Introduction

After the Earth, the Moon is much the best known body of the solar system. Almost all physical measurements that have been made on the Earth have also to some extent been made on the Moon. Artificial satellites have been placed in orbit about the Moon and have enabled the components of the gravitational potential to be estimated. The physical librations, the equivalent of the luni-solar precession of the Earth, have been observed, especially by laser ranging to the retroreflectors left on the Moon by Apollo astronauts. The Apollo astronauts took with them seismometers that have recorded impacts of meteorites and rockets on the surface and moonquakes within the Moon. The flow of heat through the surface of the Moon was measured.

The magnetic field of the Moon has been studied intensively, globally by satellites at a distance from the surface and in detail by others close to it, while the magnetization of rock samples brought back by the Apollo astronauts has been studied in the laboratory. In addition, electro-magnetic induction in the Moon has been studied. Thus, there is some prospect of being able to construct models of the interior of the Moon using much the same methods as are followed for the Earth, whereas there is at present no such prospect for any of the planets. However, there are major gaps in our knowledge of the Moon as compared with the Earth, and the principal one is that seismic data are comparatively very sparse because there are only four seismic stations on the Moon and all of them are on the same hemisphere and, furthermore, because free oscillations of the Moon have never been observed.