Rapper B.o.B. and astrophysicist Neil deGrasse Tyson made headlines last month when the two were embroiled in a high-profile back-and-forth about whether or not the Earth is flat — but apparently neither of them realized that Carl Sagan explained this whole round Earth thing to the world 36 years ago.
After B.o.B. tweeted out a series of claims that the Earth is actually flat and NASA has brainwashed the general public into thinking otherwise, Tyson responded with some science refuting B.o.B's conspiracy theories, eventually going on the Nightly Show With Larry Wilmore to shut him down with an explanation of the Earth's curvature and some strong words from Sir Isaac Newton.
But the thing is, astrophysicist Carl Sagan already debunked B.o.B. in his Cosmos series, which first aired in 1980.
In an episode of Cosmos, Carl Sagan explained how Eratosthenes, a scholar and librarian living in ancient Egypt, figured out the curvature of the Earth using only, as Sagan put it, "sticks, eyes, feet and brain, plus a zest for experiment."
Eratosthenes, Sagan explained, read an account that stated at noon on the longest day of the year, the sun in the city of Syene was directly overhead, meaning that nothing cast a shadow. Eratosthenes decided to test whether that was true at the same exact time in his home of Alexandria, which was 800 kilometers to the north.
Testing it, he realized that, at the same exact time on the same day, a stick in Alexandria casts a shadow while a stick in Syene casts none. The only way this could be true, Sagan explains, is if the Earth is curved.
Sagan explains that Eratosthenes went on to use the information he learned to calculate the circumference of the Earth — and he was pretty close to accurate. Sagan totally lays it down like an expert, so if B.o.B hasn't seen this clip yet, he probably should get on that. And if that still isn't enough to convince flat earth conspiracy theorists, how about the fact that no one has ever fallen of the edge of the planet?
Sagan's whole explanation is available below: