Ptolemy (astronomer)

Alexandrian scientist

• Born: c. 100
• Birthplace: Possibly Ptolemais Hermii, Egypt
• Died: c. 178
• Place of death: Unknown, possibly Egypt

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 hyphēgēsis (second century c.e.; The Geography of Ptolemy, 1732). Ptolemy attempted with considerable success to place the study of geography on a scientific foundation. His book, written after the Almagest, was modeled after the work of Marinus of Tyre (fl. second century c.e.), but Ptolemy added a unique dimension by placing his predecessor’s information into a scientific structure. He assumed that Earth was round, that its surface was divided into five parallel zones, and that there were other circles from the equator to the poles. He was the first geographer to write of “parallels of latitude” and “meridians of longitude.” Ptolemy, however, did make one crucial mistake. Along with other ancient geographers, he underestimated the circumference of Earth, and as a consequence few latitudes were established correctly (and, since the means were not available, no longitudes were established).

What most attracted the interest and attention of earlier geographers and of Ptolemy was the size of the inhabited world: in the north, Thule (the present Shetland Islands); in the west, the Fortunate Islands (the Canary Islands and Madeira); and in the south and east, the vast continents of Africa and Asia. Although they overestimated the size of both the eastern and southern continents, Ptolemy’s findings, and Marinus’s before him, were based on new knowledge derived from travelers’ accounts of the silk trade with China and from sea voyages in the Indian Ocean. Ptolemy revised some of Marinus’s estimates of the length and breadth of Asia and Africa, extending Asia eastward and Europe westward. More than a thousand years later, Christopher Columbus (1451-1506), who relied on Ptolemy’s Geography, was led to believe that it was possible to reach Asia by a direct route across the Atlantic Ocean.

Ptolemy’s Geography is restricted to mathematical calculations; he did not write about the physical attributes of the countries he charted or the people who inhabited them. His tables, stating the location of places in terms of latitude and longitude, gave a false impression of precision; he made frequent errors because of his basic misestimate of the size of Earth. Still, Ptolemy’s objective to draw a world map was noteworthy. His educated guess as to the location of the sources of the Nile River was remarkable, and his use of the terms “latitude” and “longitude” was a distinct contribution to the advancement of geographical knowledge.

While Ptolemy is well-known among historians of science for his volumes on astronomy and geography, it is also necessary to consider his writings on astrology, which in the ancient world was the “science” of religions. His volume Apotelesmatika (second century c.e.; Ptolemy’s Quadripartite: Or, Four Books Concerning the Influences of the Stars, 1701; commonly called Tetrabiblos, “four books”) is important partly because it was more famous than the Almagest and partly because it reflects the popular thinking of his age. The Tetrabiblos is a summary of Egyptian, Chaldean, and Greek ideas. It attributes human characteristics to the planets, such as masculine and feminine, beneficent and malevolent. It predicts the future of races, of countries and cities, and speaks of catastrophes, natural and human: wars, famine, plagues, earthquakes, and floods. It also expounds on such subjects as marriage, children, the periods of life, and the quality of death. Translated into Arabic, Latin, Spanish, and English, it influenced generations of Europeans (and, later, Americans) and formed the basis of modern astrological beliefs.

There are many historians of science who deplore the superstitions that pervade the Tetrabiblos and dismiss it as an unfortunate effort. The great historian George Sarton wrote, however, that “we should be indulgent to Ptolemy, who had innocently accepted the prejudices endemic in his age and could not foresee their evil consequences. . . .”

Significance

It would be unreasonable to expect great scientific breakthroughs during the second century c.e., and they did not happen. What did occur was the gradual advancement of knowledge to which Ptolemy contributed. Not only did Ptolemy write the Almagest and the Geography, adding new and significant materials to those of his predecessors, but he also attempted to illuminate the science of optics and the art of music. In the first case, although little was known about the anatomical and physiological structure of the eye, he devised a table of refraction, and his book reveals that he understood that a ray of light deviates when it passes from one medium into another of a different density. He addressed the role of light and color in vision, with various kinds of optical illusions and with reflection. Ptolemy’s volume on music theory, known as the Harmonika (second century c.e.; Harmonics, 2000), covers the mathematical intervals between notes and their classification. He propounded a theory that steered a middle ground between mathematical calculations and the evidence of the ear. Observation was again a guiding principle of his art as well as his science.

Other work on mechanics, dimensions, and the elements was done but has not survived. What did survive had great influence on the Arabic science of astronomy, led to the rise of European astronomy, and influenced the work of Copernicus himself in the fifteenth century. The Geography, also translated into Arabic in the ninth century, was amended to describe more accurately the territories under Islamic rule; in the West, where the work became known in the fifteenth century, it was a catalyst of cartography and to the work of the Flemish cartographer Gerardus Mercator (1512-1594). Ptolemy’s work on optics inspired the great improvements made by the Arabic scientist Ibn al-Haytham (d. 1039), and his work became the foundation of the Perspectiva of Witelo (c. 1274), the standard optical treatise of the late Middle Ages.

Just as there is no exact knowledge of Ptolemy’s birth date, there is no reliable information about when and where he died and under what circumstances. Yet those biographical facts are not that important; what is significant is the scientific legacy that was transmitted through the centuries. Ptolemy was not, as one expert has argued, an “original genius”; his forte was to take existing knowledge and to shape it into clear and careful prose.

Bibliography

Barker, Andrew. Scientific Method in Ptolemy’s “Harmonics.” New York: Cambridge University Press, 2001. This study illuminates not only the Harmonics but also Ptolemy’s observational methods in general.

Grasshoff, Gerd. The History of Ptolemy’s Star Catalog. New York: Springer-Verlag, 1989. A careful study of the relationship between Ptolemy’s work and that of Hipparchus, showing that charges of plagiarism are anachronistic and based in a misunderstanding of Ptolemy’s scientific objectives.

Irby-Massie, Georgia L., and Paul T. Keyser. Greek Science of the Hellenistic Era: A Sourcebook. New York: Routledge, 2001. Chapters cover the main scientific disciplines in the Hellenistic era, providing a historical and scientific context for Ptolemy’s work in many fields.

Ptolemy. Harmonics: Translation and Commentary. Translated by Jon Solomon. New York: Brill Academic Publishers, 2000.

Ptolemy. Ptolemy’s “Almagest.” Translated by G. J. Toomer. 1984. Reprint. Princeton, N.J.: Princeton University Press, 1998.

Ptolemy. Ptolemy’s “Geography.” Translated by Alexander Jones and J. Lennart Berggren. Princeton, N.J.: Princeton University Press, 2000.

Ptolemy. Tetrabiblos. Translated by F. R. Robbins. Loeb Library. Cambridge, Mass.: Harvard University Press, 1980. For students and readers who may want to sample the scientific works of Ptolemy. All have useful introductions and annotations.