Hipparchus (ca. 150 B.C.), perhaps the most notable of the Greek astronomers, proposed a system of circles to explain retrograde motion. By placing a planet on a small, rotating epicycle that in turn moved on a larger deferent, he was able to reproduce the behavior of the wandering stars. Furthermore, this system was able to explain the increased brightness of the planets during their retrograde phases as resulting from changes in their distances from Earth. Hipparchus also created the first catalog of the stars, developed a magnitude system for describing the brightness of stars that is still in use today, and contributed to the development of trigonometry. During the next two hundred years, the model of planetary motion put forth by Hip- parchus also proved increasingly unsatisfactory in explaining many of the details of the ob- servations. Claudius Ptolemy (ca. A.D. 100) introduced refinements to the epicycle/deferent

Hipparchus (ca. 150 B.C.), perhaps the most notable of the Greek astronomers, proposed a system of circles to explain retrograde motion. By placing a planet on a small, rotating epicycle that in turn moved on a larger deferent, he was able to reproduce the behavior of the wandering stars. Furthermore, this system was able to explain the increased brightness of the planets during their retrograde phases as resulting from changes in their distances from Earth. Hipparchus also created the first catalog of the stars, developed a magnitude system for describing the brightness of stars that is still in use today, and contributed to the development of trigonometry. During the next two hundred years, the model of planetary motion put forth by Hip- parchus also proved increasingly unsatisfactory in explaining many of the details of the ob- servations. Claudius Ptolemy (ca. A.D. 100) introduced refinements to the epicycle/deferent