Ptol·e·my1 /ˈtɒləmi $ ˈtɑː-/ (?AD100-AD170) a Greek astronomer and mathematician who lived and worked in Egypt. He studied the stars, and believed that the Earth was at the centre of the universe, and that the stars, the Sun, and the planets all travelled in circles around the Earth. This idea is known as the Ptolemaic System, and it was generally believed to be true until Nicholas Copernicus proved that it was wrong at the beginning of the 16th century.