Kepler, Tycho and the Laws of Planetary Motion

Copernicus had placed the Sun at the center, but his circular orbits still did not match the sky precisely. The Danish astronomer Tycho Brahe spent decades recording planetary positions with unprecedented accuracy, all without a telescope. After Tycho's death in 1601, his assistant Johannes Kepler used this data, especially on Mars, to test model after model.

In 1609 Kepler announced that planets move in ellipses, not circles, and that they sweep out equal areas in equal times, moving faster when nearer the Sun. In 1619 he added a third law linking a planet's orbital period to its distance from the Sun. Kepler's willingness to abandon the ancient ideal of perfect circles when the data disagreed made his work a landmark: theory had to yield to careful measurement. His laws later became a key test for Newton's theory of gravity.