Brightness Value = d2 = (D/M)2
Where the larger the relative brightness value, the better the instrument (e.g., binoculars) is for viewing in twilight or for
astronomical use after dusk (low light conditions only). Again this is only valid when comparing optics of similar quality:
d is the diameter of the exit pupil
D is the diameter of the objective
M is the magnification
Length Of A Meteor Trail
h = (Theta*D)/57.3
Where:
h is the linear height of the meteor in km
Theta is the object's apparent angular size in degrees
D is the distance of the object in km
Measuring Angles With A Micrometre Reticle
To calibrate a micrometre’s linear scale
LS = 206265/F
Where:
LS is the Linear Scale division spacing) in seconds of arc
F is the focal length of the telescope objective lens
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Estimating Angular Distance
1p piece, 4 km distant
Sun, Moon
Width of little finger at arm's length
5p piece at arm's length
2p piece at arm's length
Width of Orion's belt
Alpha Ursae Majoris (Dubhe) to Beta Ursae Majoris (Merak)
Alpha Geminorum (Castor) to Beta Geminorum (Pollux)
Width of fist at arm's length
Alpha Ursae Majoris (Dubhe) to Delta Ursae Majoris (Megrez)
Height of Orion
Length of palm at arm's length
Width of thumb to little finger at arm's length
Alpha Ursae Majoris (Dubhe) to Eta Ursae Majoris (Alkaid)
Alpha Ursae Majoris (Dubhe) to Alpha Ursae Minoris (Polaris)
1"
30'
1°
1°
2.5°
3°
5°
5°
10°
10°
16°
18°
20°
25°
27°
Estimating Magnitudes
Big Dipper (from cup
Alpha (Dubhe)
Beta (Merak)
Gamma (Phecda)
Delta (Megrez)
Epsilon (Alioth)
Zeta (Mizar)
Eta (Alkaid)
to handle)
1.8
2.4
2.5
3.4
1.8
2.2
1.9
Little Dipper (from cup to handle)
Beta (Kochab)
2.0
Gamma (Pherkad)
3.1
Eta
5.0
Zeta
4.3
Epsilon
4.4
Delta (Pherkard)
4.4
Alpha (Polaris)
2.0
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Range Of Useful Magnification Of A Telescope
D = diameter of aperture in mm
Minimum useful magnification for better contrast
0.13*D 0.2*D
Best visual acuity
0.25*D
Wide views
0.4*D
Lowest power to see all detail (resolution of eye matches resolution of telescope) 0.5*D
Planets, Messier objects, general viewing
0.8*D
Normal high power, double stars
1.2*D to 1.6*D
Maximum useful magnification
2.0*D
Close doubles
2.35*D
Sometimes useful for double stars
4.0*D
Limit imposed by atmospheric turbulence
500
The rotational period of the Earth:
86164.09890369732 seconds or,
23 hours 56 minutes and 04.09890369732 seconds of Universal Time
The rate of rotation of the Earth:
15.04106717866910 seconds of arc per second of time
The mean Sidereal day:
23 hours 56 minutes 04.090524 seconds of Universal Time
The day of Universal Time:
24 hours 03 minutes 56.5553678 seconds of mean Sidereal Time
* The lengths of a day of UT and a day of mean Sidereal time vary slightly with variations in the Earth's rotation. However, the
ratio of UT to mean sidereal time, is unaffected by these variations.
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The Periodic Table
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The Electromagnetic Spectrum
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Lens & Mirror Characteristics
Spherical
Aberration
Chromatic
Aberration
Focal Points
&
Lengths
(by kind permission NASA & STScl)
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BLANK
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