But Kansas is Still Flatter Than a Pancake
Quick quiz: When did humanity discover that the Earth is round?
If you answered Copernicus or Galileo, you're off by a millennia or two. The Ancient Greeks actually discovered this.
To prove the theory, they created an experiment using two dry wells, hundreds of miles apart. Then, at noon on the same day, the Greeks measured the shadows at the bottom of the well to see if they were at the same angle. When they weren't, they used the difference in angle to figure out the actual size of the Earth…with remarkable accuracy.
(Trigonometry is important, kids. If you don't believe me, just ask Jeff over at The Yard Ramp Guy.)
Okay, another quick quiz: When did humanity forget that the world was round?
I bet the most specific thing you can think of is the Dark Ages or the Fall of Rome, right? Well, both are incorrect: We never forgot. To quote the brilliant Stephen Jay Gould, "There never was a period of 'flat Earth darkness' among scholars (regardless of how the public at large may have conceptualized our planet both then and now). Greek knowledge of sphericity never faded, and all major medieval scholars accepted the Earth's roundness as an established fact of cosmology."
In fact, we can trace the popularization of the idea that people thought the Earth was flat back to specific historians in the 1800s.
Sadly, this is pretty common stuff when we think about history. People are swift to assume that their distant ancestors (or at least other people's distant ancestors) were dumber than people today.
It really doesn't take that much effort to realize that idea is wrong, but most people won't even try to put in the effort.
(Really don't believe me? Then let's see you design an aqueduct with pen and paper.)
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Quotable
Oh, Yard Ramp Guy: what goes around comes around:
"’I'll follow him to the ends of the earth,' she sobbed. Yes, darling. But the earth doesn't have any ends. Columbus fixed that."
— Tom Robbins, Still Life with Woodpecker