Claudius Ptolemy: A Sage Head in the Clouds

The works of Hellenistic polymath Ptolemy of Alexandria outlined the Western view of the cosmos that would survive until the Copernican revolution and defined the rational-causal view of astrology still largely ascribed to today.

Published Categorized as History
Figure of the heavenly bodies - Illuminated illustration of the Ptolemaic geocentric conception of the Universe by Portuguese cosmographer and cartographer Bartolomeu Velho (?-1568). From his work Cosmographia, made in France, 1568 (Bibilotèque nationale de France, Paris). Notice the distances of the bodies to the centre of the Earth (left) and the times of revolution, in years (right). The outermost text says: "The heavenly empire, the dwelling of God and of all of the elect"
A depiction of the Ptolemaic Universe as described in the Planetary Hypotheses by Bartolomeu Velho (1568)

Mortal as I am, I know that I was born for only one day. But when I see the stars circling their orbits, my feet no longer touch the ground.

Claudius Ptolemy

The year 100 A.D. saw the Roman Empire at its greatest territorial extent under Emperor Trajan. A territory stretching from the Iberian Peninsula in the West to the Caspian Sea in the East lay under the rule of one man, and that man employed astrology as the primary tool by which to judge his fortunes. Trajan’s predecessor, Nerva, backed his claim to the throne with his horoscope, which allegedly foretold his aptitude as emperor. Trajan’s successor, Hadrian, himself an avid astrologer, managed to accurately foretell the time of his own death. The predictions of the astrologer Antigonus of Nicaea ensured that Hadrian’s successor was Antoninus Pius and not Pedanius, an ill-fated and disinherited relative. For the better part of 70 years, the Empire was largely at peace, blessed with a series of competent rulers and a smooth process of succession in part overseen by the consultation of horoscopes and other key astrological portents.

Into this golden era, when the emperor of the Western world was chosen by the celestial bodies, Claudius Ptolemy was born. A Roman citizen of the province of Egypt, Ptolemy worked as a research professor at the Library of Alexandria. The Library, considered the foremost center of learning in the classical world, boasted some 700,000 papyrus scrolls, and had supported the work of philosophers, mathematicians, and thinkers such as Euclid, Hippocrates, and Hipparchus, who established the modern tropical zodiac and collected the first significant catalog of stars, forming the base for Ptolemy’s work.

Fostered by this environment of peace and learning, Ptolemy produced significant works in mathematics, astronomy, astrology, geography, optics, and music. In the treatise on astronomy that would later become known by its Arabic name, the Almagest, Ptolemy outlined the geocentric model of the universe that would guide astronomers until Copernicus, and his work Geography influenced European and Middle Eastern cartography until the beginning of European colonial expansion. In this essay, however, we will focus on Ptolemy’s third major work, known as Tetrabiblios, or “Four Books”, and how it outlines the vital connections between astrology and astronomy and sets forth many of the principles of modern Western astrology still in practice today. 

A Mathematician Among the Stars: Ptolemy’s Worldview

Surprising for someone who wrote seminal texts on both astronomy and astrology, Ptolemy did not consider himself either an astrologer or an astronomer. He chose instead to classify the Almagest as a work of mathematics. This was, to some extent, the norm of the day, but it helps to explain the philosophy that for Ptolemy defined the connection between the two fields.

In the Almagest, he identified 48 constellations and mapped the positions of 1000 stars, while also developing predictive models for the movement of celestial bodies (most notably accounting for retrograde, the periods in which certain planets and stars appear to move backward in the sky), all using mathematics and data as his primary tools. Astronomy was thus a method of studying “the movements of the sun, moon, and stars in relation to each other and the earth.”

Astrology, in contrast, meant understanding “the changes [the heavens] bring about in that which they surround”—in other words, how the movement of celestial bodies impacts life on Earth. In many ways, astrology was astronomy’s raison d’etre, as developing predictive mathematical models for far-off planets and stars mattered only in so much as they could help generate the horoscopes that would help the Empire determine who was most fit to rule. 

From Andreas Cellarius Harmonia Macrocosmica, 1660/61. Chart showing signs of the zodiac and the solar system with world at centre.
Scenography of the Ptolemaic cosmography

This also explains why a mathematician would pen a treatise on astrology. Arguably as influential as his Almagest, Ptolemy’s Tetrabiblios is a compendium of Hellenistic astrology that made the author’s name synonymous with astrology for millennia. For many astrology involved the interpretation of fate and divine will, but Ptolemy took a distinctly secular and scientific approach, attempting to explain astrology in terms of Aristotelian natural philosophy. He sought to provide a rational foundation for astrology by systematically presenting its principles and subjecting them to rigorous analysis.

Whereas the markedly Stoic Manilius (and most early astrologers besides) saw astrology as a method to understand the will of a deity or deities, for Ptolemy it was about understanding a simple process of cause and effect. Borrowing from earlier Greek thinkers, Ptolemy ascribed primary importance to planetary influence. To him, all life was, at its core, identical, and the diversity of life on Earth was due to the diversity of environmental factors. The constellations, planets and fixed stars were one such factor. He theorized that the planets and stars transmitted a kind of energy to the Earth, much like light, that in turn interacted with and affected the lives of men. Ptolemy also softened much of Manilius’ fatalism surrounding astrology, in a way addressing astrology’s imperfection as a predictive art. While the influence of celestial bodies may be fixed, how they manifest can change, explaining why certain predictions come to pass and others do not.

Tetrabiblios: Key Concepts

Ptolemy identifies two main predictive applications of astrology. The first, general astrology, deals with large phenomena such as the weather, the environment, cities, and countries. The eruption of Mount Vesuvius, purportedly foretold by astrological conditions, is one such example. The second, genethlialogical or natal astrology, concerns the destiny of individuals, most notably deciphered via the calculation of the horoscope. It is worth noting that Ptolemy gave prime importance to the moment of conception, but given the impossibility of knowing it exactly, relied on the time of birth as measured via an astrolabe.

In the Almagest, Ptolemy provides a comprehensive guide for mathematically calculating astronomical positions, and many of the techniques described therein were later refined by Islamic scholars. Many subsequent astrologers, unable to perform the complex calculations necessary to predict planetary or stellar movement, would rely on precalculated tables. The Tetrabiblios is in many ways an extension of the Almagest, providing readers with sufficient astrological lore to interpret mathematical data. 

Portrait of Ptolemy
Claudius Ptolemäus, Picture of 16th century book frontispiece

In Tetrabiblios, Ptolemy establishes a system of attributes for the planets, according to the most primal elements or qualities that were thought to make up the universe: hot and cold, moist and dry. This same logic underpinned the humor system, which was the foundation for Hippocratic and Galenic medicine. Instead of Manilius’ Zodiac Man, Ptolemy outlines a Planetary Man, which in much the same way draws a connection between parts of the body and celestial influences, simply with planets instead of constellations taking the leading role.

Interestingly enough, Ptolemy’s emphasis on planetary influence may have helped astrology survive into the Christian era. No more was fate attributed to machinations the Greco-Roman pantheon, but instead to ostensibly secular planets and stars. As Hellenistic philosophers and scientists had already determined, celestial bodies influence the seasons, the tides, and the weather—isn’t it only logical to suppose that they influence human life as well? As the planets moved through the signs of the tropical zodiac, this influence would wax or wane.

Ptolemy goes on to outline several other important tenets of astrology, at times diverging from Manilius and other astrologers on certain points he believed to be based more on superstition than rational analysis. In addition to the planets, he attributed positive and negative power to some fixed stars, even those outside of the Zodiac.

In place of Manilius’ dodecatemoria division of the signs, Ptolemy divided each constellation into five parts, called horia, each assigned to a certain planet and possessing planetary qualities. He also made further subdivisions of the signs known as termini. He solved the issue of division of mundane houses, or topoi, and explained why certain aspects are harmonious while others are unharmonious. A sextile, for example, connects Aires to Gemini, two masculine signs, whereas a square connects Aires to Cancer, a masculine sign to a feminine sign. Aspects connecting signs of the same gender are considered positive or harmonious, whereas aspects connecting opposite genders are considered more of a negative influence.

Ptolemy also outlines how to calculate life span via the prorogator. His system involved identifying the dominus vitae, or the dominant planet at the time of birth, and calculating the distance between its starting point (the “aphetic” place) and the ominously named point of destruction (or the “anaretic” place). The difference in degrees between these two positions could then be converted into time. 

Aristotle, Ptolemy and Copernicus are standing and discussing. A ship in the background represents one of the arguments for the movement of the Earth: just as the motion of a moving ship need not affect the motion of something moving inside the ship, so the motion of the moving Earth need not affect the motion of something moving on the Earth.
Aristotle and Ptolemy discuss with Copernicus their respective views on the movements of the Sun and the Earth.

The Ages of Man

An interesting and influential system that Ptolemy outlines—one that does not appear in the works of earlier astrologers—is the division of one’s life into seven stages.

Each stage is purportedly ruled by a planet, as Hellenistic astronomy recognized seven planets, including the Sun and the Moon. The duration of each stage could, like lifespan, be determined by the return of the ruling planet to its original position, known as apokatastasis.

The first phase, or infancy, is ruled by the Moon. The second, childhood, is ruled by Mercury, while the third (identified with the Lover) is ruled by Venus and lasts through adolescence to early adulthood. The next and longest stage is ruled by the Sun and represents the prime of life. The next fifteen or so years are ruled by Mars, and the following 12 or so are ruled by Jupiter. The last stage, representing the descent into senility, is ruled by Saturn. These stages were immortalized in Jacques’ speech from Shakespeare’s As You Like It

In addition to expanding on the principles outlined in Almagest, Ptolemy also provides a section in Tetrabiblios that represents an astrological extension of his work in Geography. Dividing the known world into four sections with the center at the site of the city of Antioch, Ptolemy assigned each quadrant astrological qualities; for example, he classified Europe, the northwestern quadrant of the map, with fire attributes, and Libya, in the southwest, with water attributes. He then went on to describe the tendencies of the various ethnic groups in the world based on these astrological alignments. 

The Influence of Ptolemy on Modern Astrology

From his post in Alexandria, Claudius Ptolemy drew on Babylonian, Chaldean, Persian, Egyptian, and Greek sources for his research. In turn, he produced three major works in three distinct fields, although Hellenistic thinkers may have classified them all simply as different applications of mathematics. The tenets in Geography, the Almagest, and Tetrabiblios held sway in each field for over 1000 years, and, refined as they were by subsequent scholars, came to underpin much of the cultural and scientific awakening of the European Renaissance. 

In terms of astrology, Ptolemy’s influence was two-fold. First, in outlining what he believed to be the parts of the field solidly based on reason, he presented a systematic account of Hellenistic astrology that would become the virtual textbook for subsequent astrologers. He also managed to settle certain astrological disputes, such as the division between mundane houses, and provide the predictive mathematical tools still used by astrologers today. Secondly, and perhaps more importantly, he ensured that the study of astrology did not end with the Roman Empire. By attempting to consolidate the field with rational inquiries and scientific processes, rather than with Greco-Roman religious beliefs, he made the field more secular, and thus more adaptable to a new religious worldview.

This shift of emphasis away from Apollo and Minerva to Jupiter and Mercury may have ensured that astrology would continue to play a large role in the lives of nobles in later Christian states, either through medical astrology or through the predictive power of horoscopes. More still, Ptolemy’s insistence on examining the universe as a rational system of cause and effect embodies the spirit of scientific inquiry that would shape the modern world we live in.


  • Astrology: A history by Dr. Peter Whitfield (2001)
  • A Scheme of Heaven by Alexander Boxer
  • The Fated Sky: Astrology in History by Benson Bobrick
  • Ptolemy in
  • Tetrabiblios by Cladius Ptolemy, trans. Frank Egleston Robbins

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