Also, it is found from observation that the sun, the planets, and the satellites are, all of them, round. This roundness cannot be the roundness of a flat plate, for instance, for then the objects in question would sometimes present their thin sides to our view. It happens, also, that upon the discs which these bodies show, we see certain markings shifting along continually in one direction, to disappear at one side and to reappear again at the other. Such bodies must, indeed, be spheres in rotation. The crescent and other phases, shown by the moon and the inferior planets, should further impress the truth of the matter upon us, as such appearances can only be caused by the sunlight falling from various directions upon the surfaces of spherical bodies. Another proof, perhaps indeed the weightiest of all, is the continuous manner in which the stars overhead give place to others as one travels about the surface of the earth. When in northern regions the Pole Star and its neighbours--the stars composing the Plough, for instance--are over our heads. As one journeys south these gradually sink towards the northern horizon, while other stars take their place, and yet others are uncovered to view from the south. The regularity with which these changes occur shows that every point on the earth's surface faces a different direction of the sky, and such an arrangement would only be possible if the earth were a sphere. The celebrated Greek philosopher, Aristotle, is known to have believed in the globular shape of the earth, and it was by this very argument that he had convinced himself that it was so. The idea of the sphericity of the earth does not appear, however, to have been generally accepted until the voyages of the great navigators showed that it could be sailed round.