Eratóstenes

Eratóstenes of Cyrene (c. 275–194 BCE) was a Greek mathematician, geographer, and astronomer who served as the chief librarian of the Library of Alexandria. He is regarded as one of the most important early scholars in geography and number theory, helping to systematize knowledge about the world known to the Hellenistic world and to advance methods of mathematical reasoning.

One of Eratóstenes’s lasting contributions is the Sieve of Eratosthenes, an efficient algorithm for identifying prime numbers. By repeatedly marking the multiples of successive primes, the method can enumerate primes up to any given limit. The sieve remains a standard example in number theory and computer science for illustrating basic algorithmic thinking.

In geography, Eratóstenes is best known for estimating the Earth’s circumference. He observed that at local noon during the summer solstice the Sun was directly overhead in Syene (today Aswan), while in Alexandria it cast a measurable shadow, forming an angle of about 7.2 degrees. Treating this angle as 1/50 of a full circle and using the distance between the two cities (about 5,000 stadia), he concluded that the Earth’s circumference was approximately 50 times that distance, close to 40,000 kilometers. The exact figure depends on the length assigned to a stadion, but the method yielded a remarkably accurate scale for the era.

Eratóstenes also contributed to geographic writing and cartography, notably through the work Geographika, which described the known world and laid groundwork for later mapmaking. He died in Alexandria, leaving a legacy as a pioneering interdisciplinary scholar whose methods influenced mathematics, geography, and scientific inquiry for centuries.