Chapter 57
The river is an immaterial
species of magnetic power in the sun. So why not have the oar to borrow something from the magnet? What if all the bodies of the planets are enormous round magnets? Of the earth (one of the planets, for Copernicus), there is no doubt. William Gilbert has proved it. -- (p.550)
Natural Principles: The Magnet
Kepler introduces magnetism as the mechanism by which the planet approaches to and recedes from the sun.
"But to describe this power more plainly, the planet's globe has two poles, of which one seeks out the sun, and the other flees the sun. So let us imagine an axis of this sort, using a magnetic strip, and let its point seek the sun. But despite its sun-seeking magnetic nature, let it remain ever parallel to itself in the translational motion of the globe, except... the progressive motion of the aphelion."
On the left side of the planet's orbit, the attracting side of the magnet is facing the sun, and the planet's distance from the sun decreases, while on the right side, the repelling side of the magnet faces the sun, and the planet's distance increases again. Note that for Kepler's idea, the sun is a magnetic monopole -- it always attracts the same side of the planet's magnet, no matter the orientation. Here you have the planet's motion, with a line from the sun to the planet drawn in to help you see the changing distance:
And again, this time with the orbit drawn in (although, truly, the circular orbit is an effect of the prinicple. The shape is secondary to the motion).
For suppose that this force of directing the axis towards the sun does detract somewhat from the retentive power [of keeping the magnet parallel to itself]... Accordingly, in the aphelial semicircle the point will gradually incline backward... Thus the aphelion will become retrograde... But in the perihelial semicircle... will then be made to move forward, and to be fast... [T]he force tending to turn the magnetic axis towards the sun is therefore stronger at perihelion than at aphelion... And so the reason is clear why the apsides progress, and do not retrogress.
The attracting side of the magnet is pulled towards the sun, while the repelling side is pushed away. Thus, at aphelion and perihelion, the magnet will be given an impulse to rotate. But this impulse is stronger at perihelion (represented by a larger arrow) than at aphelion, and thus, the net result is a progression of the aphelion in the same direction as the motion of the planet (it progresses, rather than retrogessing in the opposite direction).
