Ptolemy

← Encyclopedia

Claudius Ptolemy was a Greek-speaking scholar of Roman Egypt, active in Alexandria around the middle of the second century, whose mathematical works governed how Europe and the Islamic world pictured the cosmos for some fourteen hundred years. Almost nothing is known of his life — not his birthplace, not his teachers, not the years of his birth or death. What survives is the work, and the work was enough to make his name a synonym for a model of the universe.

His astronomical treatise, which the Arabic tradition called al-majisṭī — “the greatest” — and which came back to Latin Europe as the Almagest, set the Earth motionless at the centre and the planets, sun, and stars wheeling about it on combinations of circles. It was not a sketch but a working machine: tables, geometric arguments, and observational records that let an astronomer compute where any heavenly body would stand on any future night. That predictive power is why the system held so long. It worked, to the accuracy the instruments of the age could check, and a model that works is hard to abandon. Only Copernicus, Kepler, and Galileo unseated it, and the “Ptolemaic system” became, in their hands, the name for the order they overturned.

Ptolemy wrote across the mathematical sciences — a Geography that mapped the known world by coordinates and shaped cartography for a millennium, and works on optics, music, and the projection of the sphere. For the later esoteric tradition the decisive book was the Tetrabiblos, his treatise in four parts on astrology. There he argued that the heavens act on the sublunary world as plainly as the sun governs the seasons and the moon the tides, and that the art of reading those influences, while fallible, was a legitimate branch of natural philosophy rather than superstition. He was careful to claim less than his successors would: he framed astrology as conjecture about tendencies, not iron decree. That guarded defence, from the same author who had given astronomy its rigour, lent the practice an authority it carried through Arabic astrology and into the Latin Middle Ages and Renaissance, where the Tetrabiblos circulated as the discipline’s standard text.

Modern scholarship draws a line his readers did not. Astronomy and astrology were for Ptolemy two halves of one inquiry — the first establishing the positions of the stars, the second their meaning for earthly life — and only later centuries split the predictive science from the predictive art. Some historians have charged that he doctored or borrowed observations in the Almagest; the dispute is unresolved, and turns on standards of evidence the ancient world did not share. What is not in dispute is the reach. Few figures have furnished both a science later proved wrong and a science still practised, and stood at the head of each for as long as he did.

→ Related: Divination · Galileo Galilei · Pietro D Abano · Neoplatonism