how much a planet has been flattened at its poles by its rotational speed. If the Earth's size and rigidity were proportionately reduced, it would be similar to a basketball of butter. Now for all spinning balls, the part furthest from the spin axis spins the fastest, like the person at the end in crack the whip. And since the parts of a planet at its equator are furthest from its pole (which is its spin axis), it follows that a planet's equatorial part has the greatest tendency to fly away. Thus all spinning heavenly bodies are wider through their equators (their "equatorial diameter") than through their poles (their "polar diameter"). This effect increases as a planet spins faster, and decreases the denser the planet is. The Earth, with a density of 5.5 g/cm3, has an equatorial diameter 27 miles greater than its polar diameter, or an oblateness of 0.3%. Saturn, a gas giant that would float in water with a density of only 0.687 gm/cm3 (the density of water is 1 gm/cm3 ), is 8078 miles thicker through its equator than through its poles. Its oblateness is 10%. If a planet's equatorial diameter is De and its polar diameter is Dp, then its oblateness is defined as (De - Dp)/De.

The ellipticity of a rapidly rotating planet, expressed in terms of the increase in distance through the equator, compared to the distance through the poles. Thus, if a planet's diameter is 3% more through the equator, than through the poles, it could be said to have an oblateness of 3%. Numerically, identical to the flattening, and only differing from the flattening in terms of the viewpoint. In the case of flattening, the planet is thought of as being squashed through the poles. In the case of oblateness, it is thought of as bulging out at the equator.

The flattening of a planet from spherical form because of the centrifugal effect of rotation.

The quality or state of being oblate.