Eratosthenes of Cyrene (/ɛrəˈtɒsθənz/; Greek: Ἐρατοσθένης [eratostʰénɛːs]; c. 276 BC c.195/194 BC) was a Greek polymath: a mathematician, geographer, poet, astronomer, and music theorist. He was a man of learning, becoming the chief librarian at the Library of Alexandria. His work is comparable to what is now known as the study of geography, and he introduced some of the terminology still used today.[1]

An etching of a man's head and neck in profile, looking to the left. The man has a beard and is balding.
Etching of an ancient seal identified as Eratosthenes. Philipp Daniel Lippert, Dactyliothec, 1767.
Born276 BC[note 1]
Cyrene (in modern Libya)
Died194 BC (around age 82)[note 2]
  • Scholar
  • Librarian
  • Poet
  • Inventor
Known for

He is best known for being the first person known to calculate the circumference of the Earth, which he did by using the extensive survey results he could access in his role at the Library; his calculation was remarkably accurate.[2][3] He was also the first to calculate Earth's axial tilt, which has also proved to have remarkable accuracy.[4] He created the first global projection of the world, incorporating parallels and meridians based on the available geographic knowledge of his era.

Eratosthenes was the founder of scientific chronology;[5] he used Egyptian and Persian records to estimate the dates of the main events of the mythical Trojan War, dating the sack of Troy to 1183 BC. In number theory, he introduced the sieve of Eratosthenes, an efficient method of identifying prime numbers.

He was a figure of influence in many fields who yearned to understand the complexities of the entire world.[6] His devotees nicknamed him Pentathlos after the Olympians who were well rounded competitors, for he had proven himself to be knowledgeable in every area of learning. Yet, according to an entry[7] in the Suda (a 10th-century encyclopedia), some critics scorned him, calling him Beta (the second letter of the Greek alphabet) because he always came in second in all his endeavours.[8]

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