# Ellipsoidal coordinates

Ellipsoidal coordinates are a three-dimensional orthogonal coordinate system that generalizes the two-dimensional elliptic coordinate system. Unlike most three-dimensional orthogonal coordinate systems that feature quadratic coordinate surfaces, the ellipsoidal coordinate system is not produced by rotating or projecting any two-dimensional orthogonal coordinate system.

## Basic formulae

The Cartesian coordinates can be produced from the ellipsoidal coordinates by the equations

where the following limits apply to the coordinates

Consequently, surfaces of constant are ellipsoids

whereas surfaces of constant are hyperboloids of one sheet

because the last term in the lhs is negative, and surfaces of constant are hyperboloids of two sheets

because the last two terms in the lhs are negative.

## Scale factors and differential operators

For brevity in the equations below, we introduce a function

where can represent any of the three variables . Using this function, the scale factors can be written

Hence, the infinitesimal volume element equals

and the Laplacian is defined by

Other differential operators such as and can be expressed in the coordinates by substituting the scale factors into the general formulae found in orthogonal coordinates.

• Focaloid (shell given by two coordinate surfaces)

## Bibliography

• Morse PM, Feshbach H (1953). Methods of Theoretical Physics, Part I. New York: McGraw-Hill. p. 663.
• Zwillinger D (1992). Handbook of Integration. Boston, MA: Jones and Bartlett. p. 114. ISBN 0-86720-293-9.
• Sauer R, Szabó I (1967). Mathematische Hilfsmittel des Ingenieurs. New York: Springer Verlag. pp. 101102. LCCN 67025285.
• Korn GA, Korn TM (1961). Mathematical Handbook for Scientists and Engineers. New York: McGraw-Hill. p. 176. LCCN 59014456.
• Margenau H, Murphy GM (1956). The Mathematics of Physics and Chemistry. New York: D. van Nostrand. pp. 178180. LCCN 55010911.
• Moon PH, Spencer DE (1988). "Ellipsoidal Coordinates (η, θ, λ)". Field Theory Handbook, Including Coordinate Systems, Differential Equations, and Their Solutions (corrected 2nd, 3rd print ed.). New York: Springer Verlag. pp. 4044 (Table 1.10). ISBN 0-387-02732-7.

### Unusual convention

• Landau LD, Lifshitz EM, Pitaevskii LP (1984). Electrodynamics of Continuous Media (Volume 8 of the Course of Theoretical Physics) (2nd ed.). New York: Pergamon Press. pp. 1929. ISBN 978-0-7506-2634-7. Uses (ξ, η, ζ) coordinates that have the units of distance squared.