Tim Steinbacher, age 13, of Denton, Texas, for his question:

WHO WAS ERATOSTHENES?

Eratosthenes was a famous Greek mathematician who lived in the 200s B.C. He found a way to determine the size of the earth. He also developed the Sieve of Eratosthenes, a system for identifying prime numbers.

Eratosthenea based his measurement of the earth on the assumptions that the earth is round and the sun's rays are parallel. He knew that at noon of the day of summer solstice in Alexandria, Egypt, a vertical post casts a shadow. At the same time in Syene, a town directly south, a vertical post casts no shadow.

Eratosthenes used Euclidean geometry to determine that the angle formed by the post and an imaginary line from the end of the shadow to the top of the post equals an angle at the earth's center formed by imaginary lines from the two towns. He calculated the earth's circumference by mesuring the distance between Alexandria and Syene, and multiplying it by the number of times the angle at the earth center is contained in 360 degrees.

From the circumference, Eratosthenes found the diameter to be about 7,850 miles. The correct polar diameter is about 7,900 miles.

Eratosthenes was also a talented astronomer, athlete, historian and poet.