This article is about the geometric solid figure. For a member of a Roman Catholic religious order, see Oblate (religion).

An oblate spheroid is an ellipsoid having two equal polar semi-minor axes, shorter than the equatorial semi-major axis.

The oblate spheroid is interesting because it is the approximate shape of many planets and celestial bodies, including the Earth. It is therefore the geometric figure most used for defining reference ellipsoids, upon which cartographic and geodetic systems are based.

For a discussion of the physics that determines the shape of a spinning celestial body, see Equatorial bulge

The aspect ratio, b:a, is the ratio of the polar to equatorial lengths, while the flattening, f, is the ratio of the equatorial-polar length difference to the equatorial length: $f=\backslash frac\{a-b\}\{a\}.\backslash ,\backslash !$
These are just two of several different parameters used to define an ellipse and its solid body counterparts, all of which are ultimately trigonometric functions of the ellipse's modular angle, or angular eccentricity.

See also

• Flattening
• Reference ellipsoid
• Spheroid

Planetary science | Surfaces

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