Ptolemy, following Pythagoras, Plato, and Aristotle, acknowledged that the earth's figure was globular, and he demonstrated it by the same arguments that we employ at the present day. He also discerned how this mighty globe was isolated in space. He admitted that the diurnal movement of the heavens could be accounted for by the revolution of the earth upon its axis, but unfortunately he assigned reasons for the deliberate rejection of this view. The earth, according to him, was a fixed body; it possessed neither rotation round an axis nor translation through space, but remained constantly at rest in what he supposed to be the centre of the universe. According to Ptolemy's theory the sun and the moon moved in circular orbits around the earth in the centre. The explanation of the movements of the planets he found to be more complicated, because it was necessary to account for the fact that a planet sometimes advanced and that it sometimes retrograded. The ancient geometers refused to believe that any movement, except revolution in a circle, was possible for a celestial body: accordingly a contrivance was devised by which each planet was supposed to revolve in a circle, of which the centre described another circle around the earth. Although the Ptolemaic doctrine is now known to be framed on quite an extravagant estimate of the importance of the earth in the scheme of the heavens, yet it must be admitted that the apparent movements of the celestial bodies can be thus accounted for with considerable accuracy. This theory is described in the great work known as the "Almagest," which was written in the second century of our era, and was regarded for fourteen centuries as the final authority on all questions of astronomy.