orbits are all elliptical, has been discarded. Copernicus had required a great deal of faith to support his system in the face of enormous practical difficulties. Galileo's telescope had disposed of some of these difficulties, by showing that the stars are much farther off than the planets and the sun, and that certainly some of the planets do revolve about the sun; Jupiter's moons also showed the possibility that the earth might not be the centre of the system, although it clearly has a moon revolving round it.
But what was really required was a proof rather than a plausible hypothesis, and this Galileo considered that he had found in his tidal theory. He had an incomplete grasp of the idea of relative motion, and he thought that as the earth is rotating and revolving round the sun the actual velocity of a point on the surface would be greater when it was turned away from the sun and less when, turned towards the sun. Then assuming that the oceans behaved like water carried in a vessel, he said the water will pile up in the direction of rotation on the earth's surface, where it is moving more slowly, and will also pile up in the opposite direction, where it is moving more quickly, so that there will be high water twice a day, as is actually the case. Many other learned men, with much greater knowledge than was possible in Galileo's day, have completely failed to solve the problem of the tides, so that Galileo's want of success is quite easy to understand. It is not so clear that his opponents understood where his failure lay, but the failure is undeniable, and it seems perfectly clear that if he had produced a real proof of the earth's rotation, the "official" opposition would have been withdrawn. Even Cardinal Bellarmine himself plainly stated that if such proof were forthcoming, it would be necessary to revise the interpretation of some parts of Scripture. Clearly, then, it
