Here’s an account….

“Let’s start with the easiest measurement: the size of the Earth. High school science text books sometimes mention Eratostheses as the first person to make a good measurement of the Earth’s diameter. Eratosthenes was the librarian of the great Library of Alexandria around 250 BC. He had access to one of the largest collections of knowledge ever available up to that time (sort of like an early World Wide Web…).

While reading in the library, Eratosthenes discovered that, on a certain day in the year, the sun was directly overhead Syene, a city in the far south of Egypt. He knew the distance to Syene from other books in the library. By measuring the angle of the sun in Alexandria on the right day, he found that the Earth is about 7850 miles in diameter, very close to our modern number of 7900 miles. To be fair, Eratosthenes was not the first to try this measurement. The concept of a round Earth had been around for 200 years; Aristotle and other Greeks tried the same trick using stars, but they never got a very accurate number.”

Since his name is long and unusual, he is easy to find on the Internet, and most of the sources will tell you about the same thing, so you won’t need to search forever to get background material. I do recommend that you understand his experiment, so get a good description.