Chapter 68

Kepler's shocking genius displays itself with his removal of the last axiom left in astronomy. It lay hidden as an assumption, because, rather than being a hypothesis about the motions of the planets, it was an assumption that underlay even the means of making observations.

     In the first volume of the Progymnasmata, p. 233, Brahe demonstrated that the latitudes of the fixed stars are different today than at the time of Ptolemy...
     Copernicus put the matter very simply: the fixed stars are not subject to any motion from place to place, and thus are truly fixed forever in the same places.
     The ecliptic, in turn, is the great circle in the sphere of the fixed stars beneath which, for us on earth, the sun ever appears, and which it is seen to traverse annually. And whether this motion belongs to the sun or the earth, in either case it belongs to one of the planets. Therefore, the fixed stars do not themselves contain the cause of the ecliptic: it only results from the annual motion of the earth or of the sun about the center of the world.

Here you have the retrogression of the nodes. The poles of the earth and Mars move as on page 638. The nodes of Mars, as determined by the ecliptic, are purple. The hypothesized nodes the earth makes with the royal road are green. The location of the true Mars nodes are orange, but they could not be determined by Kepler. If they were, they could confirm his hypothesis that the same magnetic power causes the limits and the eccentricity.

Therefore, as long as we are wanting suitable observations from antiquity, circumstances compel us to leave this discussion of the motion of the nodes, along with many other matters, to posterity, if, indeed, it should please God to vouchsafe the human race a length of time in this world sufficient to work through such remaining questions thoroughly. (p.640)

Now, On To The Future!