The Greek astronomer, mathematician, and geographer Ptolemy (c. 100–170) may be best known to history for his greatest mistake: In the landmark astronomical treatise the Almagest, he claimed that the sun, stars, and planets all revolved around Earth. His conclusion was accepted by virtually all astronomers for the next 1,400 years, until it was finally debunked in the sixteenth century.
But before Nicolaus Copernicus (1473–1543) proved that it was Earth that revolved around the sun, Ptolemy was widely regarded as the greatest astronomer of history, as well as the West’s leading authority on the cosmos.
Ptolemy was born in Egypt and lived much of his life in Alexandria, the capital of the Roman province. Prior to the Romans, Egypt had been under Greek rule, and Ptolemy spoke and wrote in ancient Greek. However, he was also a Roman citizen, a privileged status granted to relatively few elite inhabitants of the territories.
In the 120s, Ptolemy began recording his astronomical observations, which he used to write the Almagest. Also containing summaries of the work of other major ancient astronomers, especially Hipparchus, an astronomer from Rhodes who studied eclipses and the motion of the sun, the Almagest was a compilation of all the astronomical knowledge in the ancient world. Completed in about 150, the text was used by astronomers and astrologers for centuries afterward to predict eclipses and compile horoscopes.
Ptolemy was also a noted cartographer who produced some of the ancient world’s most accurate maps. He was the first mapmaker to make extensive use of longitude and latitude, as well as the first whose maps reflected the curvature of the earth. Like his astronomy, Ptolemy’s geography would be considered the pinnacle of scientific knowledge for centuries after his death.
ADDITIONAL FACTS
- Ptolemy’s goal in his Geography, a world atlas and mapmaking guide, was to provide sufficiently detailed directions so that any reader could draw his or her own world map. To make the map as thorough as possible, he listed the longitudes and latitudes of 8,000 locations in the ancient world—one of the most complete historical records of ancient cities and place names in existence.
- Like those of many other ancient Greek thinkers, Ptolemy’s works were preserved thanks to the efforts of Arab scholars during the Middle Ages. Indeed, the most frequently used English title of Ptolemy’s astronomical manual, Almagest, is derived from the book’s Arabic name.
- Ptolemy believed there were only six planets, including Earth, a view that prevailed for about the next 1,700 years. The outermost planets, Uranus and Neptune, were not discovered until 1781 and 1846, respectively.
