The planets travel around the Sun in perfect circles.

Copernicus moved Earth and the other planets around the Sun, but he preserved one of astronomy’s oldest assumptions: heavenly motion should be built from perfect circles.

Tycho Brahe’s exceptionally precise observations made that increasingly difficult. Johannes Kepler inherited Tycho’s measurements of Mars and spent years trying to reproduce them with combinations of circular motion.

A stubborn discrepancy remained. It was small by ordinary standards but too large to dismiss given the precision of the observations.

Kepler eventually abandoned the circle and found that an ellipse, with the Sun at one focus, matched Mars far better. In 1609 he published what became his first two laws of planetary motion.

The change is important because heliocentrism and elliptical orbits were separate conceptual steps. Copernicus had relocated the center of planetary motion without discarding circular perfection.

Many planetary orbits are close enough to circles that circular approximations remain useful. A circle is itself a special case of an ellipse. What Kepler removed was the requirement that celestial motion must be perfectly circular.