Background

Tycho Brahe (1546-1601) was a Danish astronomer who is best known for his discovery of a supernova in the constellation Cassiopeia in 1572. This new star was so bright that it was even visible during the day. The appearance of this bright star challenged astronomers' view of the universe because it was believed to be static and unchanging. King Frederick II, King of Denmark, provided Brahe with funds in order to build an astronomical observatory on the island of Hven. It was here at the Uranienborg Castle Observatory, that with no other instruments other than a globe and a compass, Brahe discovered huge errors in the standard astronomical tables. This lead Brahe to remeasure all the 788 different stars of Ptolemy's great star catalog. The data Brahe accumulated was superior to all other astronomical measurements made until the invention of the telescope in the early 17th century.

Brahe's Theory of Planetary Motion

Brahe never really accepted the Copernician theory of the universe and looked for a new way that would resolve it with the older Ptolemaic system of the universe. Brahe devised a system that put the Earth in the center of the universe. The five known planets circled the Sun, which in turn orbited around the Earth once a year. The stars, Brahe believed, revolved around the Earth once every day. Even though Brahe's theory of planetary motion was incorrect, Johannes Kepler, who was his assistant from 1600 until Brahe's death in 1601, used the data that he collected about the stars in developing the correct description of planetary motion.