Hellenic geographer, librarian, and astronomer Eratosthenes of Cyrene (276-194 B.C.E.) made several accurate measurements of the Earth, which is why he is often known as the “father of geodesy” (the science of Earth measurement). Although he was not the very first to deduce the Earth’s circumference, Eratosthenes is thought by most historians to be the first to accurately measure it.

Eratosthenes knew the Sun’s light at noon reached the bottom of a well in Syene (now Aswan on the Nile in Egypt) on the summer solstice (which meant the Sun was directly overhead). Thanks to a helper, he compared it to a well’s shadow at the same time in Alexandria. He determined that the zenith distance—or the angle from the zenith (point directly overhead) to the point where the Sun was at noon—was 0 degrees at Syene, and at Alexandria it was about 7 degrees. By measuring these angles and the distance between the two cities, Eratosthenes used geometry to deduce that the Earth’s circumference was 250,000 stadia. The number was later revised to 252,000 stadia, or 25,054 miles (40,320 kilometers).

The actual circumference of the planet is 24,857 miles (40,009 kilometers) around the poles and 24,900 miles (40,079 kilometers) around the equator, because the Earth is not completely round. From his data Eratosthenes also determined another accurate measurement: the Earth’s diameter. He deduced the Earth was 7,850 miles (12,631 kilometers) in diameter, which is close to the modern mean value of 7,918 miles (12,740 kilometers).

Eratosthenes brilliantly used his knowledge of angles and mathematics to be the first to determine the Earth’s circumference accurately.