The Moon in Ptolemy's Tetrabiblos: moisture, soul and childhood

In the second century AD, in Alexandria, Claudius Ptolemy wrote two works that would dominate the study of the sky for more than a thousand years: the Almagest, devoted to mathematical astronomy, and the four-book astrological treatise known as the Tetrabiblos. In the latter the Moon holds a central place. Ptolemy does not describe her as a deity to be worshipped, but as a celestial body with precise physical qualities, able in his view to act on the earthly world in a natural and regular way.

A light that moistens

In the first book Ptolemy gives each planet a combination of the four Aristotelian qualities: hot, cold, moist and dry. The Moon, he writes, is above all moist, because she is close to the Earth and immersed in the exhalations rising from the ground; she warms only slightly, through the light she receives from the Sun. From this nature came her feminine and nocturnal character. Together with Venus, the Moon belonged to the nocturnal «sect», opposed to the diurnal one led by the Sun.

Ptolemy also looked for empirical arguments to support lunar influence. At the start of the work he recalls that the tides rise and fall with the Moon, and that plants and animals seem to respond to her phases. Such observations, widespread in antiquity, made plausible the idea of a sky acting upon the Earth. On this basis the author presented astrology as a conjectural science, comparable to the medicine of his day, rather than as a form of religious divination.

Body, soul and the ages of man

In the third book, devoted to nativities, the Moon takes on precise functions. When judging the qualities of the soul, Ptolemy assigns the rational part to Mercury and the sensitive, irrational part to the Moon, linked to impulses and perceptions. The Moon is also, with Venus, the significator of the mother, while the Sun and Saturn indicate the father. This division reappears almost unchanged in the medieval and Renaissance manuals that commented on the treatise.

In the fourth book Ptolemy divides human life into seven ages, each governed by a planet according to the order of the spheres. To the Moon, the closest to the Earth, belong the first four years: infancy, the age of rapid growth, moisture and changeability. Mercury, Venus and the Sun follow, and so on up to Saturn, lord of old age. The scheme enjoyed great success and was taken up by Arabic and Latin commentators throughout the Middle Ages.

The legacy of the treatise

The Tetrabiblos was translated into Arabic and then, in the twelfth century, into Latin under the title Quadripartitum; one of the first versions was made by Plato of Tivoli. Medieval astrologers often read it together with the commentary of Ali ibn Ridwan, an eleventh-century Egyptian physician known in the West as Haly. Alongside the authentic work circulated the Centiloquium, a collection of a hundred aphorisms attributed to Ptolemy but almost certainly later, in which the Moon appears frequently.

For today's scholars the value of the Tetrabiblos is above all historical. It shows how a great astronomer of the imperial age could combine Aristotelian physics, the observation of tides and the Babylonian inheritance into a coherent system. The Moon appears in it as the link between the unchanging heavens and the earthly world of change, an idea that Greek philosophy had already formulated and that Ptolemy translated into the technical language of astrology.

Reading Ptolemy's pages on the Moon today means entering a vision of the cosmos in which physics, medicine and astrology were not yet separate. Her moist qualities, her bond with the mother and with infancy, her role as guide of the night remained for centuries the starting point of every astrological discussion of our satellite, and they explain many images still alive in the European tradition.