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# How do you calculate how fast the Earth spins?

The Earth makes one rotation in 24 hours (well, not EXACTLY but close enough for argument). So take the circumference in miles (you can look this up) and divide by 24, that's how many miles per hour the surface travels [at the equator]. The above answer will give you a very good answer. This is an explanation of some of the reasons the answer above says "not exactly". This is wonderful stuff. Imagine yourself looking down at the solar system from way up north in space. (North doesn't end at the North Pole-- it keeps going). You see the sun in the middle, and earth moving along in its nearly circular orbit. From up here, the earth is turning counter-clockwise. Some point on the equator is moving from 6 o'clock to 5, then 4, the 3 and so on. Now the earth is moving in its orbit, of course. It is also moving counter-clockwise around the sun. You can see these movements in your mind's eye with a little practice. Now we can easily measure the amount of time from one solar noontime (when the sun is at is highest possible point for us) until tomorrow's solar noontime. We can use this time and the known circumference of the earth, to calculate earth's rate of rotation. But if you play with the image that you have of the solar system, you will see that the earth has to turn a little more than one full turn in order to reach the next day's solar noon. Even if you use the most accurate clocks instead of solar noon (as implied above), you will see that the clocks are meant to average out our observations of solar noon, and the earth will still have to turn a little extra each day before reaching standard noontime! There is another kind of day called a sidereal day. This is an Earth Day measured as if we are being observed from the fixed stars and not our sun. Sidereal days are a little shorter than solar days or days measured by standard time. You can see in your mind that as the earth turns, a point on the equator will face the distant stars (say straight up in your mental picture) before that point turns all the way to face the sun. Each day the gap gets a little bigger. So measuring the earth's rotation using a sidereal day will give you a measurement that is a little more accurate.