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Copernican (heliocentric/Sun-centred) system of the Universe, 1761. Illustration showing the orbits of the planets around the Sun. The orbits of the moons of Earth, Jupiter and Saturn are also shown. Prior to the work of the Polish astronomer Nicolaus Copernicus (1473-1543), the Earth was considered to be the stationary centre of the universe, a notion first advocated by the Egyptian astronomer Ptolemy. Copernicus' pioneering work The Revolutions of Celestial Spheres, 1543, describes his idea of a Sun-centred universe, in which the Earth is merely one of the planets revolving around the Sun and rotating on its axis. (Paris, 1761). (KEYSTONE/HERITAGE IMAGES/Ann Ronan Pictures)