First Calculation of the Earth Circumference

 

 Last week we talked about Pythagoras who discovered that the Earth is round about 500 BC. Well, about 300 years later, another smart guy called Eratosthenes decided to calculate the circumference of the Earth. No computer, no GPS, no map. How did he do that?

Eratosthenes is the founder of geography. He lived in Alexandria, Egypt and was among other thing the conservator of the Alexandria library. His method is based on two observations:

1-      There was a place called Syene in the South of Egypt. Nowadays we call it Aswan. At Syene, on June 21^st and at noon, the sun rays reach the bottom of a vertical well.

2-      At the same time at Alexandria, on the North coast of Egypt, the shade of a gnomon (a vertical post stuck in the ground) forms an angle of 1/50 of a circle with the post.

Looking at the picture we can see that the earth being round, its circumference is 50 times the distance between Syene and Alexandria. So this had to be measured with some accuracy. This involved several surveying trips conducted by professional “bematists”. They were surveyors trained to measure distances by counting their steps. They found that the distance was 5,000 stadia. So the Earth circumference would be 50 times that, or 250,000 stadia.

An Egyptian stadium was 155 to 160m, let’s say 157.5m. The Earth circumference is therefore 250,000 times 157.5m, or 39,375km.

More modern methods give a result of 40,076km. Eratosthenes got it minus 1.7%. Absolutely amazing!

Reference

https://en.wikipedia.org/wiki/Eratosthenes