Ptolemy & Urania. Coloured historical engraving of the goddess Urania (Goddess of Astronomy, at right) inspiring Ptolemy in his work. Ptolemy is holding a quadrant, a device for measuring the altitude of stars. Also seen in the image is an astrolabe (bottom left), a device to measure the altitude and position of celestial bodies. Ptolemy (c100-c170), was an Egyptian astronomer who devised a system that became known as the geocentric theory. This held that the Earth was at the centre of the Universe with the planets revolving about it. This view was unseated by Copernicus in 1543 when he showed that the Earth revolved around the Sun.