Eratosthenes, of Cyrene | Biography

Eratosthenes was one of the most versatile scholars of the ancient Greek world. As a mathematician and chief librarian at the Museum of Alexandria, in Egypt, he worked at the center of Greek science. He was the founder of mathematically based geography and was famous for his accurate measurement of Earth's circumference. Eratosthenes, along with his friend Archimedes, Euclid, and other third-century B.C. mathematicians, raised Greek science to unprecedented heights.

Eratosthenes was born in Cyrene, a prosperous Greek city-state in North Africa (modern-day Libya), about 276 B.C., although his birthdate is sometimes placed as early as 285 B.C.. The son of Aglaos, he studied with the grammarian Lysanias in Cyrene and perhaps with the poet Callimachus in Alexandria and Eratosthenes achieved early renown as a poet. He then spent several years in Athens, studying astronomy and geometry with Arcesilaus, and philosophy with Bion the Cynic, Ariston of Chios, and Apelles, at the Athenian Academy. Subsequently, Eratosthenes was invited by Ptolemy III (Euergetes I) to return to Alexandria as tutor to the royal family. About 235 B.C., he became head of the Alexandrian Library.

Prior to Eratosthenes, geography was based primarily on the stories written down by the poet Homer. Eratosthenes argued that poetry was for enjoyment and should not be used as the basis for scientific geography. So he set about mapping the inhabited world, from the Atlantic Ocean to the Ganges River. With the knowledge of the Great Library at his disposal, he began by dividing the known world into northern and southern divisions with an east-west line that passed through the Straits of Gibraltar and the island of Rhodes, reaching the Himalayan Mountains in the east. At right angles to this line, he placed his fundamental north-south meridian, passing through Alexandria. Parallel lines were drawn through major landmarks such as Sicily, the Pillars of Hercules, the Euphrates River, and the tip of India. He pointed out that it would be possible to sail due west from Spain across the Atlantic Ocean and reach India. In the following century, Eratosthenes's work was extended by his vocal critic, Hipparchus of Nicaea, who used astronomical observations to accurately define longitude and latitude.

Eratosthenes's most famous accomplishment was his accurate determination of the circumference of Earth. He had heard from travelers that there was a well at Syene, a Greek city in Egypt near modern Aswan, where, at noon on the summer solstice, the sun did not cast a shadow, indicating that it was directly overhead. From his map, Eratosthenes thought that Syene was due south of Alexandria, on the same meridian. Therefore, he measured the shadow of a rod in Alexandria at noon on June 21 and, using simple geometry, calculated that the sun was 7ò12' from overhead. Since this was one-fiftieth of a full circle, he deduced that Earth's circumference was 50 times the distance from Alexandria to Syene. He determined this distance based on how far a camel traveled in one day, and the number of days it took to make the journey. Although his calculation for the angle of the sun was extremely accurate, his camel calculations were less so. Nevertheless, his was the most accurate value for Earth's circumference available until modern times.

Geographica, which dealt with mathematical and physical geography as well as ethnogeography, was Eratosthenes's most influential work. His terrestrial globe divided Earth into frigid circular zones at each pole (the Arctic and Antarctic), two temperate zones, and the tropics or torrid zone bisected by the equator. The Greeks had long been puzzled by the reverse currents found in straits, but Eratosthenes analyzed them accurately and explained that they were tidal, like the ocean.

Eratosthenes discussed many different mathematical problems in his Platonicus, including proportion, progression, and the theory of musical scales. The Greeks solved problems in geometry by using proportions and Eratosthenes attacked the Delian problem, or the duplication of the cube. This was the question of how to find an infinite number of mean proportionals between two straight lines. He designed an instrument called a mesolabe to demonstrate his mathematical method for solving this problem. The sieve of Eratosthenes, his simple method for finding prime numbers, was a major contribution to number theory.

In addition to his mathematical and geographical treatises, Eratosthenes wrote works on grammar and philosophy. Ancient Comedy was his most important work of literary criticism. He also compiled a historical chronology of Greek literature and politics, Chronography, with a surprisingly accurate dating of events. To achieve this, he developed a system of dating based on the timing of the Olympiads, the original Olympic Games.

An influential member of the royal court, Eratosthenes remained at the Alexandrian Library until his death at about the age of 80. After going blind, he committed suicide by starvation in ca195 B.C. Only fragments of his writings have survived.