1. The Earth Rotates

2. Relevant Geography

3. Earth’s Rotation
23 hours and 56 minutes relative to background stars (sidereal period)
Spin axis of Earth defines points on the celestial sphere (using north and south poles)
North Celestial Pole
South Celestial Pole
Celestial Equator
Sky appears to rotate east to west about the celestial poles because Earth rotates west to east.

4. Celestial Sphere

5. Rotating Celestial Sphere

6. The Celestial Sphere

7. Definitions
Celestial Sphere: An imaginary sphere where celestial objects are projected on the basis of their direction from Earth
Celestial Poles: The two points where the spin axis of the Earth’s spin axis intersects the celestial sphere
Celestial Equator: The projection of the Earth’s equator onto the celestial sphere
Great Circle: A circle on the sphere's surface whose center is the same as the sphere’s center, and divides the sphere into two equal hemispheres

8. Definitions
Zenith: The point on the sky that is directly overhead of the observer.
Horizon: The great circle on the celestial sphere that is 90 degrees from the zenith
Hour circle: The great circle through the position of a celestial body and the celestial poles
Meridian: The hour circle that passes through the zenith and both celestial poles

9. Directions on the Local Sky
Altitude: The minimum angular distance between the position of a celestial body and the horizon
Azimuth: The angular bearing of an object, measured from North (0 degrees) through East (90 degrees), South (180 degrees), West (270 degrees), and back to North (360 degrees)
Hour Angle: The angle between the meridian and an object’s hour circle (west is positive)

10. Celestial Coordinates
Vernal Equinox: The position of the Sun when it crosses the celestial equator from south to north
Declination: The minimum angular distance from the position of a celestial body and the celestial equator
Right Ascension: The eastward angle from the vernal equinox to the intersection of an object’s hour circle with the equator
1 hour of angle = 15 degrees

11. Celestial Coordinates

12. Motion Depends on Declination

13. The Sky at the North Pole
At the North Pole, the North Celestial Pole is at the zenith
Stars never rise or set
Planets, Moon, and Sun do rise and set…Why?

14. Stars Rise and Set at the Equator

15. The Sky at Our Latitude

16. Circumpolar Constellations
Circumpolar constellations never set.
Circumpolar constellations change with latitude… sky changes with latitude

17. The Sky at Southern Latitudes

18. Tilt of Spin Axis

19. Precession of the Earth’s Axis

20. North Celestial Pole Changes

21. Solar vs. Sidereal Day

22. An Earth Day
Sidereal Day: 23 hr 56 min 4 sec Motion relative to background stars
Mean Solar Day: 24 hours The average time between meridian crossings of the Sun
Apparent Solar Day: varies The actual time between the meridian crossings