How Eratosthenes calculated the Earth's circumference

In the mid-20th century, satellites determined the exact circumference of the Earth, 40,030 km. But over 2,000 years earlier in ancient Greece, a man arrived at nearly that exact same figure by putting a stick in the ground. That man was Eratosthenes. A Greek mathematician and the head of the library at Alexandria. The idea of a spherical Earth was floated around by Pythagoras around 500 BC and validated by Aristotle a couple centuries later.