Download Session Two - A Sidewalk Astronomer in Charlottetown

Introduction to Astronomy - Session #2
By Matthew Campbell [email protected]
Naked eye (positional) astronomy
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Celestial sphere: an imaginary sphere with Earth at its center, extending out into space. The
sky overhead is the half of the sphere we see appearing as a dome. The other half of the sphere
is below the horizon.
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Celestial pole: (aka the north star) Earth's north pole or axis of rotation is projected onto the
celestial sphere at the point of the celestial pole. This point on the sky appears to remain
stationary (Earth spinning on it's axis), whereas the rest of the stars appear to travel in circular
paths around it.
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Celestial equator: Point on the celestial sphere where the plane of Earth's equator extending
out into space intersects it.
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Ecliptic: an arc on the celestial sphere that the Sun, moon, and planets appear to travel on
throughout the year. It's tilted to the celestial equator by 23.5 degrees. Throughout the year, the
Sun passes through the zodiacal constellations on the path of the ecliptic.
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Equinoxes and Solstices
◦ As the Sun makes it's 365 ¼ day circuit around the ecliptic, there are 4 main events that
occur.
▪ On Mar 21 the vernal equinox
▪ On Jun 21 the summer solstice
▪ On Sep 22 the autumnal equinox
▪ On Dec 21 the winter solstice
◦ During an equinox, the tilt of Earth's axis neither inclines away from nor towards the Sun.
Daytime and night are of approximately equal duration (first day of spring or fall). This
occurs twice a year when the Sun's motion crosses the point where the celestial equator and
ecliptic hinge together.
◦ During a solstice, the Earth's axis is tilted to it's maximum angle either toward or away from
the Sun (23.5 degrees). This occurs twice each year as the Sun reaches its highest or lowest
point relative to the celestial equator.
▪ On the summer solstice, the Sun reaches it's highest point in the sky, and that day is the
longest.
▪ On the winter solstice Dec 21, the Sun reaches it's lowest point in the sky, and that day is
the shortest.
◦ The Sun is positioned in the following constellations for each event:
▪ Vernal equinox - Pisces
▪ Summer solstice - Gemini
▪ Autumnal equinox - Virgo
▪ Winter solstice - Sagittarius
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As Earth travels around the Sun throughout the year, the apparent position of the Sun on the
ecliptic is shown in this diagram below:
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Zenith: the point on the celestial sphere directly overhead for a given observer.
Local Meridian: arc that extends from due north on the horizon, through the zenith, to due
south on the horizon.
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Circumpolar stars: are always above the horizon (never rise or set for a given latitude). These
stars are always visible throughout the night all year.
Equatorial stars: all other stars which aren't circumpolar.
◦ These stars rise in the east, achieve their highest position above the horizon on the meridian.
From the meridian, these stars move westward until they set in the west.
Celestial sphere tilt and latitude: the tilt of the celestial sphere depends on the observer's
latitude. The celestial pole is located at the same height in degrees above the horizon as the
observer's latitude.
◦ For example, in Charlottetown (46 deg N) the north star would be due north at a height of
46 degrees above the horizon.
◦ At the north pole (90 deg N), the north star would be directly overhead, at the zenith.
◦ At the equator (0 deg), the north star would be on the horizon (not visible).
◦ Circumpolar stars are located close to the celestial pole, no further than the observer's
latitude from the pole.
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Seasonal changes to evening sky – see Sky and Telescope's getting started north link
Observing the moon and planets
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When starting to observe the Moon, the two main features to see are Maria and craters
◦ Maria (Latin for sea) are the large dark grey areas on the Moon. These were formed by
ancient volcanic eruptions which settled and cooled. Early astronomers thought these were
seas of water.
◦ Craters are depressions formed by high speed impacts with the lunar surface.
◦ Diagram of the Maria and main craters:
◦ The best time to observe the Moon is when it isn't full. The “terminator line” is the moving
line that separates the day and night side of the Moon. Along the terminator, the shallow
angle of the Sun creates shadows along mountain ranges and impact craters.
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A Few Facts on the Moon
◦ The Moon can be thought as a historical museum due to the lack of an atmosphere, weather,
and geological activity. Many craters that were created close to 4 billion years old still
appear the same today.
◦ The Moon's rotation is synchronized with the time it takes to orbit the Earth. The same face
is always pointed at the Earth. This type of orbit is called “tidally locked”.
Diagram of Moon phases
◦ The Moon's face appears to change due to the sunlit portion of the Moon changing as it
orbits around the Earth. Notice in the diagram below that the Moon is always 50% lit, but
it's appearance from Earth changes as it's position changes.
How to find planets on the ecliptic
◦ Planets in the solar system all travel in orbits in roughly the same plane. They are never too
far from the plane of the ecliptic. The 5 classic naked eye planets (Mercury, Venus, Mars,
Jupiter, Saturn) will always stand out as a bright star.
◦ Mercury and Venus are always close to Sun. Outer planets are at various points on the
ecliptic. Find out where a planet will be before going to try to observe it.
◦ If a planet is too close to or behind the Sun, it may not be visible at all for a long time.
◦ You may read that a planet is in a constellation, eg. “Jupiter is in Leo”. This means that the
planet is currently located inside the boundaries of that constellation.
◦ Jupiter, and Saturn are the easiest planets to observe. Mercury is so close to the Sun that it's
mostly only visible low in the sky during dusk or dawn. Uranus and Neptune are faint and
hard to find for a beginner. Venus and Mars are usually low on the horizon.
◦ The gas and ice giants spend a long time in each constellation due to their long orbital
period. The further away a planet is, the slower it appears to move across the sky.
▪ Jupiter takes about 12 years to orbit, so it will spend about a year in each constellation.
▪ Saturn takes about 29 years to orbit, so it will spend about 2.5 years in each
constellation.
▪ Uranus takes about 84 years to orbit, so it will spend about 7 years in each constellation.
▪ Neptune takes about 165 years to orbit, so it will spend about 14 years in each
constellation.
Apparent and limiting magnitudes, surface brightness
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Apparent magnitude is the apparent brightness of a celestial body. The scale has + and –
values.
◦ The brighter an object appears, the lower the value it's magnitude is. The star Vega in the
constellation Lyra is the base line for the scale defined as apparent magnitude 0.0.
◦ Bright objects: the Sun is -26.7, the Moon is -12.7, and Venus is -4.9
◦ Medium brightness: Beehive cluster (M44) +3.7, Uranus +5.9
▪ A person with good vision can see stars down to magnitude +6 with the naked eye at a
dark country site.
◦ Dim objects: Galaxy M91 +11.0, Owl Nebula (M97) +9.9
The scale units aren't a simple relation like a thermostat. Each increase or decrease by 1 unit is
by a factor of 2.5 of the previous one.
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Limiting magnitude is the faintest apparent magnitude of a celestial body that is detectable by
a given instrument.
◦ When observing objects, this is a very important factor to consider. A 8” Dobsonian has a
limiting magnitude of 14 or 15 but you will find it hard to see objects that are magnitude 11
or fainter due to observing conditions.
Surface brightness
◦ Objects like nebulae or galaxies have their magnitudes rated the same as stars. The
difference is that a star is a point of light, whereas a galaxy has a larger apparent surface
area. The entire luminosity of the object is summed over it's area. The magnitude is then the
same as a point source like a star emitting the luminosity. Therefore, large objects appear
dimmer than stars that are the same magnitude since their light is spread out over an area.
Star names and star magnitudes on maps
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The Greek alphabet is used to name stars in each constellation. This naming system is called the
Bayer Designation. The brightest star is alpha, the next brightest beta, etc. Traditionally the
brighter stars have proper names, but all will also have a Greek letter as well.
◦ The Greek letter is combined with the genitive Latin constellation name. For example in the
constellation Ursa Major, the brightest star will be called α Ursae Majoris or α UMa, the
next brightest is β Ursae Majoris or β UMa, etc. α Ursae Majoris has the proper name
Dubhe, β Ursae Majoris is Merak.
◦ Most maps will show a proper name instead of a Bayer name.
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On star maps, the magnitudes will be shown in the legend. The scale is usually from -1 to +6.
Brighter stars appear to be a larger bold circle.
Astronomical catalogs
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An astronomical catalog is a list of astronomical objects grouped together because they share a
common type, origin, means of detection, or method of discovery. Beginning deep space
observers usually start with the Messier objects catalog.
Common astronomical distances
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Astronomical Unit (AU) – average distance between the Earth and the Sun
◦ 1 AU is 150 million kilometers, or 93 million miles. This is useful for distances in the solar
system. For example Mars orbits at a distance of 1.7 AU, where as Neptune orbits at a
distance of 30 AU
Light year (ly) – the distance light travels in one year
◦ 10 trillion kilometers, or 6 trillion miles. Used to express distances on a galactic scale and
further. Light travels 299,792 kilometers, or 186,282 miles per second. At this speed over a
period of a full year is the distance of 1 light year.
More advanced maps (use these once more familiar with the sky)
1. Sky and Telescope's pocket sky atlas
2. Norton's double star atlas
Learn right ascension and declination before using a star atlas. These coordinates specify
positions on the celestial sphere (similar to latitude and longitude). These charts also have
markings on them indicating angular distances. You can use this to determine how many
degrees you need to move your finder when star hopping.
Observing Programs
RASC Explore the Universe (start with this one)
http://www.rasc.ca/explore-universe
The Messier objects is a catalog of 110 deep space objects
http://en.wikipedia.org/wiki/List_of_Messier_objects
http://www.rasc.ca/observing/messier-certificate
http://www.solarius.net/Pages/Articles/dbArticle.aspx?artid=messier_finders
100 double stars program
http://pgrasc.org/wp-content/uploads/2012/11/AL-Double-Star-Certificate.pdf
Links
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http://www.constellation-guide.com/constellation-list/
http://www.scc.losrios.edu/pag/44-common-misconceptions-astronomy/
http://apod.nasa.gov/apod/
http://cometchasing.skyhound.com/
http://en.wikipedia.org/wiki/Meteor_shower
http://www.cloudynights.com/documents/primer.pdf
http://www.skyandtelescope.com/astronomy-resources/how-to-align-your-newtonian-reflectortelescope/