equatorial bulge

The equatorial bulge is the departure of the Earth's shape from a perfect sphere: its equatorial radius is about 6378 km, its polar radius about 6357 km — a difference of 21 km. The bulge arises because the centrifugal acceleration of rotation acts only in the equatorial plane, and a fluid or plastic body in hydrostatic equilibrium with its own gravity and its rotation deforms into an oblate spheroid.

For the Earth, the oblateness is characterised by the dimensionless quantity J_2 ≈ 0.001083, which encodes the mass distribution's departure from spherical symmetry. This small number is crucial. It is the handle by which lunar and solar gravitational torques can act on the Earth: if the Earth were a perfect sphere, there would be no torque from any external mass (a point mass outside a spherically symmetric body exerts no torque on it). The 21-km bulge is what makes axial precession and nutation possible.

Other rotating planets have their own equatorial bulges. Jupiter, rotating in 10 hours, has an oblateness ratio of 0.065 — visibly non-spherical in even amateur telescopes. Saturn is even more oblate at 0.098, again visible in photographs. Mars has an oblateness of 0.0059, larger than Earth's because its internal structure is less dense in the centre. Measuring a planet's oblateness provides a direct probe of its internal mass distribution, and spacecraft missions routinely use the gravitational effects of the oblateness on their orbits to constrain models of planetary interiors.