Equatorial bulge

An equatorial bulge is a planetological term which describes a bulge which a planet may have around its equator. The Earth has an equatorial bulge of 43 km due to the centrifugal force of its rotation. That is, its diameter measured across the equatorial plane is 43 km more than that measured between the poles. The planet with the largest equatorial bulge 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.

The differing gravitational field also affects the orbits of satellites and changes their orbits from pure ellipses.