   The manuscript of On the Revolutions was basically complete when Rheticus came to visit him in 1539. The work comprised six books. The first book, the best known, discussed what came to be known as the Copernican theory and what is Copernicus’s most important contribution to astronomy, the heliocentric universe (although in Copernicus’s model, the sun is not truly in the center). Book 1 set out the order of the heavenly bodies about the sun: “[The sphere of the fixed stars] is followed by the first of the planets, Saturn, which completes its circuit in 30 years.  After Saturn, Jupiter accomplishes its revolution in 12 years. The Mars revolves in 2 years. The annual revolution takes the series’ fourth place, which contains the earth…together with the lunar sphere as an epicycle. In the fifth place Venus returns in 9 months. Lastly, the sixth place is held by Mercury, which revolves in a period of 80 days” (Revolutions, 21–22). This established a relationship between the order of the planets and their periods, and it made a unified system. This may be the most important argument in favor of the heliocentric model as Copernicus described   it.[8]   It was far superior to Ptolemy’s model, which had the planets revolving around the earth so that the sun, Mercury, and Venus all had the same annual revolution.  In book 1 Copernicus also insisted that the movements of all bodies must be circular and uniform, and noted that the reason they may appear nonuniform to us is “either that their circles have poles different [from the earth’s] or that the earth is not at the center of the circles on which they revolve” (Revolutions, 11). Particularly notable for Copernicus was that in Ptolemy’s model the sun, the moon, and the five planets seemed ironically to have different motions from the other heavenly bodies and it made more sense for the small earth to move than the immense heavens. But the fact that Copernicus turned the earth into a planet did not cause him to reject Aristotelian physics, for he maintained that “land and water together press upon a single center of gravity; that the earth has no other center of magnitude; that, since earth is heavier, its gaps are filled with water…” (Revolutions, 10). As Aristotle had asserted, the earth was the center toward which the physical elements gravitate. This was a problem for Copernicus’s model, because if the earth was no longer the center, why should elements gravitate toward it?