I have read that the ancient Greeks calculated the cicumference of the earth nearly 2500 years ago. When compared to what we know now about the earth’s actual cicumference they were only off by a couple hundred miles.
CC
The Great Pyramid also calculates the Earth’s circumference and is only off by feet - done almost 3000 years before the Greeks.
Eratosthenes.
Opposite interior angles of parallel lines (rays of the sun). The length of the legs of a triangle formed by the sun casting a shadow on a stick on the ground is similar to a triangle formed between the city you are in and one of the tropic lines, and the radii of the earth to each city.
He calculated the circumference of the earth, and the distance between cities, within a few miles.
It’s also proof positive the earth is curved, though its not what we teach kids in school. Apparently, we needed Magellan to deliver the empirical evidence.