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.