Eratosthenes (c.276-194 B.C.), a mathematician, is known for his mathematical calculations and geometry.

Eratosthenes was called "Beta" (the second letter of the Greek alphabet) because he was never first, but he is more famous than his "Alpha" teachers because his discoveries are still used today. 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.

He made a calendar with leap years, a 675-star catalog, and maps. He recognized the Nile's source was a lake, and that rains in the lake region caused the Nile to flood.

### Eratosthenes - Life and Career Facts

Eratosthenes was the third librarian at the famous Library of Alexandria. He studied under the Stoic philosopher Zeno, Ariston, Lysanias, and the poet-philosopher Callimachus. Eratosthenes wrote a *Geographica* based on his calculations of the circumference of the earth.

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

### Writing of Eratosthenes

Much of what Eratosthenes 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 Syene.

### Eratosthenes Calculates the Circumference of the Earth

By comparing the shadow of the sun at Summer Solstice noon at Alexandria and Syene, and knowing the distance between the two, Eratosthenes calculated the circumference of the earth.The sun shone directly into a well at Syene at noon. At Alexandria, the angle of inclination of the sun was about 7 degrees.

With this information, and knowing that Syene was 787 km due south of Alexandrian Eratosthenes calculated the circumference of the earth to be 250,000 stadia (about 24,662 miles).