Kepler is best known for his three laws of planetary motion, presented in his works Astronomia nova, Harmonices Mundi, and Epitome of Copernican Astronomy. He predates Isaac Newton's as the original discoverer of universal gravitation.

His first discovered law states that planetary orbits are not circular, but elliptical.
His second discovered law states the line connecting the Sun to a planet sweeps equal areas in equal times for all cases.
His third discovered law states that the square of the orbital period of a planet is proportional to the cube of its mean distance from the Sun, with the "mean distance" being half the sum of the planet's smallest and the greatest distance from the Sun (also called the semi-major axis).

http://www-istp.gsfc.nasa.gov/stargaze/Kep3laws.htm
Kepler's Three Laws of Planetary Motion 

His discovery of the three laws comprise probably the most elegant discoveries in astronomy ever made, of universal principles governing planetary motions. The real significance of Kepler lies elsewhere. The significance is that he fundamentally shaped the nature of science. The science of astronomy up to his time was picky-backed upon theological assumptions. It was deemed that since the earth is corrupt, the heavens had to be perfect, meaning that the heavenly bodies (the planets) had to follow the motion of perfect circular paths, since the circle is a perfect geometric construct. But the evidence didn't fit. And so from Ptolemy on, to Copernicus, and also Brahe fudge factors were invented to make the evidence fit the underlying assumption, utilizing such inventions as the epicycles and the equant. Kepler compared all three and exposed the similarity of the assumption that point to a single underlying failure, namely that of superimposing a priory assumption onto scientific discovery. This is the very thing that still shackles climate science today, producing impossible perceptions enforced by dogma. 

The dogma of carbon forced manmade global warming is a case in point. It politically imposes a notion for which the scientific community is demanded to produce the fudge factors that in twisted ways support the imposed notion with their own 'epicycles' and 'equants.' That's the same folly all over again, that had plagued Ptolemy, Copernicus, and Tyco Brahe. 

The dogma of the Milankovitch cycles poses the same burden on climate science. It imposes the notion that gravity is the only force acting in the universe and determining our climate, which the observed evidence disputes. Here too, great efforts are made to conjure up 'epicycles' and 'equants' to justify the priory assumptions, closing the door to real scientific discovery in the wider sphere of the Universe and its vast and powerful dynamics.

The notion of 'epicycles' and 'equants' has also pervaded the political sphere. The fact that 15,000 delegates have assembled in Copenhagen is built on the priory assumption that manmade global warming is a real thing. They delude themselves with mountains of fudge factors that justify to them the imposed assumptions and aim to find a way of living with what is basically an imposed falsity, rather than scrapping the 'epicycles' and 'equants' and opening their eyes to what is actually observable as true. This is the process by which Johannes Kepler became the pioneer who thereby changed the face of science, which may wee be his greatest contribution for the advance of human culture.

What he achieved in this respect wasn't easily complished as his lengthy and painstaking work New Astronomy illustrates.

See: http://wlym.com/~animations/newastronomy.html