The idea that planetary orbits are not perfect circles, but rather elliptical (flattened circles), was worked out by Johannes Kepler. He formulated his three laws of planetary motion in the early 17th century:

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 intervals of time.

3. Law of Periods: The square of the orbital period of a planet is proportional to the cube of the semi-major axis of its orbit.

Kepler's work was groundbreaking, challenging the prevailing geocentric model of the universe and paving the way for Isaac Newton's later work on gravity.