In 1601 the German astronomer Johannes Kepler became director of the Prague Observatory. Kepler had been helping Tycho Brahe in collecting 13 years of observations on the relative motion of the planet Mars. By 1609 Kepler had formulated his first two laws:

i. Each planet moves on an ellipse with the sun at one focus.
ii. For each planet, the line from the sun to the planet sweeps out equal areas in equal times.