Eratosthenes was a Greek scholar who calculated the circumference of the Earth using shadows from sticks in Alexandria and Syene. He observed that on the summer solstice, a stick in Alexandria cast a shadow while a stick in Syene did not. Knowing the distance between the cities, he calculated the circumference of the Earth to within a few percent of the actual measurement. Eratosthenes made several other contributions including accurately calculating the tilt of the Earth's axis and inventing the leap day.