# The New Student's Reference Work/Ptolemy, Claudius Ptolemæus

**Ptolemy** (*tŏl'ē-mĭ*), **Clau'dius Ptolemæ'us**, an eminent mathematician and astronomer. He was born probably either in 70 or 77 A.D., and died at Alexandria in 147 A.D. As a mathematician he is especially to be remembered as having written the first treatise on trigonometry, a branch of mathematics invented by Hipparchus. In astronomy he wrote a complete treatment upon the entire subject as known in his time. This volume, *The Almagest*, remained a standard work until the time of Copernicus. Ptolemy assumed that the earth is spherical, is placed in the center of the spherical heavens and is a mere point compared with the distances of the fixed stars. As a geographer, Ptolemy was the first to point out the fact that the position of any point on the earth's surface can be accurately stated only after its longitude and latitude have been determined. In addition to these works he wrote a volume on optics, in which he stated the correct laws of reflection for plane surfaces and for concave mirrors as well as the law of refraction. Ptolemy must be considered as one of the earliest types of the modern scientific investigator.