Eratosthenes was called "Beta" because he was never first place in anything, but the third librarian at Alexandria, who studied under the Stoic philosopher Zeno, Ariston, Lysanias, and the poet-philosopher Callimachus, is more famous than his "Alpha" teachers because his discoveries are still used in science.

Eratosthenes, born in 276 B.C., in Cyrene (Syrene) in what is Libya today, is known for many observations and calculations. Chief among these are the calculation of the circumference of the earth (note: the Greeks did know the earth was spherical) and the development of a mathematical sieve named after him.

Much of what he wrote is now lost, including a geometrical treatise, On Means, and one on the mathematics behind Plato's philosophy, Platonicus. He also wrote the fundamentals of astronomy in a poem called Hermes. His most famous calculation, in the now lost treatise On the measurement of the Earth, explains how he compared the shadow of the sun at Summer Solstice noon in two places, Alexandria and Syrene. Based on the distance between the two, he calculated the circumference of the earth to be 250,000 stadia (about 24,662 miles). stadia. He calculated the distance from the sun at 804,000,000 stadia and the distance from the moon at 780,000 stadia.

He also devised a calendar with leap years, a 675-star catalogue, and maps. He recognized a lake as the source for the Nile and that rains in the lake region caused the Nile to flood.

He is reported to have starved himself to death at Alexandria in 194 B.C.

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