An equatorial bulge is the extension of the equator of a spinning body, and the subsequent flattening of the poles, caused by rotation. It is most conveniently thought of as being due to an outward centrifugal force (although this is really a fictitious force). Equatorial bulges are very noticeable in the case of the planets Jupiter and Saturn, which are large, spin rapidly, and are composed of fluids that are easily deformed. The Earth also has an equatorial bulge, one effect of which is to contribute to a variation in the acceleration due to gravity with latitude. The bulge also causes the planes of satellite orbits inclined to the equator (but not polar) to rotate slowly around Earth's axis. For more on this, see station-keeping.