Who was Eratosthenes? Astronomy and math

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Three levels of stone benches in a U shape around a block in the middle. There's an access stairway at the back of the room. Ruins are in the background. Eratosthenes of Cyrene might have studied here.

Eratosthenes of Cyrene went to school here! A classroom at the University of Alexandria (al-Ahram 2004).

Who was Eratosthenes of Cyrene?

Eratosthenes of Cyrene was born about 276 BC in Cyrene (modern Libya).

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!

(More about the library at Alexandria)

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.

(More about Aristarchus)

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.

(More about circumference)

How far away are the sun and the moon?

Erastosthenes also correctly calculated the distance from the earth to the sun and the moon.

Sieve of Eratosthenes animation.gif

Sieve of Eratosthenes animation Licensed under CC BY-SA 3.0 via Wikimedia Commons.

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.

(More about prime numbers)


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.

Copy of Eratosthenes' map of the world

Copy of Eratosthenes’ map of the world

Scandinavia hardly exists at all.

Central Asia

Eratosthenes didn’t understand how big Central Asia was. He knew Ethiopia, the Scythians, and the Arabian peninsula.

(More about the Scythians)

India, China, and the Americas

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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By |2018-05-16T18:40:23+00:00July 18th, 2017|Africa, Egypt, Science|0 Comments
Cite this page: Carr, K.E. Who was Eratosthenes? Astronomy and math. Quatr.us Study Guides, July 18, 2017. Web. December 12, 2018.

About the Author:

Dr. 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, Pinterest, or Facebook, or buy her book, Vandals to Visigoths.

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