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Astronomy & Astrophysics
Michael Reid

Supplementary Chapter 1Celestial Timekeeping and NavigationS11 Astronomical Time Periods How do we define the day month year and planetary periodsyThe length of the day corresponds to the Earths rotation the length of the month to the cycle of lunar phases and the length of the year to our orbit around the Sun y We are forced to define two different types of day month and year y We also define planetary periods in two different waysThe Lengths of Day y Earths rotation is not exactly 24 hours its about 4 minutes less y You can measure Earths rotation period by measuring how long it takes for any star to go from its highest point in the sky one day to its highest point in the next day This time periods which we call a sidereal day is about 23 hours 56 minutesy Our 24 hours day which we call a solar day is based on the time it takes for the Sun to make one circuit around the local sky You can measure this time period by measuring how long it takes the Sun to go from its highest point in the sky one day to its highest point in the next dayy A solar day represents about 361r of rotation rather than the 360r for a sidereal day Th
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