Study Guide

Ptolemy (astronomer)

by Claudius Ptolemaeus

Ptolemy (astronomer) Reference

Ptolemy (astronomer) (Historic Lives: The Ancient World, Prehistory-476)

Article abstract: Alexandrian scientist{$I[g]Egypt;Ptolemy (astronomer)} Ptolemy’s scientific work in astronomy, mathematics, geography, and optics influenced other practitioners for almost fifteen hundred years.

Early Life

Very little is known about the life of Ptolemy (TOL-uh-mee). He was born in Egypt at the end of the first century c.e., but his birth date and birth place and his life thereafter are subjects of speculation. It is thought that he might have been born in the Grecian city of Ptolemais Hermii in Upper Egypt and that he might have lived to the age of seventy-eight. It has been suggested that he studied and made astronomical observations, staying for more than half of his life among the elevated terraces at the temple of Serapis in Canopus near Alexandria, where pillars were erected with the results of his astronomical discoveries engraved on them. He was probably the descendant of Greek or Hellenized ancestors and obtained Roman citizenship as a legacy from them.

Much more is known about the age in which Ptolemy lived. It was a century during which Rome ruled the Mediterranean world and during which four successive Roman emperors, Trajan, Hadrian, Antoninus Pius, and Marcus Aurelius, built roads and bridges, opened libraries and colleges, and maintained Rome’s power and peace. It was a time when educated men spoke Greek as well as Latin, when Athens was still honored for its cultural traditions, when Marcus Aurelius wrote his Tōn eis heauton (c. 171-180; Meditations, 1634) in Greek, and Greek was still the language of science and the arts.

Ptolemy, who probably used the libraries at Alexandria, was strongly influenced by a Greek scientist, Hipparchus (fl. 146-127 b.c.e.), who propounded the geocentric theory of the universe. As far back as the fourth century b.c.e., the leading view of the nature of the universe had the sun, moon, and planets revolving around the fixed Earth in concentric spheres. The competing theory was first advocated by Aristarchus of Samos (fl. c. 270 b.c.e.). Aristarchus discovered that the sun was much larger than Earth, and this discovery was the basis for his argument that Earth and all other planets revolved around a fixed sun and stars in circles. Yet the heliocentric theory could not be demonstrated by observable phenomena as long as it was thought that the sun was the center of a circle rather than of an ellipse. Hipparchus rejected the contention of Aristarchus, insisting on “saving the phenomena,” that is, adhering to the observations. His further scientific speculations founded on the geocentric theory were the legacy to Ptolemy some two centuries later.

Life’s Work

Some historians maintain that Ptolemy merely plagiarized from Hipparchus; others have said that Ptolemy superseded Hipparchus and made the work of the earlier scientist superfluous. In fact, it could be said that Ptolemy immortalized Hipparchus by acknowledging the debt he owed to his distant predecessor and by frequently quoting from him.

Whatever historical assessment is more correct, there is no doubt that Ptolemy’s work in astronomy alone lasted for more than fourteen hundred years, until the great scientific achievements of Nicolaus Copernicus (1473-1543) and Johannes Kepler (1571-1630). Ptolemy used new instruments or improved on old ones to make his observations. In the Mathēmatikē syntaxis (c. 150 c.e.; Almagest, 1948), one of his most significant books, he utilized the mathematical methods of trigonometry to prove that Earth was a sphere and went on to postulate that the heavens were also spheres and moved around an immobile Earth in the center of the solar system. He dealt with the length of the months and the year and the motion of the sun; he covered the theory of the moon; and he figured out the distance of the sun, and the order and distances of the planets, from Earth. Much of this was not new, not original; the Almagest was essentially a restatement of astronomical knowledge available three hundred years earlier. Yet Ptolemy was able to synthesize that scientific information into a system and to expound it in a clear and understandable manner. He was a teacher, and he taught well.

Ptolemy’s contribution to mathematics was even more significant. Hipparchus had invented spherical and plane trigonometry for the use of astronomers. Ptolemy then perfected this branch of mathematics so that, unlike his astronomical system, which was finally discredited, the theorems that he and Hipparchus devised form the permanent basis of trigonometry.

The Almagest, in which trigonometry was utilized to measure the positions of the sun, Earth, moon, and planets, was later translated into Arabic and then Latin, and so also was Ptolemy’s Geōgraphikē...

(The entire section is 1992 words.)