Kepler saw it as one of the chief advantages of the Copernican system that it put order into the planetary motions. Although Copernicus had indeed noted that the planetary periods increase with their distance from the sun, he did not, as far as we know, attempt to find a relationship between the two. Believing that God would not have failed to establish some mathematically precise ratio, Kepler sought from the very first to find it. Thus we see, in some of his earliest surviving letters, his attempts to relate planetary periods to the radii of their orbits using circular quadrants intersected by straight lines. Right from the beginning, Kepler gave the sun that dynamic role that was to characterize his ‘new astronomy based upon physics’. Immediately after describing the nested polyhedra that he believed determined the number and distances of the planets, he wrote:
Next, there is a moving soul and an infinite motion in the sun, and a double decrement of motion in the movables. In the first place, there is the inequality of their circuits, caused by the unequal amplitude of the orbs, which would occur even were the moving power the same in all orbs. But (2) actually this moving power (like light, in optics) is weaker the further it is from the source.
