This gentleman used a well, a few sticks, and a lot of pondering to figure out the circumference of the Earth 2300 years ago. That doesn’t say a lot about modern people with all our technology remaining willfully ignorant about common geography.
Having heard of a deep well at Syene (near the Tropic of Cancer and modern-day Aswan) where sunlight only struck the bottom of the well on the summer solstice, Eratosthenes worked out a method by which he could calculate the circumference of the earth using basic geometry. (Greek scholars knew that the earth was indeed a sphere.) The fact that Eratosthenes was a close friend of the famous Greek mathematician Archimedes is perhaps one reason for his success in this calculation. If he did not collaborate directly with Archimedes in this exercise, he must have certainly been helped by his friendship with the great pioneer in geometry and physics.
To calculate the circumference of the earth, Eratosthenes needed two critical measurements. He knew the approximate distance between Syene and Alexandria, as measured by camel-powered trade caravans. He then measured the angle of the shadow in Alexandria on the solstice. By taking the angle of the shadow (7°12’) and dividing it into the 360 degrees of a circle (360 divided by 7.2 yields 50), Eratosthenes could then multiply the distance between Alexandria and Syene by 50 to determine the circumference of the earth.
Remarkably, Eratosthenes determined the circumference to be 25,000 miles, just 100 miles over the actual circumference at the equator (24,901 miles).
