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Lecture 3

Astronomy 1021 Lecture Notes - Lecture 3: Cosmic Dust, Dailymotion, Orbital Period

Course Code
ASTR 1021
Robert Cockcroft

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Motions in the Sky
1. Changes in daily basis are caused by earth's spin
2. Monthly changes are caused by the moons orbit
3. Annual changes are caused by earth orbit around the sun
4. Longer time periods are a number of factors
Daily motion
- of sun:
- Rises in the east
- Gets higher in the southern part of the sky
- Sets in the west
- Of Southern sky:
- Same motion for the
- Moon
- Planets
- The stars in the southern part of the sky
- of northern sky:
- Appears to rotate around North Pole star
- North on horizon immediately below North Pole star
- Circumpolar stars:
- Circum- = around
- Circumpolar = around the pole
- Circumpolar stars don’t appear to rise or set
- The circle that defines circumpolar stars changes with latitude
- When you're standing at the equator there is no circumpolar stars
- Standing at the north pole would result in the whole sky being circumpolar
- Earths motion around sun: Earth’s orbital motion around the Sun is in the same sense
as Earth’s rotation on its axis
- Motions of innner planets:
- All planets orbit around the Sun in the same direction as the Earth
- Venus rotates on its axis in the opposite direction to the other planets
- Motion of outer planets:
- All planets orbit around the Sun in the same direction as the Earth
- Uranus rotates on its side
- All the same motion:
- Solar System formed from a cloud of gas and cosmic dust
- Small initial rotation increases as cloud collapses under self-gravity
- Objects (mostly) keep initial velocities
Annual Motions:
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