The Earth is not perfectly spherical, it is slightly flattened at the poles and bulges at the equator. This shape is called an oblate spheroid. The Earth is not a perfect sphere because it is rotating on its axis. As the Earth rotates, the centrifugal force causes the material on the Earth's surface to bulge out at the equator. This bulge is about 26 miles (41 kilometers) at the equator. The Earth's polar diameter is about 7,899 miles (12,742 kilometers), while its equatorial diameter is about 7,926 miles (12,756 kilometers).