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Round and Round

How many revolutions on its axis does the earth make in the course of one year?

Answer:

366. (Or 366 1/4, if you prefer.)

Since we normally think of the definition of a day as being the earth turning once on its axis, one might expect that the answer would be 365, since that is the number of days in a year. However, the definition of a day as being a revolution of the earth on its axis is not quite accurate, because each day that passes, the earth moves a little bit farther in its orbit around the sun. Hence, if you are on a particular spot on the earth and the sun is directly overhead ("noontime"), by the time the sun is directly overhead the next day, not only did the earth have to make a complete revolution, but, since the earth has progressed a little farther in its orbit, the Earth needed to turn just a little bit more in order for your point on the earth to face the sun again. This extra little bit of a turn amounts to one extra revolution of the earth in the course of a year.

Another way to think of this is in terms of a "sidereal" day, which is defined as a day in terms of the stars, rather than the sun. In other words, it is an actual, 360-degree revolution on the earth on its axis. (Technically, the stars in the sky are moving in orbit around the center of the galaxy, so this is not absolutely accurate, but since it takes tens of thousands of years for that orbit to take place, it is insignificant for our purposes.) If you look in the dictionary for sidereal day, you will notice that it is a little bit shorter than a solar day (i.e., what we think of as a normal day). If you take the difference and multiply by 365, you will find that the result is equal to one day of time.