© Sierra College Astronomy Department
Ancient Astronomy Eratosthenes
250,000 stadia roughly translates into 50,000 km, based on our best guess as to the size of a stadium. The Earth’s actual circumference is about 40,000 km, so Eratosthenes calculation is 25% too big. But his geometrical method is correct.
Earth’s circumference of 40,000 km gives a diameter of about 13,000 km (~ 8,000 mi).