Ptolemy

Claudius Ptolemy, working in Alexandria around the middle of the second century, is the most influential author in Western astrology. He did not invent it, and the distinction is worth making.

What he wrote

Two works that belong together:

The Almagest — the astronomical handbook that makes the movements of the celestial bodies calculable. For fourteen centuries it remained the standard work of astronomy, until Copernicus.

The Tetrabiblos — “four books”, the interpretation. Books I and II cover the foundations and mundane astrology, Books III and IV natal astrology.

The order is deliberate: first calculate, then interpret.

What sets him apart

He is the first to try to ground astrology in natural philosophy instead of passing it on as tradition. His explanation runs through the four qualities: the celestial bodies act on the sublunary world through heat, cold, dryness and moisture, as the Sun visibly does with the seasons.

That is why he rejects everything that cannot be explained in those terms — the lots, part of the time-lord techniques, much of the tradition that other authors carry with them.

Why he was not the first

Centuries of practice lie behind him. Dorotheus of Sidon wrote his didactic poem in the first century; Vettius Valens, his contemporary, left a collection containing hundreds of genuine charts. The foundations — ascendant, houses, dignities, sect — are older than he is.

Ptolemy is a reformer, testing an existing practice and cutting away whatever his theory will not carry.

Why this matters in practice

Because his version is not always the widespread one — and the difference becomes tangible.

Most clearly with the terms (termini): Ptolemy gives a table of his own, one that departs from the Egyptian table almost every other author uses. By one table a planet stands in its own term, by the other it does not. Anyone who interprets a term must say which table they are following.

That Ptolemy counted as the authority in the West for so long comes down to the weight of the Almagest: whoever was right about astronomy was believed about interpretation as well.

What to read

The Tetrabiblos exists in several translations and is the best way into the sources — but it is the theoretical account. Anyone wanting to see how the work was actually done in the second century reads Valens alongside it: less orderly, full of real cases.