Eratosthenes of Cyrene

Edited by afrorama
Last updated:
May 24, 2023

Born in Cyrene, which is present-day Shahhat in Libya, Eratosthenes of Cyrene was the first to calculate the circumference of the Earth and its axial tilt with great accuracy, earning him the title of, "Father of Geography''. His expertise ranged from mathematics, geography, astronomy, to poetry and music theory, making him a polymath. His exact year of birth varies across sources, with the prevailing accounts being between 285 and 276 BC. As he preferred to explore many fields of study instead of specialising, his admirers nicknamed him “Pentathlos” by his admirers, after Olympic athletes for their well-rounded capabilities, whilst his critics preferred “Beta”, second in many endeavours but never the best.


There is great debate on Eratosthenes being of Greek origin, owing to the geopolitical status of Libya as a Greek colony established in 631 BC. Cyrene was conquered by Alexander the Great in 332 BC, and after his death in 323 BC, Cyrene’s rulership was given to Ptolemy I Soter, who was one of Alexander’s generals and the founder of the Ptolemaic Kingdom in 305 BC. Under Ptolemaic rule, Cyrene was amongst the most influential intellectual centres of the classical world.

Source: ThoughtCo

Eratosthenes moved to Athens to further his studies. It is thought that Eratosthenes was inspired by the works of his tutor, Arcesilaus, which encouraged constant questioning and testing of ‘knowledge’ rather than acceptance. Despite his works not surviving, Eratosthenes' main achievements can be reconstructed. While in Athens, Eratosthenes wrote works including Olympionikai, which detailed Olympic winners and served as a preliminary study for the chronographical tables. Chronographiai (chronographical tables) synthesised Greek events from the Trojan War to the death of Alexander the Great under a universal chronology, which had not existed until then.Eratosthenes moved to Alexandria in Egypt to be a librarian in the Library of Alexandria at the request of Pharaoh Ptolemy III Euergetesin in 245 BCE. He became the third chief librarian in 240 BCE and tutored the children of the royal family. Under his leadership, the Library of Alexandria came to be the greatest repository of learning in the Mediterranean. The position of Chief Librarian came with full access to the vast collection of knowledge found there, hence along with his curatorship and tutor responsibilities, he continued his work.


Eratosthenes' most prominent achievement was the calculation of the Earth’s circumference presented in Peri tes avametreoeos tes ges (On the Measure of the Earth). Using two of Euclid’s geometric propositions, Eratosthenes reasoned that the distance between Alexandria and Syene (now Aswan on the Nile in Egypt) represented a fifth of Earth’s circumference. Eratosthenes concluded the Earth’s circumference to be 252,000 stades (39,777 kilometres), compared to the generally accepted 300,000 stades of that time. His accuracy was debated but compared with results from the best modern measurements,  Eratosthenes’s calculation was within the top 1 percent.

Within mathematics, Eratosthenes’s most renowned contribution was “Eratosthenes’s Sieve,” an algorithm to identify prime numbers. He proposed a solution to the “Delian Problem,” the problem of doubling a cube to hold double the volume, he suggested this could be solved by constructing a mesolabio, which was a mechanical line-drawing device. None of his earlier poetry survived, but two literary pieces are recalled by name, one being Hermes inspired by astrology and the second being Erigone, on the suicide of an Athenian maiden. In Peri archaias komoidias (On Ancient Comedy), Eratosthenes looked into earlier theatre pieces. His last work, Arsinoe was said to be a memoir of Arsinoe III, the wife and sister of Ptolemy IV.

Source: Wikipedia Commons


It is said in his later years, Eratosthenes became blind and voluntarily starved to death. He was buried in Alexandria within sight of the Library. Though his works have great prominence, they were still challenged and eventually reworked by Posidonius of Rhodes, whose system was considered easier to use. Nonetheless, Eratosthenes continues to be recognised as one of the greatest intellectuals of his time.