Ptolemy's BooksSeveral of Ptolemy's works have survived, covering a broad range of subjects, and each having exerted varying degrees of influence in the history of science through antiquity, the Middle Ages, and into the Renaissance. Among these are the Harmonics, on musical theory, the Optics, and the Geography, which had a considerable influence on the work of Islamic geographers and, although it came late into the West, was immensely popular thereafter, so that although its geographical data were already becoming obsolete, its theories of map projection exerted enormous influence. The Tetrabiblos was the most popular astrological treatise in antiquity, and the Planetary Hypotheses, a cosmological work, provided a vision of the universe, of contiguous nested spherical "shells" within which the planets moved, that dominated Islamic and western medieval astronomical thought and was still present even with Kepler. These last two works were companion-pieces to the Syntaxis (also known as the Almagest), a comprehensive presentation of mathematical astronomy, and it was through this Ptolemy became one of the most important figures in all the history of astronomy.
The Syntaxis was, perhaps, the first complete synthesis of mathematical astronomy into a single work. It was certainly the most successful because it was quickly accepted as definitive, and if there were any earlier such attempts, they have not survived. This was Ptolemy's great contribution to astronomy. He did much more, however, than merely collate and systematize previous work. His models were sufficiently superior as to render his predecessors' obsolete, and in at least his development of kinematic models for the planets other than the sun and moon, he was a pioneer.
The importance of the Syntaxis in the history of astronomy is thus twofold, in its consequences for the survival of earlier material, and in its influence upon what was to follow.
Through Ptolemy's references to his predecessors the Syntaxis contains much of what we know of earlier Greek mathematical astronomy. By the fourth century AD, when commentaries had been written by Pappus and Theon, the Syntaxis had already become the definitive manual of mathematical astronomy, and earlier works were no longer copied.
Secondly, the Syntaxis dominated almost all subsequent astronomy for some 1500 years, in antiquity and the Middle Ages, in both the Islamic and Christian worlds. Islamic astronomers improved some parameters, and worked to eliminate certain geometrical elements of his models that they found philosophically distasteful (such as the equant point, seen as violating the principle of uniform circular motions in the heavens), but always these were variations from the standard models that Ptolemy had provided. Even Copernicus' heliocentric models comprised combinations of the same geometrical elements of uniform circular motion (except for the equant), and were thus cast in a very Ptolemaic mould.