How did Johannes Kepler's work challenge the belief in uniform circular motion?

Johannes Kepler challenged the long-held belief in uniform circular motion by prioritizing empirical accuracy over traditional Platonic ideals. While he initially sought to refine planetary models using circular paths, his analysis of Tycho Brahe's highly precise observations of Mars revealed that circular orbits could not account for the data within the expected margin of error.123

Kepler’s departure from this tradition was formalized in his laws of planetary motion:

• Elliptical Orbits: He discovered that planets move in ellipses with the Sun at one focus, rather than in perfect circles.143
• Variable Speed: He established that a planet’s speed is not constant; instead, a line connecting the planet to the Sun sweeps out equal areas in equal times. This meant that planets move faster when closer to the Sun and slower when farther away, directly contradicting the requirement of uniform speed inherent in circular motion.123

By abandoning these constraints, Kepler shifted astronomy toward a physical science, prompting the search for the forces that govern planetary movement.3