ASTRONOMY 1021 September 28 , 2010
Astronomy – Lecture 3
Patterns & motions in the Sky
o Reasons for Seasons
Question 2: answer – C
Question 3: answer – B
Question 4: answer – B/C? (Correct answer is D, at next question)
Question 5: answer – D
Question 6: answer – D
Question 7: answer – B
Question 8: answer – C
Question 9: answer – C
Question 10: answer – C
Question 11: answer – B
Question 12: answer – B
Question 13: answer
Question 14: answer
Question 15: answer
The local sky…
o Section of the sky you’re seeing from where you are. Always different—dependent upon your
o Animation on Mastering Astronomy.
o Horizon = altitude (0 degrees). Boundary between earth and sky
o Meridian = in between horizon and zenith
o Zenith = altitude 90 degrees). Directly above head
Patterns of stars…
o In ancient times, constellations were only the brightest starts, named after mythological gods
o All sorts of objects (sun, moon, planets, comets), are moving and thus move through
constellations. How would this work if they are patterns of stars?
o Constellations are well-defined regions on the sky, not just patterns of stars anymore
o 88 total constellations in the sky – more accurate, but artificial-looking
o Use star charts to learn, planetarium software (starry night) and Google Earth sky view
o Constellations on appear to be close to one another. This is only a projection effect. The stars
may be located at very different distances from us. We have no depth perception.
o Since we lack depth perception, all stars seem to be of equal distance, as if they are glued onto a
great sphere that surrounds Earth. This is called a celestial sphere. ASTRONOMY 1021 September 28 , 2010
o In ancient times, the celestial sphere was the view of the universe: stars attached to this sphere.
Of course, stars and planets are not really on a sphere, but appearances are still used for
describing positions and motions on the sky.
o Celestial poles are directly above earth’s north and south poles. All stars appear to lie on the
celestial sphere, but really lie at different distances. The ecliptic is the Sun’s apparent annual
path around the celestial sphere.
o Key definitions: North celestial pole – above ours, South celestial pole – above ours, Celestial
Equator: projection of our equator into space, Ecliptic: path the sun follows throughout the year.
At any time, we only see half of the celestial sphere. The other half is below Earth. This half is
the local sky!
o Appears as a band going around the celestial sphere.
o What we see are countless faint stairs in the galactic disc
Size of objects…
o We lack depth perception; we only determine angular size of objects, not true physical size.
o This is the angle the object appears to span.
o Angular Distance between two objects: the angle that appea