Ptolemy is most famous for his ground-breaking work The Almagest, which is a treatise on mathematics and astronomy. It described the structure of the heavens as it was understood at the time. In Ptolemy’s day that meant that the earth was the center of the universe, and that the sun and other planets revolved around the earth. Ptolemy’s “geocentric” system was accepted for some 1,400 years.In the Ptolemaic system, the earth was not described as flat as was the general belief but as a sphere. Although the system was incorrect in that it was an earth-centered system in which the earth did not move, and the stars fixed, it nevertheless was highly useful , such as navigation and calculating astronomical events.