* Nicolaus Copernicus (1473-1543): While Copernicus is famous for proposing the heliocentric model, he still maintained that planets moved in perfect circles around the Sun. He believed this because circular motion was considered the most "perfect" motion in the ancient Greek tradition.
* Tycho Brahe (1546-1601): A meticulous observer, Tycho Brahe collected incredibly precise data on the positions of planets, especially Mars. This data was later crucial in proving that the orbits were not perfectly circular.
* Johannes Kepler (1571-1630): Kepler analyzed Tycho Brahe's data and, after years of work, formulated his three laws of planetary motion. These laws stated:
1. Planets move in elliptical orbits with the Sun at one focus.
2. A line joining a planet to the Sun sweeps out equal areas in equal times.
3. The square of a planet's orbital period is proportional to the cube of its average distance from the Sun.
So, while Copernicus provided the framework for the heliocentric model, Kepler was the one who conclusively demonstrated that planetary orbits were elliptical, not circular. This discovery marked a significant advancement in our understanding of the solar system and paved the way for Newton's later work on gravity.
