Copernicus is said to have commenced his inquiries by an historical examination of the opinions of ancient authors on the system of the universe; but it is more likely that he sought for the authority of their great names to countenance his peculiar views, and that he was more desirous to present his own theory as one that he had received, rather than as one which he had invented. His mind had been long imbued with the idea that simplicity and harmony should characterize the arrangements of the planetary system, and, in the complication and disorder which reigned in the hypothesis of Ptolemy, he saw insuperable objections to its being regarded as a representation of nature. In the opinions of the Egyptian sages, in those of Pythagoras, Philolaus, Aristarchus, and Nicetas, he recognised his own earliest conviction that the earth was not the centre of the universe; but he appears to have considered it as still possible that our globe might perform some function in the system more important than that of the other planets; and his attention was much occupied with the speculation of Martianus Capella, who placed the sun between Mars and the moon, and made Mercury and Venus revolve round him as a centre; and with the system of Apollonius Pergæus, who made all the planets revolve round the sun, while the sun and moon were carried round the earth in the centre of the universe. The examination, however, of these hypotheses gradually dispelled the difficulties with which the subject was beset; and after the labours of more than thirty years, he was permitted to see the true system of the heavens. The sun he considered as immoveable in the centre of the system, while the earth revolved between the orbits of Venus and Mars, and produced by its rotation about its axis all the diurnal phenomena of the celestial sphere. The precession of the equinoxes was thus referred to a slight motion of the earth’s axis, and the stations and retrogradations of the planets were the necessary consequence of their own motions combined with that of the earth about the sun. These remarkable views were supported by numerous astronomical observations; and in 1530 Copernicus brought to a close his immortal work on the Revolutions of the Heavenly Bodies.