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# The Online Encyclopedia and Dictionary   ## Encyclopedia ## Dictionary ## Quotes  # Spheroid

A spheroid is a quadric surface in three dimensions obtained by rotating an ellipse about one of its principal axes. If the ellipse is rotated about its major axis, the surface is called a prolate spheroid (similar to the shape of a rugby ball). If the minor axis is chosen, the surface is called an oblate spheroid (similar to the shape of the planet Earth). Prolate spheroid. Oblate spheroid.

The sphere is a special case of the spheroid in which the generating ellipse is a circle.

A spheroid is a special case of an ellipsoid where two of the three major axes are equal.

## Volume

Prolate spheroid:

• volume is $\frac{4}{3}\pi a b^2$

Oblate spheroid:

• volume is $\frac{4}{3}\pi a^2 b$

where

• a is the major axis length
• b is the minor axis length

## Surface area

A prolate spheroid has surface area $\pi\left(2 a^2 + \frac{b^2}{e} \ln\left(\frac{1+e}{1-e}\right) \right).$

An oblate spheroid has surface area

b(b + a·arcsin(e)/e).

Here e is the eccentricity of the ellipse, defined as $\left(1-(b^2/a^2)\right)^{1/2}.$

Last updated: 02-08-2005 12:31:52
Last updated: 05-03-2005 17:50:55  