Flattening

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Ellipticity redirects here. For the mathematical topic of ellipticity, see elliptic operator.

The flattening, ellipticity, or oblateness of an oblate spheroid is the relative difference between its equatorial radius a and its polar radius b:

::[f =\frac]

It is related to the eccentricity (e) of the underlying ellipse:

::[f = 1 - \sqrt]

The flattening of the Earth is 1:298.25275 (which corresponds to a radius difference of 21.385 km of the Earth radius 6378.135–6356.75 km) and would not be realized visually from space,
whereas the flattening of Jupiter (1:16) and Saturn (nearly 1:10) can be seen even in a small telescope,
and that of the Sun is less than 1:1000.

The amount of flattening depends on

the relation between gravity and centrifugal force, and in detail on
size and density of the celestial body (see Figure of the Earth, last chapter),
the rotation of the planet or star,
and the elasticity of the body.

See also: Astronomy, planetography, Earth rotation, gravity field

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