Kepler’s job of trying to fit Brahe’s data into a scheme for planetary motions must have been extremely tedious and difficult. In fact, it took Kepler eight years to develop his model, without the benefit of many of the mathematical tools we know of today (like logarithms), not to mention calculators and computers.

Remember that Kepler lived before Newton, and hence the concept of "force" was not even established. Kepler’s three LAWS are not physical laws of motion in the modern sense, but rather a description of the way planets orbit around the Sun.

1. The orbit of a planet forms an ellipse with the Sun at one focus.
2. The line joining the Sun and a planets sweeps out equal areas in equal times.
3. The square of the period of revolution of a planet is proportional to the cube of the semimajor axis of its elliptical orbit.

Kepler's laws apply not only to the nine planets but also to asteroids and comets in their motion around the Sun. Furthermore, Kepler's laws also apply to the orbital motion of any body, for example, the revolution of the Moon around the Earth or of artificial satellites orbiting the Earth.