Eratosthenes, mathematician and librarian
c. 276 BC-194 BC
The Librarian Who Measured the Earth by Kathryn Lasky, illustrated by Kevin Hawkes (Little Brown, 1994)
Sun and shadows. Those were the keys to the Eratosthenes' discovery of how to measure the Earth's circumference. A fascinating look at ancient times and at how geometry can make sense of our world.
Learn how Eratosthenes figured it out at the Cornell website.