A model of the Solar System proposed by N. Copernicus in which the Sun lay at the centre with the planets orbiting around it; the stars lay at a vast distance beyond the planets. The model retained the circular orbits and epicycles of the Ptolemaic system, but incorporated Copernicus's own observations. It also contained elements from variants of the Ptolemaic system proposed by the Arab astronomers al-*Ṭūsī and Ibn al-Shāṭir (1304–75), which Copernicus apparently knew about. In Copernicus's model the motion of the sky results from the Earth's axial rotation. The Copernican system reproduced planetary motion no better than the Ptolemaic system did, because the concept of elliptical orbits had not yet been introduced, and until that advance by J. Kepler it found little acceptance. However, it was significant in removing the Earth from the centre of the Universe.