Why did Mars help Johannes Kepler discover that planetary orbits are elliptical?

Mars was instrumental in Johannes Kepler’s discovery of elliptical orbits because of its high orbital eccentricity. While most planets have orbits that appear nearly circular, the departures from circularity in the orbit of Mars are significant enough to be detectable through precise observation.1

When Kepler arrived in Prague to work with Tycho Brahe, he gained access to Tycho’s extensive and highly accurate observational records of Mars. Kepler initially attempted to fit these observations into traditional models based on circular orbits, but he could not reconcile the data, finding a discrepancy of about 8 minutes of arc—a margin of error he knew exceeded the accuracy of Tycho’s work.12

Because Mars and Mercury are the only planets with eccentricities large enough to make these deviations from circularity appreciable, and because Mercury is too close to the Sun to observe easily, Mars provided the necessary data to force Kepler to abandon circular models and conclude that planetary paths are ellipses.1