1. 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 of the Sun

2. Solar vs. Sidereal Day

3. Lunar Month
Synodic month: days. Defined by the period of the phases.
Sidereal month: days. Defined by the motion relative to the stars.
Anomalistic month: days. Defined as time between perigees.

4. Synodic vs. Sidereal Month

5. An Earth Year
Julian Year: mean solar days. Used in calendars before 1582.
Gregorian Year: mean solar days. Used in calendars today.
Sidereal Year: mean solar days. Defined by the motion relative to stars.
Tropical year: mean solar days. Defined from equinox to equinox. Thus, related to the seasons.

6. Earth Orbits Sun

7. 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

8. 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)

9. The Celestial Sphere

10. Celestial Coordinates
1 hour = 15 degrees
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

11. Measuring Time
Apparent Solar Time: The actual hour angle of the Sun + 12 hours (using 24-hour clock).
Mean Solar Time: The hour angle of the average Sun + 12 hours
Universal Time: Mean Solar Time at Greenwich, England
Standard Time: Time assigned by time zone

12. Time Zones

13. Measuring Time
Equation of Time: the difference between the apparent solar time and mean solar time
Local Sidereal Time: Time measured by the position of the stars.
Local Sidereal Time = Right Ascension of objects at the Merdian
Local Sidereal Time = Right Ascension + Hour Angle