Kepler (1571-1630)
1. Kepler was always a heliocentrist. However, he criticized Copernicus for some errors, for example his inconsistent notion of eccentricity (measured from the sun for the earth’s deferent, but from the earth for mars’ deferent).
2. Works with rahe and inherits the astronomical data that are almost twice as accurate as those of antiquity. He took the problem of Mars’ orbit, a traditionally difficult subject with no really satisfactory solution especially after Brahe’s data.
He realized that epicicles won’t work, and therefore tries diffent types of ovals, finally settling on ellipses.
3. In 1609, he wites On the Motion of Mars, which contains his first two laws:
- Planets move on elliptical orbits with the sun occupying one of the foci
- A planet moves with such speed that equal ares are swept in equal times.
NOTE:
- the system works very well
- the system is truly heliocentric
- circles are abandoned
- being interested on the issue of what moves the planets, and convinced by his mystical pythagorean views that the source of motion must be the sun, he leaned towards the idea that a planet’s speed must be inversely proportional to its distance from the sun. It is not the second law yet, but it opened the way fot it by breaking out of the stranglehold of uniform motion.
- his work displays the basic belief in a mathematical god, a sort of perfect engineer.
4. In 1619 (Harmonice Mundi), Kepler states his third law (the square of the ratio between the period of revolution of two planets is equal to the cube of the ratio of their mean distances from the sun). Eventually, Kepler linked the orbits of the planets to musical phrases, so that the universe is not only mathematically but musically harmonious.
NOTE:
- Kepler took this as further evidence that God is the perfect mathematician.
- In fact, Kepler focused more on God the creator-engineer than on the personal god of salvation.
- Through numerical/mystical considerations, he took the sphere to be the image of the ttrinity: the center symbolizes the father, the surface the son, and the intermediate space the holy ghost. So, the universe, which is the visible image of god, is spherical, with the sun at its center. The equality of the three person of the trinity requires that of the quantity of matter among the sun, the planets and the fixed stars.