Zone of divergence: A section through the centre of mass, O, of a planet and the single point, S, on the surface of the planet that touches its Brillouin sphere. The point O is the centre of the Brillouin sphere and the origin of the coordinate system in which the spherical harmonic expansion (SHE) of the gravitational potential, V, is expressed. S has the largest radial coordinate r of any point on the planet's surface. The SHE of V converges in free space on or above the Brillouin sphere, but diverges in free space beneath the Brillouin sphere. The pink zone is therefore the zone of divergence. (Courtesy: M Bevis)
If the Earth was a perfect sphere or ellipsoid, modelling its gravitational field would be easy. But
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Isabelle Dumé
is a contributing editor to Physics World