Almagest is a 2nd-century Greek astronomical treatise written by Ptolemy. It is one of the most influential scientific texts of all time and served as the basis for all later Western astronomy up until the 16th century. The text describes the geocentric model of the universe, in which Earth is at the center and other celestial bodies revolve around it. It also includes mathematical models for predicting the positions of planets and stars, as well as detailed descriptions of each celestial object. Almagest was one of the first works to use trigonometry to calculate astronomical phenomena and remains an important source for modern astronomers.