The stress laid upon this fundamental point by Copernicus, marks the separation of the modern from the ancient view. Not that Copernicus, indeed, had obtained any real proof that the earth is merely a planet revolving around the sun; but it seemed to his profound intellect that a movement of this kind on the part of our globe was the more likely explanation of the celestial riddle. The idea was not new; for, as we have already seen, certain of the ancient Greeks (Aristarchus of Samos, for example) had held such a view; but their notions on the subject were very fanciful, and unsupported by any good argument. What Copernicus, however, really seems to have done was to _insist_ upon the idea that the sun occupied the _centre_, as being more consonant with common sense. No doubt, he was led to take up this position by the fact that the sun appeared entirely of a different character from the other members of the system. The one body in the scheme, which performed the important function of dispenser of light and heat, would indeed be more likely to occupy a position apart from the rest; and what position more appropriate for its purposes than the centre! But here Copernicus only partially solved the difficult question. He unfortunately still clung to an ancient belief, which as yet remained unquestioned; _i.e._ the great virtue, one might almost say, the _divineness_, of circular motion. The ancients had been hag-ridden, so to speak, by the circle; and it appeared to them that such a perfectly formed curve was alone fitted for the celestial motions. Ptolemy employed it throughout his system. According to him the "planets" (which included, under the ancient view, both the sun and the moon), moved around the earth in circles; but, as their changing positions in the sky could not be altogether accounted for in this way, it was further supposed that they performed additional circular movements, around peculiarly placed centres, during the course of their orbital revolutions. Thus the Ptolemaic system grew to be extremely complicated; for astronomers did not hesitate to add new circular movements whenever the celestial positions calculated for the planets were found not to tally with the positions observed. In this manner, indeed, they succeeded in doctoring the theory, so that it fairly satisfied the observations made with the rough instruments of pre-telescopic times.