Instead of the daily motion of the heavens from east to west, Copernicus substituted the revolution of the earth itself from west to east. He explained the other phenomena of the planetary motions by supposing the sun to be fixed, and the earth and other planets to revolve about him; not, however, in simple circular orbits, according to the popular view of the Copernican theory. It was absolutely necessary to retain much of the old machinery of deferent and epicycle so long as the prejudice existed, from which Copernicus himself was not free, that nothing but circular motion is to be found in the heavens. Another step was made by the following generation, and astronomers were taught by Kepler to believe that the circular motion which they were so anxious to preserve in their theories, has no real existence in the planetary orbits. The advantage of the new system above the old, was, that by not denying to the earth the motion which it really possesses, the author had to invent epicycles to explain only the real irregularities of the motions of the other planets, and not those apparent ones which arise out of the motion of the orb from which they are viewed. It is commonly said that besides the two motions already mentioned, Copernicus attributed to the earth a third annual revolution on its axis. This was necessary from the idea which he had formed of its motion in its orbit. He conceived the earth to be carried round as if resting on a lever centred in the sun, which would cause the poles of the daily motion to point successively to different parts of the heavens; the third motion was added to restore these poles to their true position in every part of the orbit. It was afterwards seen that these two annual motions might be considered as resulting from one of a different kind, and in this simpler form they are now always considered by astronomical writers.