This is a story about an Ancient Greek mathematician called Eratosthenes who calculated the earth's circumference over 2000 years ago using only simple tools and logic. The people of Ancient Greek already suspected that the Earth was round, because they observed ships that go over the horizon disappear from view from the bottom of the hull to the tip of the mast, implying there was a curvature present on Earth. But no one really knew how big the Earth really was, and here is where Erathostenes made his mark on history.

Eratosthenes noticed that the reflection of the sun can be seen inside a deep well of water when the sun was directly overhead, so the ray of sunlight therefore pointed directly at the centre of the earth. Since the Sun is so far away, sunlight travels parallel to the Earth. Greeks before him have created the concept of dividing a circle into degrees, and to create a degree you first need to create an angle. Using this knowledge, he designed an experiment, to measure the shadows cast by an obelisk in two different places at the same time. How differently the shadows appear in these two places will produce a quantifiable angle made by the curvature of the Earth. For his experiment he measured the obelisk in Alexandria and another in Syene during the summer solstice. It was a success, and there was a noticeable difference in shadow length, and after some calculations it’s determined that there is a 7 degree of curvature between Alexandria and Syene. Eratosthenes also charted the distance between these two cities, which is 800 kilometers, which is important for the next part of this experiment.

7 degrees is about 1/50th of a full 360 degree circle, therefore the distance around the world should be 50 times the distance between Alexandria and Syene. So all that’s left to do is multiply the distance we’ve acquired 50 times, and that would give Eratosthenes the theoretical full circumference of the earth. It makes perfect, logical sense, and he ended up with the figure of 40.000 kilometers for the circumference of the earth, which is actually insanely close to the real number of 40.075 kilometers. This all goes to show the genius of the human mind, how we can use our mind and a few simple tools to figure out amazing facts and knowledge about the universe we live in.