Introduction

Abstract

The study of the shape and gravitational attraction of celestial bodies has a longstanding history. Already more than 2000 years ago Greek astronomers realised that the Earth is basically spherical in shape, and efforts were made to determine its radius by means of geometric methods. Galilei was the first man who perceived that mathematics and physics were going to join forces, and he was able to unify celestial and terrestrial phenomena into one theory, destroying the traditional division between the world above and the world below the Moon. After Galilei articulated gravity as a uniform acceleration, Huygens suggested it was the natural quantity to define the unit of length, being the constant relating the period of oscillation with the length of a pendulum. This idea was, however, short-lived since oscillation periods were soon found to exhibit latitudinal variations. Following Newton’s discovery of the law of gravitation, Huygens and Newton postulated a pole-flattened equilibrium figure for the Earth, an idea which even today is used for simple geometrical modelling of the Earth’s figure. Another major breakthrough soon came with Clairaut’s study of equilibrium shapes of rotating fluids, which led to the concept of a rotating ellipsoidal Earth. His observations advanced both a geodetic application (the shape of the Earth) and provided a possible geophysical interpretation (hydrostatic equilibrium), and, more importantly, marked the beginning of physical geodesy and potential theory.

‘...a mission that includes either satellite-to-satellite tracking or a gravity gradiometer to permit direct gravity mapping of the farside should remain a high priority for lunar science.’

F. G. Lemoine et al., 1997