Physical Hypothesis of the Latitudes
Just as Kepler hypothesizes a physical, magnetic axis causing the planet’s reciprocation towards and away from the sun, why could a magnetic axis not exist that causes the planet to move above and below the ecliptic? Kepler poses two questions: 1. What could hold this axis in its direction? 2. Could this axis be the same as the magnetic axis causing the reciprocation?
On the first question, we’ll have an easier time holding this axis still than we did in chapter 57, since the magnet in chapter 57 is always attracted towards the sun, and the sun’s direction as seen by the earth is always changing. Here, however, the magnetic axis hypothesized to cause inclination is always drawn in the same direction. Thus, if a mind is to be needed, it need be brought in only to account for the slow motion of the nodes over the ages.
The answer to the second question would appear to be: no. The apsides do not coincide with the nodes for Mars. In fact, the two are 78° apart for Mars, and 90° apart for Jupiter! So it would appear that the same magnetic axis could not cause both motions. But then Kepler poses something remarkable:
“We shall also see whether, if some mean ecliptic be proposed for the six planets, that which we were requiring a little earlier is accomplished, namely, that the nodes of each of the planets correspond to the apsides.” (p.617)
There will be more on this in chapter 68. Kepler also leaves open the possibility that the physical cause for this motion is something whose analogy has yet to be discovered by mankind:
“Or is it rather to be believed that there are some possible modes of celestial motion which, though physical like the magnetic powers, cannot be comprehended by anoyone on earth owing to the lack of examples? For if we had lacked the example of the magnet (which was indeed unknown at one time), we could have been ignorant of many of the causes of the celestial motions.”