Claudius Ptolemaeus, or Ptolemy, was a Greek geographer and astronomer who lived from around A.D. 90-168. Ptolemy was born in one of Egypt's Greek settlements and was a scholar in Alexandria. His best known works were the compilations Geography and Almagest. These were considered the authoritative sources on geography and astronomy, respectively, during the Middle Ages. "Geography" was based mainly on the earlier works of others and, it turns out, was full of errors, which led to wide acceptance of incorrect information. (Ptolemy, of course, had only the geographical details of his day with which to work -- he couldn't just punch up Google Earth in his Web browser -- so the fact that his geography was a bit askew shouldn't exactly be a revelation.)
The Almagest describes the mathematical theory behind the motion of the planets as well as the sun and moon in our solar system. His effort to detail the math behind the motion of each individual planet really put him on the map, and his theories became the gospel truth on the subject for some 1,400 years. This effort by Ptolemy to make mathematical predictions of planetary motion is well known today by science buffs for something we now know is entirely wrong. Ptolemy used an Earth-based solar system for his modeling of the heavens, wherein everything, including the sun and planets, revolved around the Earth. It was the prevailing view of his time, after all, backed by the earlier thinking of Aristotle himself. It would be nearly a millennium and a half before Copernicus came along and argued that we actually live in a sun-centered, "heliocentric" solar system [source: University of St. Andrews].
Even though Ptolemy didn't exactly have the right celestial body at the center of the solar system, his calculations were nonetheless able to predict the motion and positions of the planets with a high degree of reliability. He could, in short, predict where planets would be at points in the future, using motions called epicycles to determine their paths. This was unheard of before his work, which perhaps helps explain why it lasted so long unchallenged.
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