The Greeks new the Earth was round 2500 years ago.

It was not until the mid 20th century that we managed to launch satellites into space to determine the exact kilometers and circumference of the planet.

But how did the Greek know that the Earth was spherical 2500 years ago?

Eratosthenes heard that in Syene, a city south of Egypt’s Alexandria, no vertical shadows were cast at noon on the summer solstice as the sun was directly overhead. So, the Greek Mathematician wondered if this was the case in Alexandria too, a few hundreds of miles to the North of Syene.

So, he decided to conduct an experiment. On June 21, he went to Alexandria and he put a stick directly in the ground and waited to see if a shadow would be cast at noon. It turns out there was one and he tried to measure it. The shadow casted was about 7 degrees.

Now, Eratosthenes made a very logical conclusion: If the sun’s rays are coming in at the same angle at the same time of day, and a stick in Alexandria is casting a shadow of 7 degrees while the stick in Syene is not casting a shadow at all, it means that the Earth’s surface is curved.