While Nicolaus Copernicus was the first to propose that planets orbit the Sun (heliocentric model), he believed those orbits were perfect circles.
Kepler, through meticulous analysis of astronomical observations made by Tycho Brahe, realized that planetary orbits were actually elliptical, not circular. He formulated his three laws of planetary motion, with the first law stating that the planets orbit the Sun in elliptical paths with the Sun at one focus.
