The circumference of the Earth, measured along a line of longitude, is just under 24,860 miles. Earth's equatorial circumference, however, is somewhat greater, at slightly less than 24,902 miles. This difference is a result of the planet's rotation.
As Earth spins on its axis, centrifugal force along a line perpendicular to its poles pushes the equator out into a bulge. The reverse happens at the poles, which gives the planet a slightly flattened shape. Technically, the Earth is not a true sphere, but an oblate spheroid. The difference, however, is minor and very difficult to perceive from the planet's surface.