Johannes Kepler discovered that the paths planets follow around the sun are ellipses, not perfect circles as previously thought.

This was a groundbreaking discovery and part of his famous three laws of planetary motion:

1. Law of Orbits: Planets move in elliptical orbits with the Sun at one focus.

2. Law of Areas: A line joining a planet and the Sun sweeps out equal areas in equal times.

3. Law of Periods: The square of a planet's orbital period is proportional to the cube of its average distance from the Sun.