Erathostenes

Eratosthenes was born about 276 BC in Cyrene (modern Libya). When he was still young he moved to Alexandria, in Egypt, and in 236 BC, when he was forty years old, he became the chief librarian at the great library there. Eratosthenes spent the rest of his life in Alexandria, where he seems to have known both Aristarchus and Archimedes.

Eratosthenes built on the work of Thales and Aristarchus, who had shown that the earth was round and that the earth went around the sun. Erathosthenes was able to add a fairly accurate calculation of the circumference of the earth - how big the earth was - based on calculations using the angle of the sun at noon on the summer solstice in two different locations. Erastosthenes also correctly calculated the distance from the earth to the sun and the moon.

Eratosthenes also drew an early map of the world, inventing the idea of latitude and longitude to help him in this project. Although he had a pretty good idea of what the Mediterranean looked like, and he made sure to include Libya (where he was born), there's a lot Eratosthenes doesn't seem to know. England (Britain) were much too big on the map he created, and so was Ireland.

Scandinavia hardly exists at all. Eratosthenes didn't understand how big Central Asia was. He knew the Arabian peninsula, and he knew a little about India and Sri Lanka, but he doesn't seem to have heard of China. Of course he didn't know about the Americas or Australia either.

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

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