Stock Photo - Ptolomy and Astronomy, 1503. - Description Ptolomy and Astronomy. Ptolemy was an ancient Greek astronomer and mathematician. He is widely credited with establishing the foundation of modern Western astronomy and astrology. Ptolemy’s work, Almagest, is a collection of books which discuss his astronomical theories. His major contributions included making use of the geocentric model of the universe, which places the Earth at the center of the universe, and introducing the use of epicyles (or small circles) to describe the motions of the planets. He also made many observations of the stars and planets, compiling them into star catalogs. Ptolemy’s work served as the foundation for the development of modern astronomy for centuries following his death.