His masterpiece is known by its Arabic title of the Almagest, a complete treatment of astronomical knowledge in thirteen books. This work dominated astronomical theory in Byzantium, the Islamic world, and medieval Europe. Ptolemy acknowledges debts to Apollonius and Hipparchus of Rhodes, and his writing is an important source for the history of astronomy. Ptolemy also wrote extensively on geography, where he was probably the first to use systematic coordinates of latitude and longitude, and his work was not superseded until the sixteenth century. Similarly in musical theory his treatise on Harmonics is a detailed synthesis of Pythagorean mathematics and empirical musical observation.