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An Earth Day

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. Solar vs. Sidereal Day. Lunar Month.

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An Earth Day

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  1. 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

  2. Solar vs. Sidereal Day

  3. 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.

  4. Synodic vs. Sidereal Month

  5. 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.

  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

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