PTOLEMY

ca. A.D. 100-ca. 170

Astronomer, mathematician, and geographer

Claudius Ptolemaeus, or Ptolemy, was a Greek astronomer, mathematician, and geographer. Between A.D. 146 and A.D. 170, Ptolemy wrote several major works, including a 13-volume textbook of astronomy that remained the standard work in the field until the end of the Middle Ages. An equally influential work was his 8-volume Geography, in which Ptolemy attempted to map the known world.

Little is known about Ptolemy’s life. Born in Upper Egypt, he lived most of his life in Alexandria, where he served as superintendent of the Museum. Around A.D. 150, Ptolemy completed his major astronomical work, called the Syntaxis. (This work is now known as the Almagest, from the Arabic for “the greatest.”) In the Syntaxis, Ptolemy recorded the information needed to determine the positions in the sky of the sun, the moon, the stars, and the five planets that were known at the time. He incorporated the work of the great Greek astronomer Hipparchus, who lived in the 100s B.C. Using observations of the stars made by the ancient Babylonians, Hipparchus had compiled a catalog that contained the positions in the sky of more than 850 stars. Ptolemy made his own observations of the stars between A.D. 127 and A.D. 147, which enabled him to create a star catalog that plotted the positions of more than 1,000 stars.

Along with most other ancient astronomers, Ptolemy believed that the earth was the center of the universe. According to Ptolemy, the earth is surrounded by hollow, transparent spheres that support and move the other planets, the moon, the sun, and the stars around the earth. However, because heavenly bodies move in irregular patterns across the sky, Ptolemy devised a complicated theory of interlocking movements of the planetary spheres. His theory explained the movement of heavenly bodies so well that it was not until the 1400s that scientists challenged the geocentric theory.

In his Geography, Ptolemy attempted to locate the known regions of the world by listing places by their latitude and longitude. In addition to this list of about 8,000 places, Ptolemy described important physical features of the land, such as rivers and mountain ranges. Book 1 of the Geography includes instructions on how to draw a map of the world using two different projections*.

The Geography also contains an atlas of maps. Although the maps accurately depict the outline of the Roman empire, areas beyond the boundaries of the empire are frequently distorted. Ptolemy made a number of mistakes in creating his maps. For example, he calculated that the Mediterranean Sea was much longer than it is in reality, and he connected the continent of Africa to China, making the Indian Ocean a large inland lake. Despite these mistakes, the Geography was the most accurate depiction of the known world until the 1500s.

Ptolemy wrote many other books on astronomy, astrology, optics, and music. In his Planetary Hypotheses, for example, he described the physical models that explain the movements of the planets he detailed in the Syntaxis. In the Astrological Influences, a companion work to the Syntaxis, Ptolemy attempted to show how the movements of the planets affect life on earth. These works had an important influence on astronomy and astrology throughout the Middle Ages. (See also Astronomy and Astrology; Eratosthenes; Maps, Ancient.)

* projection representation of the earth’s surface upon a flat surface, such as a grid

THE CENTER OF THE UNIVERSE

Not all ancient astronomers believed in the geocentric theory— that the earth is the center of the universe. The astronomer Aristarchus of Samos suggested that the movements of the heavenly bodies could be explained if the sun were at the center and the earth and the other planets revolved around it But Ptolemy and most other ancient astronomers rejected this argument for rational physical reasons. According to the scientific theory of the time, if the earth moved, an object thrown straight up into the air would not fall down in the same spot. It was not until after the work of Nicolaus Copernicus in the 1500s that astronomers adopted the sun-centered system.