Kepler realized that the orbit of Mars is not a perfect circle, but an ellipse. This was a groundbreaking discovery that challenged the prevailing geocentric model of the universe, which assumed that planets moved in perfect circles around the Earth.

Here's what Kepler observed:

* Mars's position in the sky didn't always match predictions based on a circular orbit. He meticulously analyzed the observations of Tycho Brahe, his predecessor, and found discrepancies between the expected and actual positions of Mars.

* He experimented with different geometric shapes for the orbit. After years of calculations and trial and error, Kepler realized that an ellipse, not a circle, best fit the observations.

This led to Kepler's First Law of Planetary Motion, which states that planets move in elliptical orbits with the Sun at one focus. This discovery revolutionized our understanding of the solar system and paved the way for Newton's law of universal gravitation.