Ancient Science · Part 10 · Alexandria · B1–B2
Eratosthenes Measures the Earth: A Shadow, a Well and the Size of the World
Part 10 of Ancient Science: Eratosthenes, head of this very Library, measured the whole Earth with a stick, a shadow and one distance he could not check very well. Sophia shows how his method worked, why his books are lost, and why we still cannot say exactly how close he came. About, roughly, approximately and how far, how long, with Philo, Sophia, Marcos and Demetria.
Tonight we meet a man who worked in this very Library, as its chief librarian, and measured the whole Earth without leaving Egypt.
How can you measure the Earth without walking round it?
With a stick, a shadow, and a little geometry. And with one number he could not check very well.
Eratosthenes was born in Cyrene, west of Egypt, studied in Athens, and was invited to Alexandria by King Ptolemy III to become head of the Library. He was a mathematician, a geographer, a poet and a historian. Some people called him “Beta”, the second letter of the alphabet, because he was second best in everything; others called him “Pentathlos”, the all-round athlete.
He wrote a book of geography, with a map of the known world; a book of chronology, which tried to give dates to events from the fall of Troy onwards; and a method for finding prime numbers, now called his “sieve”. Almost all of his books are lost. We know them from later writers, especially the geographer Strabo, who often disagreed with him.
Or try our free games and lessons.