An Earth Day

• Sidereal Day: 23 hr 56 min 4 secMotion relative to background stars

• Mean Solar Day: 24 hoursThe average time between meridian crossings of the Sun

• Apparent Solar Day: variesThe actual time between the meridian crossings of the Sun

Solar vs. Sidereal Day

Lunar Month

• Synodic month: 29.53 days. Defined by the period of the phases.

• Sidereal month: 27.32 days. Defined by the motion relative to the stars.

• Anomalistic month: 27.55 days. Defined as time between perigees.

Synodic vs. Sidereal Month

An Earth Year

• Julian Year: 365.25 mean solar days. Used in calendars before 1582.

• Gregorian Year: 365.2425 mean solar days. Used in calendars today.

• Sidereal Year: 365.2564 mean solar days. Defined by the motion relative to stars.

• Tropical year: 365.2422 mean solar days. Defined from equinox to equinox. Thus, related to the seasons.

Earth Orbits Sun

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

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)

The Celestial Sphere

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

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

Time Zones

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