    Moreover, the whole space of the planetary heavens is either at rest (as is commonly believed) or uniformly moved in a straight line, and similarly the common centre of gravity of the planets (by Law 4) is either at rest or is moved at the same time. In either case the motions of the planets among themselves (by Law 3) take place in the same manner and their common centre of gravity is at rest with respect to the whole space, and so it ought to be considered the immobile center of the whole planetary system. Thence indeed the Copernican system is proved a priori. For if a common centre of gravity is computed for any position of the planets, this either lies in the body of the Sun or will always be very near it.