Tychonic System

Tychonic system, scheme for the structure of the solar system put forward in 1583 by the Danish astronomer Tycho Brahe. He retained from the ancient Ptolemaic system the idea of Earth as a fixed centre of the universe around which the Sun and Moon revolved, but he held that, as in the newer system of Copernicus, all other planets revolved around the Sun. In both the Tychonic and the Ptolemaic systems, an outer sphere containing the fixed stars was considered to revolve every day around Earth. The Tychonic theory explained the observed variations of phase of Venus, for which the Ptolemaic system had no explanation.

In the 19th century it was believed that a system somewhat similar to Tycho’s had been proposed in the 4th century bce by the Greek philosopher Heracleides Ponticus in which Venus (and likely Mercury) went around the Sun. However, closer reading of classical sources has suggested it is most likely that this was a misreading of Heracleides Ponticus’s discussion of Venus appearing in the sky sometimes in the evening after sunset and other times in the morning before sunrise.