in astronomy, model of the solar system centred on the Sun, with Earth and other planets moving around it, formulated by Nicolaus Copernicus (Copernicus, Nicolaus), and published in 1543. It appeared with an introduction by Rhäticus (Rheticus) as De revolutionibus orbium coelestium libri VI (“Six Books Concerning the Revolutions of the Heavenly Orbs”). The Copernican system gave a truer picture than the older Ptolemaic system, which was geocentric, or centred on Earth. It correctly described the Sun as having a central position relative to Earth and other planets. Copernicus retained from Ptolemy of Alexandria, although in somewhat altered form, the imaginary clockwork of epicycles and deferents (orbital circles upon circles), to explain the seemingly irregular movements of the planets in terms of circular motion at uniform speeds.