Circular Martian Orbit

Like scientists from two thousand years before, Kepler also believed that each planet revolving around the sun had a circular orbit. When Kepler first started studying the orbit of Mars, he tried to show that the planet had a circular orbit. However, his research and two of Tycho Brahe's extensive observations of the orbit of Mars showed that this orbit was a different geometric curve and not a perfect circle.

"Kepler's Mars Orbit Drawing" was used to help come to the conclusion about the martian orbit. (Britannica)