The oblateness, ellipticity, or flattening of an oblate spheroid, or oblatum, is a measure of the "squashing" of the spheroid's Geographical pole, towards its equator. If is the distance from the spheroid center to the equator——the transverse radius——and the distance from the center to the pole——the conjugate radius——then .

First, second and third flattening

The first, primary flattening, f, is the versine of the spheroid's angular eccentricity, :  • The flattening ratio for Earth is 1:298.257223563 (which corresponds to a radius difference of 21.385 km of the Earth radius 6378.137 - 6356.752 km) and would not be realized visually from space, since the difference represents only 0.335 %.
• The flattening of Jupiter (1:16) and Saturn (nearly 1:10), in contrast, can be seen even in a small telescope;
• Conversely, that of the Sun is less than 1:1000 and that of the Moon barely 1:900.

The amount of flattening depends on

• the relation between gravity and centrifugal force;

and in detail on

• size and density of the celestial body;
• the rotation of the planet or star;
• and the elasticity of the body.

There is also a second flattening, f' , and a third flattening, f'' (more commonly denoted as "n" and first used in 1837 by Friedrich Bessel on calculation of meridian arc length), that is the squared half-angle tangent of : Prolate valuations

The above formations apply to an ellipse and oblatum, which is an ellipse rotated about its polar, or conjugate, axis, resulting in . If it is rotated about its equatorial, or transverse, axis, it is a prolate spheroid, or prolatum, where .
With a prolatum, and are reversed in all of the flattening formation elements, except for the denominators of f and f' , which means their function assignments are reversed:  