Claudius Ptolemy (TOL-uh-mee) was known as the world's greatest astronomer, geographer, and mathematician. In the second century c.e., everyone in the Alexandrian world believed that. There would be a long interlude when Ptolemy was forgotten, but then he would bounce back stronger than ever. During the Middle Ages and the Renaissance, Ptolemy was revered. People believed he was the greatest in 1492, when Columbus sailed the ocean blue and happened upon what was to him the New World. So put Ptolemy in your head as another of the most influential authors of all time. He wrote huge books that organized all the known math, geography, and astronomy of the day.
This Ptolemy was not an emperor or a general, as were the Ptolemys who ruled Egypt. We hardly know anything about his personal life. We do know that he was born in North Africa and was part of the Greek-speaking Hellenistic world. Most authorities believe that he was Greek, but a few think he may have been Egyptian. He lived and studied in Alexandria more than three centuries after Eratosthenes.
But unlike Eratosthenes, he believed that Earth is mostly dry land rather than ocean. (That was an idea that went back to Aristotle.) He thought the stars and planets orbited in perfect circles. (That was Plato's idea.) And he thought Earth much smaller than it actually is. When Ptolemy calculated, Earth came out 30 percent smaller than what Eratosthenes had figured it to be—and what it really is.
Besides that, Ptolemy made Asia huge, stretching it far beyond where it actually is. (Later, all those errors would make Christopher Columbus think that Asia was just across an ocean that wasn't terribly big.)
Ptolemy wasn't much of an original theorist, but he was a solid thinker who compiled and organized and extended the work of the past and left a base for future scientific study. His great contribution was in explaining the world mathematically. Those who followed would be compelled to do the same thing.
Ptolemy wrote massive volumes on science, geography, and mathematics. And his ideas seemed to work. With his model and his mathematics, you could predict the motions of the Sun, stars, and planets. You'd be close enough so that farmers, sailors, and teachers could use the results—which they did. Today we're apt to look back at Ptolemy and forget how important he was for centuries. Those who turned to Ptolemy were rejecting superstition and magic and using a solid work of scholarship.
His monumental book was called Megale mathematike syntaxis ("Great Mathematical Composition"). Sometimes it was just called Megiste or Almagest ("The Greatest"). In addition to making a significant contribution to trigonometry, it mapped and charted the visible stars. Ptolemy's star charts became enormously helpful to anyone trying to navigate a ship at sea.
In fact, Christopher Columbus relied on his copy of Almagest when he set out to find a new route to Asia by sailing west. Columbus read Eratosthenes, too, but he believed Ptolemy. That means he thought the Earth was smaller than it actually is and Asia, larger. When he reached the Caribbean Islands, he was sure he was near Asia (and he would have been if Ptolemy had been right).
Now consider this: If Columbus had paid attention to Eratosthenes—and if he had known how big the world really is—he might not have dared sail west. Where would we be now?
Joy Hakim, former teacher, reporter, and editor, writes nonfiction for children. Her latest project is a six-volume series called The Story of Science, from which this article is excerpted with permission. ©2004 The Story of Science, Smithsonian Books. Her 10-volume series, A History of US, won the Michener Prize in Writing and was made into a PBS special called "Freedom."
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