Eratosthenes

Eratosthenes of Cyrene was a Hellenistic scholar, geographer, mathematician, and librarian of Alexandria. Around the third century BC he estimated the circumference of the earth with remarkable accuracy.

Born: c. 276 BCE
Died: c. 194 BCE
Role: Scholar and geographer

Alexandrian scholar (c. 276 BC-c. 194 BC)

c. 276-c. 194 BCE

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Portrait of Eratosthenes with maps and astronomical diagrams

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Full name: Eratosthenes of Cyrene
Region: Egypt, Cyrene, Greek world, Alexandria and Cyrene

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Eratosthenes timeline facts

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c. 276 BC: From Cyrene
third century BC: Library of Alexandria
c. 240 BC: Measuring the earth
After c. 240 BC: Geography as science

Life Journey

Measuring the world from Alexandria

Follow Eratosthenes from Cyrene to the Library of Alexandria and his famous calculation of the earth.

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c. 276 BC

From Cyrene

Eratosthenes was born in Cyrene and became part of the Greek-speaking scholarly world of the Hellenistic age.

third century BC

Library of Alexandria

Eratosthenes became head of the Library of Alexandria, one of the ancient world's greatest centers of learning.

c. 240 BC

Measuring the earth

Using shadows at Syene and Alexandria, Eratosthenes estimated the earth's circumference with remarkable accuracy.

After c. 240 BC

Geography as science

Eratosthenes helped make geography a mathematical and historical discipline, not just a list of places.

c. 276 BC

From Cyrene

Eratosthenes was born in Cyrene and became part of the Greek-speaking scholarly world of the Hellenistic age.

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c. 276 BC

From Cyrene

Eratosthenes came from Cyrene in North Africa, a Greek city connected to the wider Mediterranean world. His education placed him within the intellectual networks that linked Athens, Alexandria, and other Hellenistic centers. He became known for range rather than narrow specialization: mathematics, poetry, geography, chronology, astronomy, and criticism all belonged to his scholarly world.

The Hellenistic world rewarded scholars who could connect many fields at once.

third century BC

Library of Alexandria

As librarian at Alexandria, Eratosthenes worked at the heart of Ptolemaic scholarship. The Library and Mouseion gathered texts, scholars, observations, and debates from across the Greek-speaking world and beyond. This environment made large-scale comparison possible. Geography, astronomy, literature, and mathematics could inform one another because the city collected people and knowledge in the same place.

Institutions can make individual brilliance more cumulative.

c. 240 BC

Measuring the earth

Eratosthenes used a famous comparison between the sun's angle at Alexandria and reports from Syene, where the sun was said to shine nearly overhead at noon on the summer solstice. By combining geometry with the distance between the two places, he calculated the earth's circumference. The exact figures depend on the length of the stadion he used, but the method was extraordinary: local observations became a measurement of the whole planet.

A global fact can be reached from two local measurements and a disciplined question.

After c. 240 BC

Geography as science

Eratosthenes wrote on geography, mapped inhabited regions, studied chronology, and developed tools for thinking about the earth systematically. Later ancient writers sometimes disagreed with him, but they worked in a world he helped define. His legacy is not just the famous circumference calculation. It is the idea that the world could be measured, mapped, dated, compared, and reasoned about as a connected whole.

Geography becomes powerful when maps, measurement, history, and mathematics meet.

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