Johannes Kepler proposed the three laws of planetary motion using observations by Tycho Brahe.
Kepler published his first two laws in 1609 in Astronomia nova, after analyzing the unusually precise measurements collected by the Danish astronomer Tycho Brahe. The first law states that planets move in ellipses with the Sun at one focus. The second says that a line from a planet to the Sun sweeps out equal areas in equal times.
Kepler published his third law in 1619. It states that the square of a planet’s orbital period is proportional to the cube of its orbit’s semimajor axis. Together, the laws replaced the older assumption that heavenly motion had to consist of perfect circles.
Kepler’s work built on Nicolaus Copernicus’s Sun-centered model and later helped Isaac Newton formulate universal gravitation. Kepler was not the first person to propose heliocentrism, and he did not use a telescope for the observations underlying his laws. His mathematical description of elliptical orbits was the decisive breakthrough.