Length of a Da1

# Length of a Da1 - week after the first rise time it will...

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Length of a Day - Solar versus Siderea l How long does it take the Earth to spin around exactly once? We could figure that out by timing how long it takes something in the sky to get back to its original position from one day to the next. If we time the motion of the Sun, we see that it takes almost exactly 24 hours for the Sun to get back to where it started from one day to the next. I guess that answers it, right? Before we jump the gun, let's time another object - a bright star, for example. How long does it take a star to get back to the same place in the sky from one day to the next? Does it take 24 hours for one complete rotation? No it doesn't. It takes 23 hours and 56 minutes. Big deal; that's almost 24 hours; there is only a four minute difference; does it really matter? You bet your banana skin it matters!The basic upshot is stars rise or set four minutes earlier each day. If a star rises tonight at 8 P.M., it will rise at 7:56 the next night, then 7:52 the night after, and then 7:48 the next night. A
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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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## This note was uploaded on 12/15/2011 for the course AST AST1002 taught by Professor Emilyhoward during the Fall '10 term at Broward College.

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