# Oblate

An oblate spheroid is ellipsoid having a shorter axis and two equal longer axes.

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An oblate spheroid

Oblateness is a measure for a planet that is a spheroid in shape, bulging outward in the center due to its rotation. (Earth is slightly oblate.) Effectively the (fictitious) centrifugal force fights against gravity causing the planet to look more like a sphere stretched at the equator.

Mathematically, oblateness may be defined as

[itex]\epsilon = {R_{e}-R_{p} \over R_{e}} \approx {3 \pi \over 2 G T^{2} \rho}[itex]

where [itex]R_{e}[itex] the equatorial radius and [itex]R_{p}[itex] is the polar radius. The approximation is valid in the case of a fluid planet of uniform density; it is a function of the Newtonian constant of gravitation [itex]G[itex], the rotation period [itex]T[itex] and the density [itex]\rho[itex].

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