Watch Carl Sagan Prove Flat Earthers Wrong With Just a Piece of Cardboard

It's a question that has plagued the minds of human beings since the beginning of time: Is the Earth flat, or is it round?

The answer has been floated around for centuries. The ancient Greek philosopher Pythagoras first floated the then-controversial idea that the Earth was the shape of a sphere in 500 BC. A few centuries later, Aristotle perpetuated the theory in his book, On the Heavens, writing, "Again, our observations of the stars make it evident, not only that the Earth is circular, but also that it is a circle of no great size. For quite a small change of position to south or north causes a manifest alteration of the horizon."

The Greek mathematician and astronomer Eratosthenes took the notion a step further and developed the equation that determined the actual circumference of the Earth, proving that the planet was indeed round.

And yet, the argument that the Earth is flat is one still made by many people, all these thousands of years later. The conversation is buzzing with people on Reddit forums like r/flathearth and on Twitter who makes claims that satellite photos of Earths and other facts that have proven Earth's roundness are part of a round Earth conspiracy comprised by the government. If the Earth looks flat while walking on it, and it feels flat, then it must be flat, right?

Well, according to an experiment conducted by astronomer, cosmologist and astrophysicist Carl Sagan, the idea of the Earth being in the shape of a circle can be easily proven just by merely tracking shadows. He explained on a 1980 episode of his popular TV series Cosmos and used a cardboard map to elaborate on the experiment Eratosthenes first conducted before he set out to determine the circumference of the Earth all those moons ago.

"Here's a map of ancient Egypt. I've inserted two sticks or obelisks. One up here in Alexandria and one down here in Syene [known today as Aswan]. Now if at a certain moment, each stick casts no shadow—no shadow at all—that's perfectly easy to understand provided the earth is flat. If the shadow at Syene is a certain length and the shadow at Alexandria is the same length, that also makes sense on a flat earth. But how could it be, Eratosthenes asked, that at the same instant there was no shadow at Syene and a very substantial shadow at Alexandria? The only answer was that the surface of the earth is curved. Not only that, but the greater the curvature, the bigger the difference in the lengths of the shadows," Sagan said.

He continued: "The sun is so far away that its rays are parallel when they hits the Earth. Sticks at different angels to the sun's rays will cast shadows at different lengths. For the observed difference in the shadow lengths, the distance between Alexandria and Syene had to be about seven degrees along the surface of the Earth. By that I mean, if you would imagine these sticks extending all the way down to the center of the Earth they would there intersect at an angle of about seven degrees."

Since Eratosthenes knew the distance between Alexandria and Syene was 800 kilometers—only because he hired someone to pace out the entire distance to aid with his calculation—he was able to determine the circumference of the Earth given that seven degrees is essentially a 50th of the full circumference of the Earth at 360 degrees. S

"Now 800 kilometers times 50 is 40,000 kilometers, so that must be circumference of the Earth. That's how far it is to go once around the Earth," Sagan explained.

Let Sagan further explain by checking out the full video below.