The increased equatorial diameter of a rapidly rotating planet, caused by centrifugal force. The size of the effect depends on the body's density and its rotation period. The Earth, which rotates in 24 hours, has a slight equatorial bulge, its equatorial diameter being about 0.3% greater than its polar diameter. Saturn, which has a rotation period of just over 10 hours and a mean density less than that of water, has the greatest equatorial bulge of all the planets, with an equatorial diameter nearly 11% greater than its polar diameter. See also Oblateness.
Subjects: Astronomy and Astrophysics.