In the year 1600, Johannes Kepler initially becomes the assistant of Tycho Brahe in Prague. A year later, after Brahe’s death, he advances to the position of imperial court mathematician. This gives him access to Brahe’s life’s work, the most extensive and accurate observations of star movements of his time. From the data, he can deduce that the motion of the planets in the heliocentric system is considerably simplified, the planetary orbits become approximately circular. We show how these orbits can be described by Kepler's laws and finally trace these laws back to Newtonian mechanics.