Download Definitions

SPACE
1.
GPE – is the energy of a mass due to its position within a gravitational field
2.
Escape Velocity – is the initial velocity required by a projectile to rise vertically and just
escape the gravitational field of a planet
Uniform Circular Motion – is circular motion with a uniform orbital speed
Centripetal Acceleration – is associated with the centripetal force of uniform circular motion
and is directed at the centre of circle.
LEO – is an orbit higher than 250km and
lower than 1000km
Geostationary Orbit – is an altitude at which
the period of the orbit matches that of earth
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250<LEO<1000
period much smaller than Earth
can pass any pt on earth
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Geographical mapping, remote sensing,
meteorology.
Closer view of earth surface
Low orbit = faster info transmission
Launchings are easier and cheaper, less fuel
Its rapid movement is hard to track
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Inevitable atmospheric drag and orbital
decay
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Fixed point above equator
Same period as earth.
Limited view of Earth (~ 1/3). Requires
network for full coverage
Global info relay, communication satellite,
weather monitoring
Easy to track
No orbital decay
Info transmission delay
High altitude makes launching more difficult
and expensive (fuel)
Suffers more damage form incoming cosmic
rays
Orbital Velocity – is the instantaneous direction and speed of an object in circular motion along its
paths (orbital energy = KE + GPE)
3.
Gravitational Field – is a field within which any mass will experience a gravitational force
Newton’s Law of Universal Gravitation – the force of attraction between 2 objects is
proportional to the product of their masses in inversely proportional to the square of their
distance apart
Slingshot effect – is a strategy used with space probes to achieve a change in velocity w/
little fuel consumption
4.
Luminiferous Aether – was the proposed medium for light and other EM waves before it
was realised that these waveforms do not require a medium for travel. (filled all space, low
density, transparent)
A Frame of Reference – is anything with respect to which we describe motion and take
measurements.
IFR – is a non accelerated environment. Only steady/no motion is allowed. A N-IFR
experiences acceleration.
Principle of Relativity – Motion cannot be determined without a reference to an outside pt
Relative simultaneity – Two events that are simultaneous to one observer may not
necessarily appear simultaneous to another observer who is in a frame that is moving at a
relativistic speed.
Time Dilation – Is the slowing down of events as observed from a FoR in relative motion
Length contraction – is the shortening of an object in the direction of its motion as observed
from a FoR in relative motion
Mass Dilation – is the increase in the mass of an object as observed from a FoR in relative
motion
Rest energy – is the energy equivalent of a stationary object’s mass, measured within the
object’s rest frame
Standard of length – Equator to geographic north pole, SI units / bar of plat-iridium in
vacuum at zero deg, atomic length of orange-red light from isotope kryton 86 [incr precision,
greater accessibility, invariability], one second of light measured by caesium clock [more
accurate as a def of time, not length. Easier to measure]
ASTROPHYSICS
1.
Sensitivity – of a telescope is its ability to pick up faint objects for observation or its light
gathering power. Varies with the collecting are of the lens/mirror.
Theoretical Resolution – of a telescope is its ability to distinguish two close objects as
separate images. Measure at an angle (arcsec) and depends on the wavelength of EMR
collected as well as telescope diameter
Seeing – refers to the twinkling and blurring of a star’s light due to atmospheric distortion
Interferometry – is a technique used to combine data form several elements of an antenna
array in order to achieve higher resolution. (eg VLA [res of 130 km] and VLBA)
Active Optics – uses a slow feedback system to correct sagging or other deformities in the
primary mirror of large modern reflector telescopes. (0.5sec, actuators)
Adaptive Optics – uses a fast feedback system to attempt to correct for effects of
atmospheric disturbances. (1000Hz, tip tilt/deformable)
2.
Astrometry – is the accurate of positional movement of celestial objects.
Parallax – is the apparent shift in position of a close object against a distant background due
to a change in the position of the observer (d = 1/p [pc, arcsec], annual parallax, 6 months,
half angle, @ = sin @ = tan @)
Parsec – is the distance that corresponds to an annual parallax of one arcsec
Light Year – is the distanced travelled through space in one year by light or other EMR
(9.46 x 1012 km)
3.
Spectroscopy – is the systematic study of spectra and spectral lines
Blackbody – is a hypothetical body that is a perfect absorber and emitter of EMR
C spectrum – consists of a continuous range of frequencies w/o either bright or dark lines,
appearing as a continuous range of colours
E spectrum – consists of only radiation at a number of discrete wavelengths, appearing as
bright lines against a dark background
A spectrum – consists of a C spectrum w/ discrete gaps at particular wavelengths as dark
line against a continuous background of colours
Emission nebulae – are regions of hot gas, mainly H, and dust heated by the radiation from
nearby young stars (UV, visible, IR and radio – characteristic of elements)
Quasars – are very distant objects that produce vast quantities of C radiation at all
wavelengths but with most of their energy at longer radio wavelengths (board high intensity
E lines on C spectrum, large red shift)
BB radiation – is the EMR that is emitted by a BB at a particular temp. Distributed
continuously but unevenly. (cool, low luminosity, red  hot, high luminosity, blue)
Doppler effect – refers to the apparent shift in the wavelength (and frequency) of a wave
when there is relative motion between source and observer.
4.
Photometry – is the measurement of the brightness of stars and other astronomical objects
Apparent magnitude – is the magnitude given to a star as viewed from the earth
Absolute magnitude – is the magnitude that a star would have if viewed from a distance of
10pc.
Spectroscopic parallax – is a method of using the HR diagram and D-M formula to determine
the approximate distance to a star
Visual magnitude – refers to the magnitude as judged by the eye
PG magnitude – refers to magnitudes determined by PG emulsions
PG PM – uses PG film to record a section of the sky
PE PM – uses a combination of a filter and electronic sensor (CCD, photomultiplier tubes) to
detect light signals and convert these into electrical signals that can be multiplied, digitised,
analysed and stored electronically.
A CCD – is a device for the movement of electrical charge, usually form the device to an area
where the charge can be manipulated
PE effect – electrons are emitted from matter as a consequence of their absorption of
energy from EMR of v short wavelengths. (light hits CCD and due to PE effect, electrical
signal is created)
Secondary emission – ability of electrons in a vacuum tube to cause the emission of
additional electrons by striking an electrode
5.
A Binary Star System – consists of 2 stars in orbit about their common centre of mass (each
star has same period, can determine mass)
Visual Bi – both stars are visible through a telescope (A > B)
Spectroscopic Bi – detected due to a periodic Doppler shift in spectral lines
Astronometric Bi – one star is significantly brighter/other is too faint (wobble)
Eclipsing Bi – One star may periodically pass in front of the other, blocking light. (plot m over
time, determine existence of 2 stars, ratio of surface temp, duration of ellipse = relative radii
and orbits)
Extrinsic Variables – vary in brightness for some reason external to the star
Rotating Bi – vary in brightness as they rotate due to the star’s ellipsoidal shape or presence
of large cool/hot areas on star’s surface.
Intrinsic Variables – vary in brightness due to actual changes in the luminosity of the star
itself.