Why did Johannes Kepler reject circular orbits for Mars?

Johannes Kepler rejected circular orbits for Mars because they could not accurately account for the precise observational data he inherited from Tycho Brahe.12

While Kepler initially attempted to model the orbit of Mars using circular paths—consistent with the traditional Ptolemaic and Copernican models—he found that these circular theories failed to match Tycho’s observations within a margin of 8 minutes of arc. Because he trusted the accuracy of Tycho’s data to within 2 minutes of arc, he concluded that the circular model was fundamentally flawed.2

Mars was particularly critical to this discovery because it possesses one of the most eccentric orbits in the solar system. By reexamining the fundamentals of planetary motion, Kepler determined that planets do not move in circles but rather in elliptical orbits with the Sun at one focus. This realization, detailed in his work Astronomia Nova, allowed him to reconcile the theoretical model with the high-precision observations of the time.132