An
equatorial bulge is a bulge which a planet may have around its
equator, distorting it into an
oblate spheroid. The
Earth has an equatorial bulge of
42.72 km (26.5 miles) due to its rotation its diameter measured across the equatorial plane (12756.28&_160;km, 7,927 miles) is 42.72&_160;km more than that measured between the poles (12713.56&_160;km, 7,900 miles).
An often-cited result of Earth's equatorial bulge is that the highest point on Earth, measured from the center outwards, is the peak of Mount Chimborazo in Ecuador, rather than Mount Everest. But since the ocean, like the Earth and the atmosphere, bulges, Chimborazo is not as high above sea level as Everest is.
To get a feel for the type of equilibrium that is involved, imagine someone seated in a spinning swivel chair, with weights in his hands. If the person in the chair pulls the weights towards him, he is doing work and his rotational kinetic energy increases. The increase of rotation rate is so strong that at the faster rotation rate the required centripetal force is larger than with the starting rotation rate.
Something analogous to this occurs in planet formation. Matter first coalesces into a slowly rotating disk-shaped distribution, and collisions and friction convert kinetic energy to heat, which allows the disk to self-gravitate into a very oblate spheroid.
