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Imagined portrait of Ptolemy[1]

Claudius Ptolemaeus (his real name) made major contributions to both astronomy and astrology.


Little is known about the life of the most famous astronomer and astrologer of Antiquity. He probably lived between 100 and 170 CE. He was a Roman citizen of Greek origin who wrote in Greek and lived in Alexandria, Egypt. This Ptolemy should not be confused with kings of Egypt under Greek and Roman rule who were also named Ptolemy, although medieval astrologers sometimes referred to Ptolemy as a king. It is unclear whether they did so to stress the stature of his work, or because they did not know he was unrelated to the Ptolemaic dynasty.


Ptolemy wrote on a wide variety of topics: music theory, optics, and geography, in addition to his work on astronomy and astrology. His book on world geography and maps of the known world were the most extensive of any produced until the Age of Exploration 1500 years later. Although he built on the work of earlier authors, Ptolemy essentially invented our modern system of latitude and longitude. This was a major advance not only for navigation, but also for accurately locating birthplaces in horoscope construction.

The Ptolemaic system


Ptolemy's work the Almagest (or Megale Syntaxis tes Astronomias) is a collection of the writings of earlier Greek astronomers (he can also be shown to have utilized Babylonian astronomical data). It is the only surviving comprehensive ancient treatise on astronomy and includes the calculation of solar and lunar eclipses and contains theories on the sun, moon, planets with epicycles, and the Fixed Stars together with an astronomical catalogue of 1028 celestial objects. This star catalogue is a version of a catalogue created by Hipparchus. Its list of forty-eight constellations is ancestral to the modern system of constellations, but it covers only the sky Hipparchus could see. He presented his astronomical models in convenient tables, which could be used to compute the future or past position of the planets. His Handy Tables provided the model for later astronomical tables.

Ptolemy advanced the geocentric model of our solar system (alternatively termed the Ptolemaic model) which, along with most of his other theories, influenced scientific thought in this field until the Copernican revolution. The widespread speculations among his contemporaries (the most famous being the astronomer Aristarchos) about the possibility that the solar system might be heliocentric were soon forgotten after the publication of the Almagest which helped to establish the geocentric model for roughly 1500 years.

In the Phaseis (Risings of the Fixed Stars), he gave a parapegma, a star calendar or almanac, based on the appearances and disappearances of stars over the course of the solar year. This knowledge helped modern astronomers to calculate the movement of the fixed stars. His writings were preserved, like most of Classical Greek science, in Arabic manuscripts. They presented the universe as a set of nested spheres.


There is no evidence that Ptolemy had his own astrological consulting practice, unlike his contemporary Vettius Valens. Ptolemy contributed to astrology as a compiler and systematizer of existing astrological Greek, Babylonian, and Egyptian information. His astrological goal was apparently to organize material that met the criteria of science as it was understood in his day, and to set it down in a logical fashion, especially according to the principles of Aristotle.

Although other astrologers wrote extensive works in late Antiquity, Ptolemy's books are noteworthy because they were highly regarded by later Hellenistic authors such as Porphyry, transmitted to Arab astrologers, and from them to Europe, at least by the 13th century. Some of Ptolemy's contemporaries were not so well known, so Ptolemy's version of astrology was the one that greatly influenced how the field developed in Europe during the Middle Ages and Renaissance.


Ptolemy's work Tetrabiblos (meaning four volumes) became one of the most important works in the history of astrology. As a source of reference, the Tetrabiblos is said to have "enjoyed almost the authority of a Bible among the astrological writers of a thousand years or more"[2]. The book was concerned with the influences of the celestial bodies in the sublunar sphere (i.e. on earth). He systematised the astrological knowledge of his time and made important contributions to the fields of natal astrology and mundane astrology, the houses, zodiac, elements, planetary interpretation, prediction and the aspects. He promoted the tropical zodiac and planetary positions identified by sign rather than star groupings. Although Ptolemy and his contemporaries considered fixed stars as valid in horoscope interpretation, his work furthered a process begun in Babylon of decoupling astrological signs from constellations.

In his Introduction to the Tetrabiblos (I:1) Ptolemy wrote that the interpretation of the heavens is based on two scientific branches. One (astronomy) teaches about the movement of the sun, moon and stars, their position at any given moment, both in regard to each other and to the earth. The other (astrology) observes the influences caused by the heavenly bodies, depending on their inner powers and the influence of their respective positions. The former (astronomy) requires an independent mental penetration and is valuable, even though it is not interested in interpretation. The validity of astrology may not be so established and complete, but it is possible to understand it by way of philosophical reflection.

Although many astrologers of his day were influenced by the fatalistic, deterministic views of the stoic philosophers, Ptolemy argued that people could ameliorate, to some extent, difficult predictions about their lives. He stated (I:3) that astrology could produce many benefits for people if they knew what to expect in advance of an event so that they could prepare for it.

See also

Prima Europe tabula[3]



  • F.E. Robbins, translated 1940, Ptolemy Tetrabiblos, Loeb Classical Library
  • J.G. Toomer and Owen Gingerich, eds., 1998, Ptolemy’s Almagest, Princeton University Press

Notes and References

  1. By Joos van Wassenhove, ca. 1475
  2. Robbins, Introduction p. xii.
  3. A C15th copy of Ptolemy's map of Britain