Kepler's System of Astronomy

In Italy, the unfortunate Galileo added so many probabilities to the system of Copernicus.

In Germany, Kepler corrected and improved it.

Kepler had great genius but none of the taste or order and method of Galileo.

Like other Germans, he was a hard worker who had an excessive passion for discovering proportions and resemblances between the different parts of nature.

He had been instructed, by Micheal Maestlin in the system of Copernicus. He wondered:

• why there were 6 planets
• why they were at such irregular distances from the Sun
• whether there was any uniform proportion between their distances, and the timespan of their revolution

He tried to find it in:

• the proportions of numbers and plain figures,
• then in those of the regular solids,
• finally, in those of the musical divisions of the Octave.

Whatever the science Kepler was studying, he seems constantly to have pleased himself with finding some analogy and thus, arithmetic and music, plain and between it and the system of the universe.

Solid geometry, came all of them by turns to illustrate the doctrine of the Sphere, in the explaining of which he was, by his profession, principally employed.

He had presented one of his books to Tycho Brahe who disapproved of his system. But Tycho was pleased with Kepler’s genius and diligence in making the most laborious calculations.

Brahe invited the obscure and indigent Kepler to come and live with him as soon as he arrived.

He employed Kepler to observe, arrange, and methodize Mars.

Kepler, upon comparing them with one another, found that:

• Mars’ orbit was an ellipse with the Sun as one of its foci.
• Mars’ motion was not equable
  • It was swiftest when nearest the Sun and slowest when farthest from it
  • Its velocity gradually encreased, or diminished, according as it approached or receded from it.

He found that these 2 facts were true for all the other Planets.

The calculations of Kepler destroyed the circular orbits of the planets.

An ellipse is, of all curves lines after a circle, the simplest and most easily conceived.

Kepler took from the motion of the Planets the easiest of all proportions, that of equality, he did not leave them absolutely without one, but ascertained the rule by which their velocities continually varied; for a genius so fond of analogies, when he had taken away one, would be sure to substitute another in its room.

Notwithstanding all this, notwithstanding that his system was better supported by observations than any system had ever been before, yet, such was the attachment to the equal motions and circular orbits of the Planets, that it seems, for some time, to have been in general but little attended to by the learned, to have been altogether neglected by philosophers, and not much regarded even by astronomers.

Gassendi began to become famous during the latter days of Kepler. But he

did not understand the importance of Kepler’s alterations to that of Tycho Brahe.

Kepler’s rule for the planetary movement was intricate and difficult to comprehend.

He said that if a straight line were drawn from the center of each Planet to the Sun, and carried along by the periodical motion of the Planet, it would describe equal areas in equal times, even if the Planet did not pass over equal spaces.

The same rule took place nearly with regard to the Moon.

The imagination, when acquainted with this law of motion, can follow it more easily.

When not acquainted with this law, the mind would wander in uncertainty with regard to the proportion which regulates its varieties.

The discovery of this analogy therefore made Kepler’s system more agreeable to the natural taste of mankind. But it was still too difficult to be comprehended.

Besides this, he introduced another new analogy into the system. It first discovered that there was one uniform relation observed between=

• the distances of the Planets from the Sun, and
• the times employed in their periodical motions.

He found that their periodical times were greater than in proportion to their distances, and less than in proportion to the squares of those distances.

but, that they were nearly as the mean proportionals between their distances and the squares of their distances.

In other words, that the squares of their periodical times were nearly as the cubes of their an analogy, which, though, like all others, it no doubt rendered the system distances;

somewhat more distinct and comprehensible, was, however, as well as the former, of too intricate a nature to facilitate very much the effort of the imagination in conceiving it.