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Why did Kepler reject circular orbits for Mars?

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Geocentric system — Celestial mechanics Ptolemaic diagram of a geocentric system, from the star atlas *Harmonia Macrocosmica* by the cartographer Andreas Cellarius, 1660.

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

While Kepler initially assumed a circular path, he found that his models consistently failed to match Tycho’s observations by about 8 minutes of arc. Because he trusted the accuracy of Tycho’s data—which he believed was precise 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, making its departure from a perfect circle more pronounced and easier to detect than that of other planets. By reexamining the fundamentals of planetary motion, Kepler determined that planets move in ellipses with the Sun at one focus, a conclusion that formed the basis of his first law of planetary motion.12