# The gravitational potential of ellipsoid

In the literature (Kirchhoff G. - Mechanic (1897), Lecture 18 or Lamb, H. - Hydrodynamics (1879)) one can find the following analytical closed form expression for the gravitational potential of homogeneous ellipsoid of unit density, whose surface is given by $$\frac{x^2}{a^2}+\frac{y^2}{b^2}+\frac{z^2}{c^2}=1 \;.$$ Gravitational point for internal points is $$\Omega=\pi abc\int_0^\infty\left(1-\frac{x^2}{a^2+\lambda}-\frac{y^2}{b^2+\lambda}-\frac{z^2}{c^2+\lambda}\right)\frac{d\lambda}{\Delta}$$ and for external points $$\Omega=\pi abc\int_u^\infty\left(1-\frac{x^2}{a^2+\lambda}-\frac{y^2}{b^2+\lambda}-\frac{z^2}{c^2+\lambda}\right)\frac{d\lambda}{\Delta} \;,$$ where $$\Delta=\sqrt{(a^2+\lambda)(b^2+\lambda)(c^2+\lambda)}$$ and $u$ is the positive root of equation $$\frac{x^2}{a^2+u}+\frac{y^2}{b^2+u}+\frac{z^2}{c^2+u}=1 \;.$$

The expressions in these formulas appear similar to confocal ellipsoidal coordinates.

How can these formulas be derived? (perhaps something more readable than the original papers) Can they be derived in terms of ellipsoidal harmonics?

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Consider the equation you have to solve, $\Delta \phi \propto \rho$. Since the Laplace is separable in these coordinates, the above expression can be derived. I am sure that it is done in a lot of standard textbooks on continuum mechanics where I would point you. Greets – Robert Filter Oct 31 '11 at 12:45
Is there a standard textbook which contains such a derivation? – liberias Oct 31 '11 at 21:16
still not found, any references and ideas are welcome :) – liberias Nov 2 '11 at 23:42
The other way around: springerlink.com/content/p866587864k0wg83 – liberias Nov 18 '11 at 10:15

This is a very late answer. There is the book

Ellipsoidal Figures of Equilibrium

by the god himself in this field S. Chandrasekhar. Chapter 3 is devoted fully to understanding the gravitational potentials of ellipsoids. Theorems 3 and 9 are what you are looking for.

Chandrasekhar does not derive the equations in terms of ellipsoidal harmonics. In fact, he states that very early on in the introduction (section 16). Instead he employs spherical polar coordinates and proceeds by establishing a series of lemmas on the moments of the mass distribution. This amounts to considering integrals of the form $$I(u) = a_1 a_2 a_3 \int_u^{\infty} \frac{du}{\Delta}; \qquad A_i(u) = a_1 a_2 a_3 \int_u^{\infty} \frac{du}{\Delta (a_i^2 + u)}$$ where $\Delta^2=(a_1^2+u) (a_2^2+u) (a_3^2+u)$ and $a_i$ are the semi-major axes of the ellipsoid. Then come the two theorems you need
Theorem 3: At a point $x_i$ interior to the ellipsoid, the potential is $$\Phi = \pi G \rho \Big[I(0) - \sum_{i=1}^3 A_i(0) x_i^2 \Big]$$
Theorem 9: At a point $x_i$ exterior to the ellipsoid, the potential is $$\Phi = \pi G \rho \Big[I(\lambda) - \sum_{i=1}^3 A_i(\lambda) x_i^2 \Big]$$ where $\lambda$ is the positive root of $$\sum_{i=1}^3 \frac{x_i^2}{a_i^2 + \lambda} = 1$$