An equatorial bulge is a planetological term which describes 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 due to the centrifugal force of its rotation. That is, its diameter measured across the equatorial plane (12756.28 km) is 42.72 km more than that measured between the poles (12713.56 km). Earth's atmosphere is distorted as well, causing low air pressure at the South Pole, which has an atmospheric pressure consistent with an altitude around 3300 to 4000 meters rather than the actual 2900 m; this has led to illness among those working at the pole, especially immediately after arrival. Ironically, the pole itself is technically at the center of a high pressure zone due to Hadley circulation
The planet with the largest known equatorial bulge (11808 km) is Saturn.
Because of a planet's equatorial bulge, its gravitational field is weaker at the equator than its poles. In the 17th century, following the invention of the pendulum clock, French scientists found that clocks sent to French Guiana, on the northern coast of South America, were slower than their exact counterparts in Paris. However, measurements of the acceleration due to gravity at the equator must also take into account the centrifugal force due to the planet's rotation. On Earth, 35% of the difference in acceleration from pole to equator is due to the equatorial bulge, and the other 65% is due to the centrifugal effect.
Many rotating astronomical bodies other than planets also exhibit equatorial bulges.