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Unformatted text preview: week after the first rise time, it will rise 4 x 7 = 28 minutes earlier (7:32). In one week, a star will be rising about half an hour earlier - that's a pretty big difference, so don't ignore those four minutes. Why is there a four minute difference? Which of these values tells us what the rotation period of the Earth is? Remember, it is the spinning of the Earth that causes the observed motions of the Sun and the stars over the course of the day (or night) - but there are two different time spans here - which one corresponds to the rotation period of the Earth? Believe it or not, it is the stars, not the Sun, that determine the amount of time for one rotation of the Earth. While all clocks on the Earth are based on the 24 hour time scale of the Solar Day , it is the more subtle Sidereal Day (or "star" day) that tells us how fast the Earth is spinning. It takes the Earth 23 hours and 56 minutes to complete one rotation....
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- Fall '10