Kepler discovered that the orbits of planets around the Sun are not perfect circles, as was previously believed, but elliptical.

This was a revolutionary discovery, and it was a key step in understanding the motion of celestial bodies. Kepler's First Law of Planetary Motion states:

"The orbit of every planet is an ellipse with the Sun at one of the two foci."

This law challenged the long-held geocentric model of the universe, which placed the Earth at the center of everything. Kepler's findings helped pave the way for the development of the heliocentric model, which places the Sun at the center of the solar system.