6 - Rotation, figure of the Earth and gravity

Summary

Preamble

The shape of the Earth is referred to as its figure. For some purposes it suffices to assume that the Earth is spherical. In this approximation it would interact with other astronomical bodies only by a purely central gravitational force, indistinguishable in its effect from an equal force operating on a point mass at the Earth's centre. No external torques could act on it, angular moment would be conserved, and the rotational axis would remain fixed in space even if internal motions are allowed. In this circumstance, several important geophysical effects would not occur and information about the Earth's interior derived from them would be missing. To a much better approximation the Earth is an oblate ellipsoid, quite close to the equilibrium shape resulting from a balance between the gravitational force pulling it towards a spherical shape and the centrifugal effect of rotation. The consequences of the equatorial bulge (or, equivalently, the polar flattening) are far reaching. The most important of these, from the point of view of our understanding of the Earth, is the precession, considered in the following chapter. This gives a direct measure of the moment of inertia, a crucial constraint on estimates of the internal density profile. In this chapter we consider the balance of forces causing the ellipticity and the consequent latitude variation of gravity.

How close is the Earth to the equilibrium ellipticity? A slight excess ellipticity is well documented and it is slowly decreasing.