23 hours and 56 minutes As our Earth whirls through space around the sun, its motions cause night and day, the four seasons and the passage of the years. If we were to synchronize our clocks using the motions of the stars as a reference, we would discover that the Earth completes a single turn on its axis not in 24 hours, but actually four minutes shy of that oft-quoted figure: 23 hours 56 minutes. As a result, the stars appear to rise, cross the sky, and set four minutes earlier each night. This amounts to a whole hour earlier in 15 days and two hours earlier in 30 days. A little quick arithmetic shows that with a difference of two hours per month, that in one year the cycle will come full circle (12 months times two hours equals 24 hours), since each star completes a full circle around the sky during the course of one year. This can be made clearer by trying an experiment. Suppose you look skyward tonight and pick out a bright star, then line it up with a nearby landmark (like a telephone pole or the peak of your neighbor’s roof). Make sure you note the exact time and the exact spot when you lined up the star. Then come back the next night at the exact same time and stand in the exact same place. You’ll see that the star has apparently shifted slightly to the right (west) of the position that it was at the previous night. Had you arrived four minutes earlier, the star would have lined up exactly with the nearby landmark just as you had seen the previous night. This apparent westward drift of the stars, incidentally, is a motion that is in addition to the daily rising, circling and setting, because the Earth does not simply stand in the same spot in space and spins, but is constantly rushing eastward along in its orbit around the sun. It carries us steadily toward and under the stars to the east and away from the stars that we are leaving in the west, until we make a complete circle around the sun, bringing us back to our original position in one year. And then … the whole performance starts again.
Posted on: Thu, 26 Sep 2013 00:47:34 +0000
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