Oblate spheroid

An oblate spheroid is a sphere-like shape that gives the appearance of being flattened, to some degree, on the top and bottom of the object so that the circumference around the poles is less than the circumference around the equator. The Earth is an oblate spheroid, as are at least a few other planets. An oblate spheroid is a type of ellipsoid.

Diagram of an oblate spheroid

Oblate spheroids have rotational symmetry around an axis from pole to pole. An example of an oblate spheroid is an M&M.

Many planets, including the Earth and Saturn, are oblate spheroids. The difference between a sphere and the Earth's shape is small, only about one part in 300.

Flattening of astronomical objects

Stars spin, and some spin very fast. The faster the spin, the flatter the oblate spheroid. The Sun rotates at 2 kilometers per second and neutron stars have speeds of thousands of kilometers per second. In other words, the Earth is a spheroid because it rotates. Gas giants like Jupiter and Saturn are flattened by rotation more than the Earth.

Origin

In 1687 Isaac Newton published the Principia. He included a proof that a rotating self-gravitating fluid body in equilibrium takes the form of an oblate ellipsoid of revolution (a spheroid). The amount of flattening depends on the density and the balance of gravitational force and centrifugal force.