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Liber VII · Divination

Liber VII · Divination · Chapter I: Astrology

Ptolemy's Tetrabiblos: The Classic Textbook of Astrology

Ptolemy's Tetrabiblos explained: the second-century treatise on astrology, its four books, natural philosophy, tropical zodiac, transmission and legacy.

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The Tetrabiblos ("Four Books") is a treatise on astrology written in Greek by the Alexandrian astronomer Claudius Ptolemy in the second century CE. It explains astrology as a natural science of celestial influences, sets out the qualities of the planets and signs, and applies them to whole peoples, to weather and to individual nativities, and for some fifteen hundred years it was the most authoritative astrological book in the Islamic world and Europe.

Claudius Ptolemy and the Purpose of the Work§

Claudius Ptolemy (c. 100–c. 170 CE) worked in Alexandria under Roman rule; the observations recorded in his works date from the 120s to the 140s CE. He wrote on astronomy, geography, optics and music, and his Mathematical Syntaxis, known from its Arabic title as the Almagest, became the standard model of planetary motion until Copernicus. The Tetrabiblos, whose Greek manuscripts call it Apotelesmatika ("on effects" or "prognostics"), was conceived as its sequel. The Almagest shows how to calculate where the heavenly bodies are; the Tetrabiblos asks what their positions mean.

Ptolemy opens by distinguishing two kinds of knowledge of the heavens. The first, astronomy proper, is exact. The second, prognostication, is less certain because it deals with the changeable nature of earthly things. He then answers two common objections: that astrology is impossible, and that even if possible it is useless. To the first he replies that everyone sees the Sun govern the seasons and the Moon affect tides and growth, so it is reasonable that the other planets also act on the air and on living beings. To the second he replies that foreknowledge, like medicine, lets people prepare for what is coming and soften what can be softened. The defence places astrology among the natural sciences of his day, a theme introduced in our guide to astrology.

The Natural Philosophy of Celestial Influence§

Ptolemy explained astrology through Aristotelian physics and the medical theory of the four qualities: hot, cold, moist and dry. The Sun heats and somewhat dries; the Moon chiefly moistens, because of her nearness to the moist vapours of the earth; Saturn chiefly cools and somewhat dries, being far from the Sun; Mars dries and burns; Jupiter and Venus are temperate, warming and moistening, and so are benefic; Mercury changes its nature according to the planets it is joined with. Planets are classed as beneficent or maleficent, masculine or feminine, diurnal or nocturnal.

The signs of the zodiac receive their natures from the seasons they mark and from the planets that rule them. The great Ptolemaic schemes of domicile rulership, exaltation, triplicity and "terms," the unequal divisions of each sign assigned to the five planets, are laid out here. Ptolemy criticised the Egyptian and Chaldean systems of terms that he had received and proposed a third set, which he said he had found in an old and damaged manuscript. Aspects, the angular relationships between planets, are justified by the ratios of musical harmony: opposition, trine, square and sextile. The system of signs and planets is summarised in our guide to the zodiac and planets.

The tropical zodiac§

One short passage had long consequences. Ptolemy stated that the signs take their beginnings from the equinoxes and solstices, so that Aries begins at the spring equinox whatever the stars behind it. Because of the precession of the equinoxes, this zodiac slowly drifts away from the constellations. Western astrology has generally followed Ptolemy; Indian astrology uses a sidereal zodiac tied to the stars, as described in our article on Vedic astrology. On the Western horoscope pages of this site, the signs are tropical in Ptolemy's sense.

The Four Books: Nations, Weather and Nativities§

The work divides into general and particular astrology. Book I contains the foundations already described. Book II treats "universal" or mundane astrology: events affecting whole countries and cities. It begins with an astrological ethnography, assigning the regions of the inhabited world to triplicities and planets and deriving the supposed characters of peoples from them; historians read it today as a window on Roman-era ideas of climate and ethnicity, including their prejudices. It continues with eclipses as the chief signs of public events, and ends with forecasts of weather from new and full moons, the solstices, comets and the appearance of the sky.

Books III and IV concern genethlialogy, the reading of an individual's birth chart. Ptolemy discusses whether the moment of conception or of birth is decisive, and gives a method for correcting an uncertain ascendant from the preceding new or full moon. He then works through a sequence of topics: parents, siblings, sex, twins, abnormal births, length of life, bodily form and illness, the quality of the soul, wealth, rank, profession, marriage, children, friends and enemies, travel and the manner of death. The final chapter divides human life into seven ages, each ruled by a planet in order, from the Moon's infancy to Saturn's old age.

Ptolemy's Place in Hellenistic Astrology§

Ptolemy was not a practising astrologer in the way that his near contemporary Vettius Valens was. Valens's Anthology is full of example charts, technical shortcuts and personal remarks; the Tetrabiblos contains no example horoscopes at all and is written as a rational synthesis. Ptolemy drops or downplays much of the working practice of his time, including most of the "lots" (calculated points such as the Lot of Spirit, though he keeps the Lot of Fortune) and much of the system of places or houses, which he treats only briefly. Modern historians, such as those who have retranslated the Hellenistic astrologers since the 1990s, therefore caution against treating the Tetrabiblos as typical of ancient practice. It represents what a philosophically trained astronomer thought astrology ought to be.

Ptolemy's authority also attracted spurious works. The Centiloquium (in Greek Karpos, "Fruit"), a collection of a hundred aphorisms, circulated under his name throughout the Middle Ages but is now regarded as a later compilation. A Greek Paraphrase attributed to the Neoplatonist Proclus restated the Tetrabiblos in simpler language and was itself influential.

Arabic, Latin and Renaissance Transmission§

The Tetrabiblos was translated into Arabic by the ninth century and became a foundation of the astrology of the Abbasid period. The Egyptian physician and astrologer ʿAlī ibn Riḍwān (d. c. 1061) wrote a full commentary on it, which was later translated into Latin and printed alongside the text. In 1138 Plato of Tivoli made a Latin translation from Arabic, under the title Quadripartitum, and other versions followed in the thirteenth century. For scholastic writers it was the classic proof that astrology rested on natural causes, which helped distinguish acceptable "natural" astrology from forbidden forms of divination.

The Greek text was first printed at Nuremberg in 1535 by Joachim Camerarius, and Philipp Melanchthon, who regarded astrology as part of natural philosophy, supported later Latin editions. Renaissance critics such as Giovanni Pico della Mirandola attacked astrology but still had to reckon with Ptolemy. Johannes Kepler, who cast horoscopes, proposed reforms of astrology on more strictly physical principles, adding new aspects while keeping the Ptolemaic idea of celestial influence. In seventeenth-century England, William Lilly's horary astrology drew on the Arabic and Latin traditions that had grown around Ptolemy.

Modern Translations and Readers§

English translations appeared from the eighteenth century. J. M. Ashmand's version of 1822 was based largely on the Proclus paraphrase and was much used by Victorian astrologers. The standard scholarly translation, by F. E. Robbins, appeared in the Loeb Classical Library in 1940 with the Greek text on facing pages. Later translations by practising astrologers have tried to render Ptolemy's technical vocabulary more precisely.

For historians of science the Tetrabiblos is a key document for how ancient thinkers related mathematics, physics and prediction. For modern astrologers it remains a reference point, whether for those who follow Ptolemy's naturalistic method or for those who see in Valens and other Hellenistic authors a more complete picture of ancient practice. The history of the horoscope itself, from Babylonian tablets to the modern chart, is traced in our guide to horoscopes.

Frequently asked questions§

What does Tetrabiblos mean?§

Tetrabiblos is Greek for "four books," referring to the work's division into four parts. Its Greek manuscripts also call it Apotelesmatika, roughly "on effects," and medieval Latin readers knew it as the Quadripartitum.

When was the Tetrabiblos written?§

It was written in Alexandria in the second century CE, probably after Ptolemy completed the Almagest, so around or after the middle of that century. An exact date is not known.

Did Ptolemy use the tropical zodiac?§

Yes. He stated that the signs begin from the equinoxes and solstices, so that Aries starts at the spring equinox. This definition underlies the tropical zodiac of Western astrology, while Indian astrology uses a sidereal zodiac fixed to the stars.

Is the Centiloquium by Ptolemy?§

No. The hundred aphorisms of the Centiloquium circulated under Ptolemy's name for centuries, but scholars regard the work as a later compilation, probably from the early medieval period.

Sources and further reading§