Maestlin was a proponent of the heliocentric model of the solar system and planets introduced by astronomer Nicolaus Copernicus. Johannes Kepler, too, accepted this theory immediately, seeing the hand of God in its simplicity and becoming interested in astronomy.
Giving up his plan of becoming a clergyman, Johannes Kepler departed the University of Tubingen in 1594 to accept a chair in mathematics and astronomy at the university in Graz, Austria. After taking this position, he developed a complex hypothesis to explain the distance between the orbits of planets. (He, like so many before him, mistakenly believed these orbits were circular. Still, his early calculations matched the observational evidence within 5%. In his later work, he altered his thinking to believe that planetary orbits are elliptical.)
Johannes Kepler next theorized that the sun emitted a constant force across the planes of a planet's orbit, which diminished with distance. He believed this force pushed the planets around their orbits. In 1596, he published these theories in a treatise called Mysterium Cosmographicum (Cosmographic Mystery). This was the first written defense of the Copernican model, which used geometric calculations as evidence.
Although Danish astronomer Tycho Brahe did not subscribe to the Copernican theory, himself, he was so impressed with the work of Johannes Kepler that in 1600, he invited Kepler to become his assistant. Brahe, the mathematician at the court of Emperor Rudolph II at Prague, was one of the most prolific observers of the cosmos. At the time of his death, one year after Johannes Kepler became his assistant, the data he collected during his lifetime was far superior to any others made prior to the invention of the telescope.
After Tycho Brahe's death, Johannes Kepler stepped into his role of imperial mathematician and court astronomer. He remained in this position until he became mathematician to the states of Oberosterreich (upper Austria) in 1612.
During his working years, Johannes Kepler was a brilliant astronomer and mathematician and a prolific writer. His first major contribution came in the form of a treatise on the theory of optics, just two years after he stepped into Brahe's shoes.
His next major work was entitled Astronomia Nova (New Astronomy), which was published in 1609. Using the observations of his predecessor along with his own, he was able to calculate the orbit of the planet Mars. Forgetting all previous notions of circular planetary orbits, he theorized that the planets move in elliptic orbits with the sun at one focus. This was the first of Kepler's so-called Laws of Planetary Motion. The second, which also appeared in this work is called the area rule, which says that a hypothetical line from the sun to a planet sweeps out equal areas of an ellipse during equal intervals of time, meaning that the closer a planet comes to a sun, the faster it moves.
Some of the other work Johannes Kepler accomplished during his stint in Prague included a book enthusiastically accepting and expanding on Galileo's observations using a telescope. He also completed a treatise on optics as telescope lenses the year before his departure to Linz.