For centuries, the shape of the Earth was a leading topic of thought among philosophers, historians, poets and others — from various civilizations ranging across time.

The question has been definitively settled since the age of space exploration began in the mid-20th century — largely driven by US-Soviet tensions during the Cold War.

Since then, humans have launched satellites into space and determined that — not only is the Earth round — but its exact circumference is 40,030 km.

But more than 2000 years prior, an ancient Greek mathematician came up with nearly the exact same figure — using only a stick and his mind.

That mathematician was Eratosthenes, the head of the library of Alexandria, who calculated Earth’s circumference by putting a stick in the ground.

How did he do it?

In the video below created by Business Insider, narrator Alex Kuzoian explains exactly that.

See the video