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How do scientists measure the Earth’s rotational speed?

The Earth’s rotational speed is based on the sidereal period of the Earth’s rotation, but it differs depending on where the observer is located. By dividing the distance traveled once around the Earth by the time it takes to travel that distance, the speed can be determined.

For example, a person on the Earth’s equator will travel once around the Earth’s circumference—or 24,900 miles (40,079 kilometers)—in one day. To get the speed, divide the miles by the time it takes to get back to the same place (around 24 hours), or just over 1,000 miles (1,609 kilometers) per hour. A person at one of the poles is hardly moving at any speed. This is because there is so little distance traveled in a day (a stick stuck vertically in the ice exactly at the North or South Pole will only travel about 0.394 inch [1 centimeter] per day.)

What about other places on Earth? Traveling north or south from the equator toward the poles decreases one’s tangential rotational speed. Thus, the rotational speed at any point on the Earth can be calculated by multiplying the speed at the equator by the cosine of the point’s latitude.

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