Ptolemy’s Almagest

The last great astronomer of antiquity was Ptolemy, who lived in Alexandria in the second century A.D. He too made a survey of the heavens, being assisted by Hipparchus’s catalogue of stars and the fact that Aratus and others had grouped the stars into forty-eight star pictures or constellations. His great book, later known as the Almagest, summarized the main Greek achievements in astronomy and recounted in detail how all the heavenly bodies were supposed to revolve about the earth. The treatment was extremely involved, but it could scarcely have been otherwise. Ptolemy adopted without question the cherished Greek idea that the heavenly bodies moved with uniform motions in circular orbits or combinations of circular orbits. Actually, the moon and planets pursue elliptical orbits, their motions in those orbits are by no means uniform, and we observe them from an earth which is itself a moving body. But the great scope and rigorous treatment of the work ensured success, and it remained the basic text in astronomy until the close of the Middle Ages.

The Muslims, to whom we owe most of the star names, studied, commented upon, abridged, and modified the Almagest with great enthusiasm. Many readers, however, were baffled by its geometrical complexity. One classic example was Alfonso X, King of Castile and Leon, surnamed “The Wise”. His reaction was to remark that if God had asked his advice when He created the universe he would have suggested something less complicated. In Western Europe the Church gave almost complete approval to the cosmological ideas of Aristotle and Ptolemy. In fact, these ideas became so sacrosanct that to question them came dangerously near to blasphemy. Not until the 16th century did the first ripples of rebellion against the dogmas of Aristotle disturb the complacency of astronomical and religious thought.


Ptolemy, Renaissance Portrait