Planets don't move in circles — one astronomer proved they trace ellipses

Using decades of naked-eye observations inherited from Tycho Brahe, Johannes Kepler discovered that planets sweep out equal areas in equal times along elliptical, not circular, orbits. His third law linked orbital period to distance from the Sun, and later gave Newton the pattern gravity had to explain.

— Johannes Kepler, Astronomia Nova — 1609 (first two laws); Harmonices Mundi, 1619 (third law)