Kepler (1571-1630) was born, and spent his early life, in Germany. He gained a master's degree in 1591, in mathematics and astronomy, and went on to study theology. In 1594, while on the course, he was offered a mathematics and astronomy teaching post in Graz in the south of Austria. In 1596, he published his "Mysterium Cosmographicum", a theory of the sizes and motions of the planets based on the Copernican view. In 1600, he, as a Protestant, was expelled from Catholic Graz, and moved to Prague where he became chief assistant to the imperial mathematician, Tycho Brahe. On Brahe's death in 1601, he took over the post of imperial mathematician. In 1612, again due to religion, he was excommunicated, and left Prage for Linz, while remaining imperial mathemetician. He went on to use Brahe's observations with his calculations to complete and publish the "Tabulae Rudolphinae", the Rudolphine Tables, in 1627. The tables comprised a star catalogue and planetary tables. Prior to that, he found that Brahe's observations of planetary motion followed three relatively simple rules. Published in 1609, in the "Astronomia Nova", and 1619, in the "Harmonicae Mundi", these three laws of planetary motion are:
Kepler's Three Laws of Planetary Motion
1. The orbit of every planet is an ellipse with the sun at a focus.
2. A line joining a planet and the sun sweeps out equal areas during equal intervals of time.
3. The square of the orbital period of a planet is directly proportional to the cube of the semi-major axis of its orbit.
Essentially, they describe the motion of any two bodies orbiting each other. Prior to this, geocentric (Earth-centered) models of the Universe predominated, so he was challenging the likes of Aristotle and Ptolemy, as well as his mentor Tycho Brahe's Geo-Helio-Centric model. Early astronomers found it difficult to conceive of the Earth in motion, so had to construct models where the Earth was stationary, and the other bodies moved. All assumed that the stars were eternally fixed, and did not change in any way. Kepler's helio-centric model, together with his proof that planetary velocites changed during their orbits of the sun, challenged all the cherished beliefs of astronomers of the day. His laws are one of the foundations of Isaac Newton's gravitational theories.
At this time, what we would describe as astronomy and physics were two very distinct disciplines. Astronomy was a branch of mathematics, and physics a branch of natural philosophy. Kepler described his astronomy as "Celestial Physics", although it included as its basis that God created the Universe and all it contained in a way that could be understood through reason.