Tychonic system

The Tychonic system (or Tychonian system) is a model of the universe published by Tycho Brahe in the late 16th century, which combines what he saw as the mathematical benefits of the Copernican system with the philosophical and "physical" benefits of the Ptolemaic system. The model may have been inspired by Valentin Naboth and Paul Wittich, a Silesian mathematician and astronomer. A similar cosmological model was independently proposed in the Hindu astronomical treatise Tantrasamgraha (c. 1500 CE) by Nilakantha Somayaji of the Kerala school of astronomy and mathematics.

It is conceptually a geocentric model, or more precisely geoheliocentric: the Earth is at the centre of the universe, the Sun and Moon and the stars revolve around the Earth, and the other five planets revolve around the Sun. At the same time, the motions of the planets are mathematically equivalent to the motions in Copernicus' heliocentric system under a simple coordinate transformation, so that, as long as no force law is postulated to explain why the planets move as described, there is no mathematical reason to prefer either the Tychonic or the Copernican system.