Ptolemy

Who they were

Claudius Ptolemy (c. 100 – c. 170 CE) worked in Alexandria and produced the two books that defined celestial study in the West for well over a thousand years: the Almagest, a complete mathematical astronomy, and the Tetrabiblos, the foundational textbook of horoscopic astrology.

He also wrote on geography, optics and music theory. Almost nothing is known about his life.

Where they came from

Alexandria in the second century was the intellectual centre of the Mediterranean, with access to Babylonian observational records, Egyptian calendrical material and the whole Greek mathematical tradition. Ptolemy's achievement was synthesis: he took several centuries of accumulated observation, much of it from Hipparchus, and built a single geometrical system that predicted planetary positions accurately enough to be useful.

What they taught

The Almagest. A geocentric model in which planets move on epicycles carried by deferents, with a device called the equant to account for varying speed. It was mathematically sophisticated and it worked — it predicted positions well enough that it was not seriously displaced until Kepler, fourteen hundred years later.

The Tetrabiblos. Ptolemy's case is that astrology is a natural science, difficult and fallible but legitimate, and his argument is by physical analogy: the sun manifestly drives the seasons and ripens crops, the moon manifestly moves the tides and affects living things, so celestial bodies evidently act on terrestrial ones — and the remaining planets should be expected to do so too, in subtler ways.

He was careful about the limits. He compares astrology to medicine and to navigation: expert practices dealing with variable material, in which error is common and the practitioner's skill matters. He explicitly denies rigid determinism, allowing that celestial inclination can be resisted. And he attacks bad practitioners at length — the opening book is partly a defence of astrology against the charge that it is what its worst exponents make of it.

The tropical zodiac. Ptolemy defined the zodiac from the equinoctial point rather than from the stars, knowing about precession, and gave the reasoning. Western astrology follows him in this, which is why the disagreement with the Indian tradition in tropical and sidereal traces back to a decision he made and justified.

What we can and cannot say

We can say the Almagest is one of the great works of ancient science — superb mathematics built on a false cosmology, which is a combination worth sitting with. The model was wrong about what the solar system is and right enough about where things would appear to be that it served navigators and calendar-makers for over a millennium.

We can say the Tetrabiblos is still the reference point for Western astrology, and that Ptolemy's argument for it is the best pre-modern one available. It is also where the argument fails. The analogy holds for the sun and moon, which exert large and measurable effects, and breaks for everything else: the planets' gravitational and electromagnetic influence at the earth's surface is negligible — the delivering midwife exerts more gravitational force on a newborn than Mars does. Ptolemy could not have known that, and his reasoning was sound given what he did know.

We should note a live scholarly dispute. Robert Newton's The Crime of Claudius Ptolemy (1977) argued that he fabricated observations, presenting calculated positions as observed ones. Most historians of astronomy regard the charge as overstated and shaped by anachronistic expectations about data reporting, but there are genuine problems with some of his figures, and the question of how much he observed versus adopted from Hipparchus is not settled.

We cannot separate the two halves of his work in the way modern categories want. He wrote them as companion volumes, and to him they were one enterprise: establish where the celestial bodies are, then determine what their positions signify. The division that makes one of those books science and the other superstition was drawn fifteen hundred years after he died.

Further reading

• Tetrabiblos, Book I — the defence of astrology, and shorter than its reputation.
• Almagest, in G.J. Toomer's translation (1984).
• Alexander Jones (ed.), Ptolemy in Perspective (2010) — on the fabrication dispute and much else.