How Eratosthenes calculated the Earth's circumference

In modern times, satellites determined the exact circumference of the Earth at 40,030 km. 2,000 years earlier in ancient Greece, a man arrived at nearly that exact figure by putting a stick in the ground. Eratosthenes was a Greek mathematician and the head of the library at Alexandria. The idea of a spherical Earth was floated by Pythagoras around 500 BC and validated by Aristotle a few centuries later.

The method of measuring the Earth's circumference was carried out in Alexandria in the 3rd century BC. Eratosthenes read that there was no shadow at the stick-erected upright at noon on June 21 in the city of Syene near the Nile River. There was a shadow of the stick at noon on June 21st in the city of Alexandria. Why were the shadows of the sticks different in the two cities? There was only one answer: The Earth was curved. The distance between Alexandria and Syene had to be 7 degrees on the surface of the Earth according to the difference between the shadow lengths of the two sticks.