Biography of EratosthenesEratosthenes was born around 276 B.C.E. at a Greek colony in Cyrene, Libya. He was educated at the academies of Athens and was appointed to run the Great Library at Alexandria in 240. While serving as head librarian and scholar, Eratosthenes wrote a comprehensive treatise about the world, called Geography. This was the first use of the word, which literally means "writing about the earth" in Greek. Geography also introduced the climatic concepts of torrid, temperate, and frigid zones.
Eratosthenes' ExperimentHaving heard of a deep well at Syene (near the Tropic of Cancer and modern Aswan) where sunlight only struck the bottom of the well on the summer solstice, Eratosthenes determined that he could discover the circumference of the earth. (Greek scholars knew that the earth was indeed a sphere). To calculate the circumference, Eratosthenes needed two things. He knew the approximate distance between Syene and Alexandria, as measured by camel-powered trade caravans. He then measured the angle of the shadow in Alexandria on the solstice. By taking the angle of the shadow (7°12') and dividing it into the 360 degrees of a circle (360 divided by 7.2 yields 50), Eratosthenes could then multiply the distance between Alexandria and Syene by 50 to determine the circumference.
Remarkably, Eratosthenes determined the circumference to be 25,000 miles, just 100 miles over the actual circumference at the equator (24,901 miles). While Eratosthenes made mathematical errors in his caculations, these fortunately canceled each other out and yielded an amazingly accurate answer.
A few decades later, the Greek geographer Posidonius thought Eratosthenes' circumference was too large. He calculated the circumference on his own and obtained 18,000 miles, 7,000 miles too short. During the middle ages, most scholars accepted Eratosthenes' circumference though Christopher Columbus used Posidonius' circumference to convince his supporters that he could quickly reach Asia by sailing west from Europe.
In old age, Eratosthenes became blind and died of self-induced starvation in either 192 or 196 B.C.E. He had thus lived to be about 80 to 84 years old.