## The Ancient Greek Originator of Ptolemaic Astronomy and Geography

Ptolemy was an ancient Greek scholar who originated the Ptolemaic system in astronomy and made important contributions to geography and optics.

Despite his important scholarly contributions, no one knows much about Ptolemy’s life. Historians must guess the years of his birth and death from the publication dates of his published work. Estimates range from about 85 to 100 for his birth date and from 150 to 170 for the year of his death. These estimates are based on the fact that his earliest known recorded astronomical observation was made in 127 and the last was in 150. The extent of Ptolemy’s work after his last observation probably took about 20 years to finish.

Theodore Meliteniotes wrote, in 1360, that Ptolemy was born in Hermion, Egypt. However Meliteniotes cited no evidence to support this dubious claim. Most historians therefore think that Ptolemy was born and lived out his life in Alexandria, Egypt, the recorded site of all his astronomical observations. Because Alexandria was the site of the great ancient library, Ptolemy had access to the best scholarly writings available at the time.

Ptolemy lived in Egypt. His Greek name, however, suggests that he had Greek ancestors, so he is considered an ancient Greek scholar rather than an Egyptian scholar. His full name was Claudius Ptolemaeus, which is a Roman name. It suggests that a Roman emperor had given one of Ptolemy’s ancestors Roman citizenship.

### Ptolemaic Astronomy

Ptolemy is most famous for his contribution to astronomy, which is in his 13-book work called the *Almagest*. The Ptolemaic system of astronomy placed the Earth at the center of the universe and explained all the observed motions of the Sun, Moon, planets, and stars with a system of uniform circular motions. Ptolemy borrowed heavily from the previous work of Hipparchus. Ptolemy built on this previous work, but also added new original contributions that more accurately explained the available observations. The Ptolemaic astronomical system was wrong, but it was a good enough scientific model to last over 1400 years until the time of Copernicus.

### Ptolemy’s Geography

In addition to astronomy, Ptolemy wrote an important book on geography. He added new original ideas to previous work. Despite significant errors Ptolemy made contributions to geography that still survive. Ptolemy developed latitude and longitude as a grid system to determine locations on Earth. He was also the first to develop a projection to map the spherical Earth onto flat maps. The major error in Ptolemy’s geography was his underestimation of the Earth’s size.

### Other Contributions

Ptolemy’s astronomical and geographical work required a lot of mathematics. Ptolemy therefore made contributions to mathematics, especially in the area of trigonometry. He also did some work on music theory. Unlike today, in Ptolemy’s time, people considered astrology a scholarly pursuit, and Ptolemy wrote a book on astrology. Ptolemy wrote an early book on optics. He developed a theory of refraction which was approximately correct for small angles.

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