Ptolemy

Claudius Ptolemy (c. 100-170 CE) was a Greek-speaking astronomer, mathematician and geographer working in Alexandria under Roman rule, and one of the most influential scientific authors who ever lived. Little is known of his life; his importance lies entirely in his books, which became standard authorities for well over a thousand years.

His astronomical masterwork, the Mathematike Syntaxis, is universally known by the name Arabic translators gave it, the Almagest, the Greatest. In it Ptolemy set out a complete mathematical model of the heavens on a geocentric basis: the Earth motionless at the centre, the Sun, Moon and planets carried on combinations of circles, deferents and epicycles, with the refinement of the equant, calibrated to reproduce the observed motions with real precision. It also preserved a catalogue of over a thousand stars arranged in the constellations.

The achievement was that the system worked: it predicted planetary positions well enough to serve astronomers, astrologers and calendar-makers, and so it endured. The Ptolemaic cosmos remained the accepted picture of the universe throughout the medieval Islamic and Christian worlds, until Copernicus displaced the Earth from the centre in 1543 and Kepler replaced the circles with ellipses.

If the Almagest governed how the heavens were mapped, Ptolemy's other great astronomical work governed how they were read. The Tetrabiblos, the Four Books, also called by its Latin name Quadripartitum, is the most important single treatise in the history of Western astrology, the work that gave the subject a systematic and quasi-scientific form.

Ptolemy presented astrology as a natural science, continuous with the astronomy of the Almagest: the heavenly bodies, he argued, exert real physical influences on the sublunary world, as the Sun governs the seasons and the Moon the tides, and the astrologer's task is to read those influences. He organized the doctrine of the planets, the signs, the aspects and the houses, and treated both the general astrology of weather and nations and the particular astrology of the individual nativity.

Because it came from the same towering authority as the Almagest, the Tetrabiblos lent astrology enormous prestige and became its foundational textbook, transmitted through the Islamic world and back into medieval and Renaissance Europe. Much of the technical vocabulary and structure of traditional Western astrology, material this atlas documents across its astrological entries, descends from Ptolemy's ordering of it.

Ptolemy's range extended well beyond the heavens. His Geography attempted to map the entire known world by coordinates of latitude and longitude, laying out methods of projecting the sphere onto a plane and listing the positions of thousands of places. Its coordinates were often inaccurate, and its underestimate of the size of the Earth would, much later, encourage Columbus to think Asia within reach across the Atlantic, but the idea of a mathematical, coordinate-based cartography was enormously influential when the work was recovered in the Renaissance. He also wrote on optics, on the mathematics of musical harmony, and on astronomical instruments.

His legacy is double-edged in a revealing way. The astronomical system of the Almagest, for all its long dominance, was in the end overthrown: the geocentric universe gave way to the heliocentric, and Ptolemy became the great example of an authoritative science later proved wrong in its foundations.

Yet the Tetrabiblos was never refuted in the same way, because astrology does not rest on the geocentric error alone, and it continued as a living practice long after the cosmos it assumed had been abandoned. Ptolemy thus stands at the head of two traditions with opposite fates, a discarded astronomy and an enduring astrology, both flowing from the same Alexandrian pen.