The centre of mass of the EARTH-MOON system. If two bodies move under the influence of their mutual gravitational attractions, they will describe orbits around a point between them, the centre of mass of the two bodies. The relative distances of the two bodies from this point will be inversely proportional to their masses. Thus, for example, if one body is twice the mass of the other, the centre of mass will lie one third of the way from the more massive body to the less massive one (i.e. their distances are in the ration 1:2).
