# Ptolemy : biography

Ptolemy’s astrological outlook was quite practical: he thought that astrology was like medicine, that is *conjectural*, because of the many variable factors to be taken into account: the race, country, and upbringing of a person affects an individual’s personality as much as, if not more than, the positions of the Sun, Moon, and planets at the precise moment of their birth, so Ptolemy saw astrology as something to be used in life but in no way relied on entirely.

A collection of one hundred aphorisms about astrology called the *Centiloquium*, ascribed to Ptolemy, was widely reproduced and commented on by Arabic, Latin and Hebrew scholars, and often bound together in medieval manuscripts after the *Tetrabiblos* as a kind of summation. It is now believed to be a much later pseudepigraphical composition. The identity and date of the actual author of the work, referred to now as Pseudo-Ptolemy, remains the subject of conjecture.

## Astronomy

The *Almagest* is the only surviving comprehensive ancient treatise on astronomy. Babylonian astronomers had developed arithmetical techniques for calculating astronomical phenomena; Greek astronomers such as Hipparchus had produced geometric models for calculating celestial motions. Ptolemy, however, claimed to have derived his geometrical models from selected astronomical observations by his predecessors spanning more than 800 years, though astronomers have for centuries suspected that his models’ parameters were adopted independently of observations. Ptolemy presented his astronomical models in convenient tables, which could be used to compute the future or past position of the planets.Bernard R. Goldstein, "Saving the Phenomena: The Background to Ptolemy’s Planetary Theory", *Journal for the History of Astronomy*, 28 (1997): 1–12 The *Almagest* also contains a star catalogue, which is an appropriated version of a catalogue created by Hipparchus. Its list of forty-eight constellations is ancestral to the modern system of constellations, but unlike the modern system they did not cover the whole sky (only the sky Hipparchus could see). Through the Middle Ages, it was spoken of as the authoritative text on astronomy, with its author becoming an almost mythical figure, called Ptolemy, King of Alexandria.S. C. McCluskey, *Astronomies and Cultures in Early Medieval Europe*, Cambridge: Cambridge Univ. Pr. 1998, pp. 20–21. The *Almagest* was preserved, like most of Classical Greek science, in Arabic manuscripts (hence its familiar name). Because of its reputation, it was widely sought and was translated twice into Latin in the 12th century, once in Sicily and again in Spain.Charles Homer Haskins, *Studies in the History of Mediaeval Science*, New York: Frederick Ungar Publishing, 1967, reprint of the Cambridge, Mass., 1927 edition Ptolemy’s model, like those of his predecessors, was geocentric and was almost universally accepted until the appearance of simpler heliocentric models during the scientific revolution.

His *Planetary Hypotheses* went beyond the mathematical model of the *Almagest* to present a physical realization of the universe as a set of nested spheres,Dennis Duke, in which he used the epicycles of his planetary model to compute the dimensions of the universe. He estimated the Sun was at an average distance of 1,210 Earth radii, while the radius of the sphere of the fixed stars was 20,000 times the radius of the Earth.Bernard R. Goldstein, ed., *The Arabic Version of Ptolemy’s* Planetary Hypotheses, *Transactions of the American Philosophical Society* 57, no. 4 (1967), pp. 9–12.

Ptolemy presented a useful tool for astronomical calculations in his *Handy Tables*, which tabulated all the data needed to compute the positions of the Sun, Moon and planets, the rising and setting of the stars, and eclipses of the Sun and Moon. Ptolemy’s *Handy Tables* provided the model for later astronomical tables or *zījes*. In the *Phaseis* (*Risings of the Fixed Stars*), Ptolemy gave a *parapegma*, a star calendar or almanac, based on the hands and disappearances of stars over the course of the solar year.