Oblateness of a spheroid is the situation in which two of its axes of symmetry have an equal length greater than that of the third. Earth, in common with the other planets of the Solar System, is oblate, its polar diameter being some 45 km greater than its equatorial diameter. The Sun, too, is an oblate spheroid but one in which the equatorial bulge is very small so that it is close to being a perfect sphere. In fact, although the Sun is 1.4 million km across, the difference between its diameter at the equator and between the poles is a mere 10 kilometers. Jupiter, on the other hand, is conspicuously oblate, being almost 7% wider across its equator than the distance from pole to pole.

A prolate spheroid is one with two axes of symmetry equal in length and shorter than the third.