THE PHILOSOPHER Eratosthenes worked at the Library of Alexandria. It was common knowledge that there was a well in the town of Syene, some distance away, where on midsummer’s day you could see the reflection of the sun at the bottom.
Eratosthenes sent a rider on a camel to Syene, timing how long it took to get there. Because he had already measured how far a camel walks in a day, he was able to calculate the distance from Alexandria to Syene as 500 miles.
The next midsummer’s day, he placed a vertical pole outside the Library and measured the angle between the tip of the pole and the tip of its shadow. It was just over seven degrees, or one-fiftieth of the 360° in a circle.
So Eratosthenes multiplied the distance to the well (500 miles) by the number of times the 7° angle would fit into a circle (50), and got a result for the circumference of the Earth of 25,000 miles.
The true circumference is 24,901.5 miles. He was 2% out.
