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 bodies available are the stars. Here at once we are met by the difficulty that the stars are themselves in movement. These movements affect the value of the stars, as points of reference, so seriously, that if there were only one or two stars available for the inquiry it would be utterly impossible for us ever to discover the movements of the solar system. But there are, of course, hundreds, or rather thousands, of stars, which can be made to render assistance. No doubt these stars are themselves endowed with movements, but their journeys are so varied that the effects they produce tend to neutralise each other, so far as our present purpose is concerned. We are thus enabled to investigate the problem as if the stars were at rest, when a sufficiently large number of them are considered together.

Supposing the solar system to be bound on a journey through the celestial spaces, it is obvious that in the course of time apparent displacements would be thereby produced in the relative positions of the stars. The nature of the effects produced may be seen from the following illustration, which has often been given before, but may serve us once again. Let us think of a harbour, the entrance of which is marked by two lights, one on either hand. As the ship approaches the harbour the two lights, which, while the vessel was still a long way off, seemed close together, begin to open out. As the vessel approaches still nearer the lights spread wider and wider until at last, just as the ship enters the port, the two lights have opened so completely that one is on the right hand and the other on the left. In this manner we become familiar with the conception that the lights seem to spread away from the point towards which the motion