Spheroid

From Freepedia

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 or cigar). If the minor axis is chosen, the surface is called an oblate spheroid (similar to the shape of the planet Earth).

A spheroid can also be characterised as an ellipsoid having two equal semi-axes, as represented by the equation

<math>\frac{x^2}{a^2}+\frac{y^2}{a^2}+\frac{z^2}{b^2}=1</math>

A prolate spheroid has one semiaxis longer than the other two, (b > a); an oblate spheroid has one semiaxis shorter than the other two (b < a) and can resemble a disk.

Image:ProlateSpheroid.PNG Prolate spheroid.

Image:OblateSpheroid.PNG Oblate spheroid.

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

Volume

Prolate spheroid:

• volume is <math>\frac{4}{3}\pi a b^2</math>

Oblate spheroid:

• volume is <math>\frac{4}{3}\pi a^2 b</math>

where

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

Surface area

An oblate spheroid has surface area

<math>\pi\left(2 a^2 + \frac{b^2}{e} \ln\left(\frac{1+e}{1-e}\right) \right).</math>

A prolate spheroid has surface area

<math>2\pi b\left(b + a \frac{\arcsin{e}}{e}\right)</math>.

Here e is the eccentricity of the ellipse, defined as

<math>\sqrt{1-\frac{b^2}{a^2}}.</math>