Much of what we know about Greek astronomy comes to us through Claudius Ptolemy (c. 100-170) , whose 13-volume treatise on astronomy, THE ALMAGEST, compiled the achievements of his predecessors. But he is most famous for advancing the first general theory of cosmology—“the study of the structure and motions of the universe”—although his work drew heavily on that of perhaps the greatest Greek astronomer, Hipparchus. His geocentric model explained both the structure of the known universe (essentially the seven “planets”) and how those planets moved through space.