Kepler used observations to show that the orbits of planets are elliptical, not circular as was previously believed.

He formulated three laws of planetary motion based on the detailed observations of Tycho Brahe:

1. Law of Ellipses: Planets orbit the Sun in elliptical paths, with the Sun at one focus of the ellipse.

2. Law of Areas: A line segment joining a planet to 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.

These laws revolutionized our understanding of the solar system and laid the foundation for Newton's law of universal gravitation.