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Eratosthenes (ca. 275-ca. 195 B.C.E.), who was born in coastal North Africa (now Libya), went to school in Athens and then was called to Egypt by Ptolemy III, who asked him to become director of the library/museum/university at Alexandria. He was the perfect person for that job. Eratosthenes (er-uh-TOS-thuh-neez) was a great scholar himself, and he energized others. His nickname was "Beta," which is the second letter in the Greek alphabet. Some people thought him second only to Aristotle in broad talent. We know Archimedes was one of his friends, because Archimedes dedicated a book to him. But today Eratosthenes is remembered mostly because of some measuring he did.

Eratosthenes discovered that at noon on the summer solstice (the longest day of the year), at Syene (an Egyptian city near modern Aswn on the southern Nile) the Sun's rays lit the very bottom of a deep well, and a stick cast no shadow at all.

That wasn't true at Alexandria, on the northern mouth of the Nile, where at the same time on the same day, the Sun's rays did cast a shadow.

Eratosthenes must have asked himself, "Why a shadow at Alexandria and no shadow at Syene?" Then he figured out that the Sun must be right overhead at Syene but not quite overhead at Alexandria, and he realized that he had hit upon valuable information. He saw it as indicating that the Earth is curved. He wanted to prove that idea.

Eratosthenes is said to have put a stick in the ground in Alexandria and measured the angle of its shadow. It was 7.2 degrees, which is 1/50 of a circle. If the Earth is truly a globe - as he believed - then the distance between Alexandria and Syene is 1/50 of that globe. To find the size of the whole globe, all he had to do was measure the distance between the two cities. So he paid someone to walk from Alexandria to Syene, counting his steps, and then he multiplied that distance by 50. The answer was very close to the best modern measurements of the circumference of the Earth. That was more than two millennia ago, and the only equipment Eratosthenes used was a stick in the ground, his brain, and a hired walker.

Given what he found to be the large size of the Earth and the small size of the known land, Eratosthenes surmised that there was a huge interconnected ocean. (That would be verified by the voyage of Magellan 18 centuries later.)

Most of Eratosthenes' works, like those of most of the ancient thinkers, has been lost. Much of what we know of him and his accomplishments comes from the written comments of others. And they tell us that he was a geographer, historian, and literary critic, as well as an astronomer. He was the first man, of whom we know, who was concerned with accurate dating. He set up a chronology that began history with the Trojan War. He devised a system for determining prime numbers that is still called the Eratosthenes' Sieve. But it was when he came up with that close-to-accurate measurement of the Earth that he demonstrated that the universe is understandable. Given some brainpower, we can figure out how it works. What would his contemporaries and future generations do with that insight? For a long time they just forgot all about it.

-- From The Story of Science, by Joy Hakim. Published by Smithsonian Books (800) 233 4830. 10/11/2004

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