Despite what flat-earth people around the world might say, the earth is round and this knowledge is not a modern invention. Many ancient cultures were aware of this fact – you don’t need satellites or lasers to prove that our planet is round. Among many we remember the name of the Greek polymath and the first person to determine the circumference of the earth, Siren’s Eratosthenes. He calculated it in the third century BC and it was incredibly accurate for the time being.

His exact method is not known, but a simplified version was reported by the Greek astronomer Cleomedes. Eratosthenes knew that the distance between the cities of Alexandria and Syene (modern Aswan) was 5000 Egyptian stadia (157.5 meters or 517 feet). He further assumed that the two cities were under the same meridian (not the stars) and the sign was exactly under the summer of Cancer (nearby, but not exactly). The rest is a bit geometric and well timed. At noon on the summer solstice in Seneca, the sun is perpendicular to the tropics of Cancer, so a rod planted in the ground can cast no shadow.

In Alexandria the sun will appear at a slight angle instead, so a stick of a certain length will cast a shadow of a length depending on the tendency of the sun’s rays. Eratosthenes measured the angle and it worked with about 1/50 of the circumference, so knowing the distance between Alexandria and Sinai is 5,000 stadia, the whole circumference of the earth must be 50 times: 250,000 stadia. This is exactly 1.4 percent of the original number.

He further assumed that the two cities were under the same meridian (not the stars) and the sign was exactly under the summer of Cancer (nearby, but not exactly). The rest is a bit geometric and well timed. At noon on the summer solstice in Seneca, the sun is perpendicular to the tropics of Cancer, so a rod planted in the ground can cast no shadow.

Eratosthenes measured the angle and it worked with about 1/50 of the circumference, so knowing the distance between Alexandria and Sinai is 5,000 stadia, the whole circumference of the earth must be 50 times: 250,000 stadia. This is exactly 1.4 percent of the original number.