An
equatorial bulge is a difference between the equatorial and polar
diameters of a planet, due to the
centrifugal force of its
rotation. A rotating body tends to form an
oblate spheroid rather than a
sphere. The
Earth has an equatorial bulge of : that is, its diameter measured across the equatorial plane is 42.77 km more than that measured between the poles . An observer standing at sea level on either pole, therefore, is 21.36 km closer to
Earth's centrepoint than if standing at sea level on the equator. The value of Earth's radius may be approximated by the average of these radii.