The Size of the Earth

The earth is essentially spherical in shape but it is not a perfect ball. As it travels through space around the sun, it also spins on its axis like a top. This latter motion is fairly quick. During one day, the earth makes almost one complete revolution and thus a point on the equator travels close to 25,000 miles, at a rate of slightly more than 1000 miles per hour.

The centrifugal force that results from this constant circular motion of the earth causes the middle of the earth, the parts at and near the equator, to bulge slightly and the extreme ends of the ball, the parts near the north and south poles, to flatten in. Thus, the earth's circumference at the equator is 24,902 miles, 42 miles more than the circumference around the north and south poles (24,860 miles).

For the same reason, the diameter of the earth through the equator is 7927 miles, while the diameter through the poles is only 7900 miles.