Ptolemy
ca. 100–170 Egyptian scientific scholar

The ancient Egyptian scientist Claudius Ptolemy wrote works on a wide range of topics, including astronomy, geography, mathematics, and music. Although Ptolemy's writings were over a thousand years old by the time the Renaissance began, they had a great influence on the scientific thought of the time.

Modern scholars know little about Ptolemy's life. He appears to have spent his whole life in Alexandria, a city in Egypt, which was then part of Roman territory. His first name is Roman in form and his last name Egyptian, suggesting that he came from a mixed family.

Ptolemy believed that the study of the skies was more than a science. In his major work on astronomy, The Almagest, he argued that studying astronomy led people to think about God, whom he considered to be the cause of all heavenly motions. This view of astronomy was very influential during the Renaissance. In another work, the Quadripartitum (Four Parts), Ptolemy expressed his belief in astrology*. He claimed that the heavenly bodies had a physical influence on all earthly matters, including geography, climate, and even human lives.

The Almagest covered all aspects of mathematical astronomy known to ancient scholars. It also explains Ptolemy's vision of the universe—a sphere with a tiny, unmoving Earth at its center. Other ancient astronomers believed in this geocentric, or earth-centered, view of the universe, but they had trouble making it fit with their observations of the movements of heavenly bodies. The irregular motions of the planets, for example, did not match perfectly circular orbits around Earth. Various astronomers came up with theories to fix this problem. Some suggested that the orbits of the planets were not quite centered on Earth, while others proposed more complicated adjustments. Ptolemy cleverly combined these different schemes in a new mathematical theory that fit very well with observations of the heavens.

Ptolemy's system defined the common view of the Earth and the heavens for much of the Renaissance. A few Renaissance astronomers argued with some of the points of the system, but their work was inferior to Ptolemy's. By the early 1500s, however, the Polish astronomer Nicolaus Copernicus had begun to formulate a heliocentric, or Suncentered, model of the solar system. His work sowed the seeds of a new system that would in time destroy the Ptolemaic vision of the universe.

(See alsoScience. )

* astrology

study of the supposed influences of the stars and planets on earthly events