Orbits

Abstract

Perhaps the most seductive application of dynamics is to the motion of astronomical bodies. At first sight it seems crazy that bodies such as the Moon and even the Earth can be treated as particles. However the vast distances of space make the approximation not only good but possibly better than almost all terrestrial applications of the particle model. The Sun is so massive compared to the small inner planets (Mercury, Venus, Earth and Mars) that these planets can be assumed to orbit the stationary Sun. Similarly, satellites can be assumed to orbit the stationary Earth. The Moon however does not orbit the stationary Earth. The latter ‘wobbles’ as the Moon orbits, as both travel about the common centre of mass which is just below the surface of the Earth. Jupiter also acts on the Sun, causing a wobble but, in this case, the wobble is much less and entirely negligible.