Orthogonal Coordinate System

An orthogonal coordinate system is a system of curvilinear coordinates in which each family of surfaces intersects the others at right angles. Orthogonal coordinates therefore satisfy the additional constraint that

(1)

where is the Kronecker delta. Therefore, the line element becomes

			(2)
			(3)

and the volume element becomes

			(4)
			(5)
			(6)
			(7)
			(8)

where the latter is the Jacobian.

The gradient of a function is given in orthogonal curvilinear coordinates by

			(9)
			(10)

the divergence is

(11)

and the curl is

			(12)
			(13)

For surfaces of first degree, the only three-dimensional coordinate system of surfaces having orthogonal intersections is Cartesian coordinates (Moon and Spencer 1988, p. 1). Including degenerate cases, there are 11 sets of quadratic surfaces having orthogonal coordinates. Furthermore, Laplace's equation and the Helmholtz differential equation are separable in all of these coordinate systems (Moon and Spencer 1988, p. 1).

Planar orthogonal curvilinear coordinate systems of degree two or less include two-dimensional Cartesian coordinates and polar coordinates.

Three-dimensional orthogonal curvilinear coordinate systems of degree two or less include bipolar cylindrical coordinates, bispherical coordinates, three-dimensional Cartesian coordinates, confocal ellipsoidal coordinates, confocal paraboloidal coordinates, conical coordinates, cyclidic coordinates, cylindrical coordinates, elliptic cylindrical coordinates, oblate spheroidal coordinates, parabolic coordinates, parabolic cylindrical coordinates, paraboloidal coordinates, prolate spheroidal coordinates, spherical coordinates, and toroidal coordinates. These are degenerate cases of the confocal ellipsoidal coordinates.

Orthogonal coordinate systems can also be built from fourth-order (in particular, cyclidic coordinates) and higher surfaces (Bôcher 1894), but are generally less important in solving physical problems than are quadratic surfaces (Moon and Spencer 1988, p. 1).

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