Who was Eratosthenes? - Greek Astronomy
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Who was Eratosthenes?

Carl Sagan on Eratosthenes

Eratosthenes was born about 276 BC in Cyrene (modern Libya). When he was still young he moved to Alexandria, in Egypt, and in 236 BC, when he was forty years old, he became the chief librarian at the great library there. Eratosthenes spent the rest of his life in Alexandria, where he seems to have known both Aristarchus and Archimedes.

Eratosthenes built on the work of Thales and Aristarchus, who had shown that the earth was round and that the earth went around the sun. Erathosthenes was able to add a fairly accurate calculation of the circumference of the earth - how big the earth was - based on calculations using the angle of the sun at noon on the summer solstice in two different locations. Erastosthenes also correctly calculated the distance from the earth to the sun and the moon.

Eratosthenes also worked as a mathematician. Among other things, he invented an efficient way to discover all the prime numbers in a set of whole numbers by multiplying smaller prime numbers to eliminate all numbers that were not prime, as in the animation here.

Eratosthenes map
Copy of Eratosthenes' map of the world

Eratosthenes also drew an early map of the world, inventing the idea of latitude and longitude to help him in this project. Although he had a pretty good idea of what the Mediterranean looked like, and he made sure to include Libya (where he was born), there's a lot Eratosthenes doesn't seem to know in this map. England (Britain) is much too big, and so is Ireland.

Scandinavia hardly exists at all. Eratosthenes didn't understand how big Central Asia was. He knew Ethiopia, the Scythians, and Arabian peninsula, and he knew a little about India and Sri Lanka, but he doesn't seem to have heard of China. Eratosthenes didn't know about the Americas or Australia either.

Learn by doing: the earth-sun-moon dance
More about the Egyptian astronomer Ptolemy

Bibliography and further reading about Eratosthenes:

Greek and Roman Science, by Don Nardo (1998). Nardo has written a lot of good books about the ancient world for kids; this one is no exception.

Ancient Science: 40 Time-Traveling, World-Exploring, History-Making Activities , by Jim Wiese (2003). Activities, as the title says - how to make your own sundial, and so on. The author is a science teacher.

Early Greek Science: Thales to Aristotle, by Geoffrey Lloyd (1974).

History of Greek Mathematics: From Aristarchus to Diophantus, by Thomas L. Heath (1921, reprinted 1981). A lot of Euclid, but also describes who the other major Greek mathematicians were and what they did.

Episodes from the Early History of Mathematics, by Asger Aaboe (1997).

More about Ptolemy
Hellenistic Egypt
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Karen Carr is Associate Professor Emerita, Department of History, Portland State University. She holds a doctorate in Classical Art and Archaeology from the University of Michigan. Follow her on Instagram or Twitter, or buy her book, Vandals to Visigoths.
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