Johannes Kepler

Johannes Kepler was a German astronomer and mathematician best known for formulating the three laws of planetary motion that describe how planets move around the Sun. Kepler worked as an assistant to the Danish astronomer Tycho Brahe and, after Brahe's death in 1601, inherited his extensive and unusually precise set of naked-eye observational data, which Kepler used to determine that planetary orbits are ellipses rather than perfect circles, a conclusion he published in his 1609 work Astronomia Nova as the first of his three laws. His second law described how a planet sweeps out equal areas in equal times as it orbits, moving faster when closer to the Sun, and his third law, published in 1619, established a precise mathematical relationship between a planet's orbital period and its distance from the Sun. Kepler's laws were later shown by Isaac Newton to follow directly from the law of universal gravitation, and they remain a foundational part of classical mechanics and modern astronomy.