SPACE.com -- Kepler and Mars

Kepler's discovery was based on observational data. Basing his work on Copernicus' then-revolutionary theory that displaced Earth from the center of the solar system (and the universe, too), Kepler's calculations added mathematical credibility to Copernicus' idea that the Sun was actually at the center (actually, at one of the two foci of the ellipse) with Earth and the other planets revolving around it.