Fracastor, in his _Homocentrics_, says that nothing is more monstrous or absurd than all the excentrics and epicycles of Ptolemy, and proposes to explain the difference of velocity in the planets at different parts of their orbits by the medium offering greater or less resistance, and their alteration in apparent size by the effect of refraction. The essential element of this system was that it took appearances for realities, and was founded on the assumption that the earth is fixed in the centre of the universe, and of course therefore neglected all the appearances produced by its motion, or had to explain them by some peculiarity in the other planets. Although it was corrected from time to time to make it accord better with observation, it was the same essentially that was taught officially everywhere. It reigned supreme in Egypt, Greece, Italy, and Arabia, and in the great school of Alexandria, which consolidated it and enriched it by its own observations. But though the same in essence, the details, and especially the means of overcoming the difficulties raised by increased observations, have much varied, and it will be interesting and instructive to record some of the chief of them. One of the most important influences in modifying the astronomical systems taught to the world has been that of the Fathers of the Christian Church. When, after five centuries of patient toil, of hopes, ambitions, and discussions, the Christian Church took possession of the thrones and consciences of men, they founded their physical edifice on the ancient system, which they adapted to their special wants. With them Aristotle and Ptolemy reigned supreme. They decreed that the earth constituted the universe, that the heavens were made for it, that God, the angels, and the saints inhabited an eternal abode of joy situated above the azure sphere of the fixed stars, and they embodied this gratifying illusion in all their illuminated manuscripts, their calendars, and their church windows. The doctors of the Church all acknowledged a plurality of heavens, but they differed as to the number. St. Hilary of Poitiers would not fix it, and the same doubt held St. Basil back; but the rest, for the most part borrowing their ideas from paganism, said there were six or seven, or up to ten. They considered these heavens to be so many hemispheres supported on the earth, and gave to each a different name. In the system of Bede, which had many adherents, they were the Air, Ether, Fiery Space, Firmament, Heaven of the Angels, and Heaven of the Trinity. The two chief varieties in the systems of the middle ages may be represented as follows:--