Johannes Kepler discovered the laws of planetary motion using observations by Tycho Brahe.
Kepler was a German mathematician and astronomer, while Tycho Brahe was a Danish nobleman whose exceptionally precise naked-eye observations provided the essential evidence. After joining Brahe in Prague in 1600, Kepler gained access to years of measurements, especially those recording the motion of Mars.
Kepler published his first two laws in Astronomia nova in 1609 and his third law in Harmonices Mundi in 1619. The laws showed that planets travel in ellipses, sweep out equal areas in equal times, and have orbital periods related to their distances from the Sun. These results replaced the older assumption that planetary paths had to be perfect circles.
Kepler’s work supplied a mathematical foundation for Isaac Newton’s later theory of universal gravitation. A common mix-up is crediting Brahe with the laws themselves: Brahe collected the observations, but Kepler analysed them and formulated the laws.