The credit for figuring out that orbits are ellipses goes to Johannes Kepler. He was a German astronomer who, building upon the work of Tycho Brahe, spent years meticulously analyzing Brahe's astronomical observations of Mars.

Here's how Kepler arrived at his conclusions:

* Tycho Brahe's Observations: Brahe had collected incredibly detailed and accurate data on the position of Mars in the sky over many years. Kepler inherited this data after Brahe's death.

* Challenging Circular Orbits: Kepler spent years trying to fit the data to a circular orbit, but he couldn't quite make it work. The discrepancies were small but significant.

* The Elliptical Solution: Kepler finally realized that the data fit perfectly to an elliptical orbit, where the Sun is not at the center but at one of the foci of the ellipse.

* Kepler's Laws: This discovery led him to formulate his three laws of planetary motion:

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

* Law of Areas: A line joining a planet to the Sun sweeps out equal areas in equal time intervals.

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

Kepler's work revolutionized our understanding of the solar system and paved the way for future discoveries in astronomy and physics.