Who was Eratosthenes of Cyrene?
Chief librarian in Alexandria
When he was still young he moved to Alexandria, in Egypt. He studied mathematics and astronomy at the University of Alexandria. You can see one of the classrooms in the picture!
Then in 236 BC, when he was forty years old, Eratosthenes of Cyrene became the chief librarian at the great library of Alexandria. Eratosthenes spent the rest of his life in Alexandria. He studied at the library and helped other scientists with their projects too.
Eratosthenes got to know other scientists
Many other scholars also worked at Alexandria’s big university and library. Eratosthenes had the chance to get to know them. For example, he seems to have known both Aristarchus and Archimedes. They were Greek astronomers who came to Alexandria, as he did, to study mathematics and astronomy.
Eratosthenes and the circumference of the earth
Eratosthenes built on the work of Thales and Aristarchus. Aristarchus 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. He did his calculations using the angle of the sun at noon on the summer solstice in two different locations.
How far away are the sun and the moon?
Erastosthenes also correctly calculated the distance from the earth to the sun and the moon.
The sieve of Erathosthenes
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’ world map
Eratosthenes also drew an early map of the world, inventing the idea of latitude and longitude to help him in this project.
The Mediterranean Sea and Europe
He had a pretty good idea of what the Mediterranean looked like. And he made sure to include Libya (where he was born). But there’s a lot Eratosthenes doesn’t seem to know about what the world looks like. England (Britain) is much too big, and so is Ireland.
Scandinavia hardly exists at all.
India, China, and the Americas
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).