Eratosthenes
Eratosthenes
Circa 285-194 b.c.e.
Librarian, Astronomer, Geographer
Encyclopedic Knowledge. Eratosthenes was born in Cyrene (present-day Libya) and became the librarian at Alexandria during the second half of the third century b.c.e. after studying with the philosopher Ariston of Chios, who himself was a student of Zeno, the founder of the Stoic school of philosophy. The period during which Eratosthenes lived, the Hellenistic period, was one that encouraged eclectic scholars, as many of the personalities connected with the Library at Alexandria demonstrate. In his post as librarian Eratosthenes followed such Hellenistic scholars as Apollonius of Rhodes, who is most noted for his adaptation of classical poetry. (Apollonius wrote the Argonautica, a famous poem on the myth of Jason and the Argonauts that brilliantly incorporates allusions to earlier Greek poetry.) Like Apollonius, Eratosthenes demonstrated the encyclopedic knowledge that characterized his time; he wrote on every subject from comedy to philosophy. More importantly, Eratosthenes composed a seminal thesis on geography in three books, which includes the title Peri tês anameîrêseôs tês gês (“On the Measurements of the Earth”), a work that, although subsequent authors regarded it as pivotal, unfortunately only survives through citations in other works.
Circumference. Eratosthenes is credited with being the first person to have calculated the Earth’s circumference. He observed that at Syene (present-day Aswan) in Egypt, the Sun’s rays fell vertically at noon on the summer solstice. Meanwhile, on the same date and at the same time in Alexandria, sunlight fell at an angle of 7° from the vertical. Correctly assuming that the Sun’s distance from the Earth is great, he concluded that the rays are nearly parallel when they reach the Earth. Eratosthenes then calculated the Earth’s circumference by measuring the distance between the two cities (five hundred miles). Because the exact unit of measurement he used is uncertain, astronomers today believe that his calculation varies anywhere from 0.5 percent to 17 percent from the modern figure.
Sources
Hipparchus Bithynius, The Geographical Fragments of Hipparchus, edited by D. R. Dicks (London: Athlone, 1960).
Peter Marshall Fraser, “Eratosthenes,” in The Oxford Classical Dictionary, edited by Simon Hornblower and Antony Spawforth, third edition (Oxford: Oxford University Press, 1999), pp. 553–554.