Claudius Ptolemaeus was an Egyptian mathematician, astronomer, and geographer who prospered in Alexandria during the second century CE. He was the author of numerous scientific treatises, of which three of those are still of importance to European and Islamic science. Almagest (astronomical treatise) is the first, the second being the Geography, a complete discussion of the knowledge of geography of the Roman world. Tetrabiblos (astrological treatise) is the third one. In Ptolemy’s mathematical field, the one that is the most famous may be Ptolemy's Theorem, which is the relation between two diagonals of a cyclic quadrilateral and the four sides (a quadrilateral whose vertices lie on a common circle).This theorem has several useful corollaries;
