Geocentrism is the belief that the Earth is fixed at the centre of the Universe. Geocentrists accept that the earth is round. Before the 16th century most people believed in the theory of geocentrism. From Earth, it looks like the Sun and stars are moving across the sky. In about 150 AD the Greek astronomer, Ptolemy wrote a book to explain in great detail how the spherical Earth is surrounded by things that move in the sky. From the time of Ptolemy through to the 16th Century AD educated people who knew the earth is round almost always believed the Geocentric theory of Ptolemy.