# Johannes Kepler (1571–1630)

Wanted to show that God had created the heavenly bodies according to a regular, beautiful, symmetric design. Went through the observations of Tycho Brahe, postulated an increasing number of circles in Mars' orbit, until he had got it down – but it was a complicated theory. There were small errors ascribable to measurement, and thankfully Kepler did not want to believe that Brahe had measured imperfectly, so he kept working until in desperation he postulated an elliptical orbit. This fit perfectly!

Before, everyone assumed the orbits were circles – even if you had to give the planets many different circular orbits they could be in at different times. Shaping it as an ellipse enabled there to be only one orbit. But few people took such a thing as an elliptical orbit seriously – it seemed too imperfect for heavenly bodies.

He explained the elliptical orbit to Galileo himself, who rejected it. Now we accept it as Kepler's First Law.

- Kepler's First Law: The orbit of any planet is elliptical. They can be
*almost*circular, but never perfectly circular. - Kepler's Second Law: planets move fastest when they are closest to the sun, and slower when far away.
- Kepler's Third Law: The time for an orbit increases the further the planet is from the sun.