During the last quarter of the 16 century, the Danish nobleman Tycho Brahe provide careful observation of the planets over an extended period of time (20 years).
He hoped to use his data to verify his own model of the solar system, in which the sun orbited th earth and all other planets orbited the sun.
At Brahes death at 1601, his assistant Johannes Kepler inherited the data that Brahes had accumulated.
Kepler spend some 20 years analyzing these data, looking for mathematical regularities.
He came to the conclusion that the ideal of circular orbits should be discarded and replaced with elliptical orbits.
Kepler summerized his laborious study of planetary motion with the following three laws:
1. The planets move in elliptical orbits with the sun at one focus.
2. A line from the sun to any planets sweeps out equal areas in equal time.
3. The square of a planet's period is proportional to the cube of the planet's mean distance from the sun.
This java applet let you play with Kepler's laws and learn more physics insight.