Hey Bill Nye, "How Do We Know the Earth Is Round?" #TuesdaysWithBill | Big Think

Matt Harden: Hey Bill, can you prove beyond a reasonable doubt that the earth is a sphere and not flat? I have a lot of Facebook friends who say that it's an act. Thank you.

Bill Nye: Is the earth flat or round? It's round. Now, let's see, how do we go about proving that? Go to the seashore. Go to the seashore and figure out why you can't—if you live on the East Coast—figure out why you can't see Spain from the East Coast or North America? It looks like you might have a St. Louis hat on. I'm not sure. Just go to the middle of the Mississippi River and look south. Why can't you see Louisiana? Why can't you see New Orleans? What's the problem there?

Then climb a tower or go to the top of a hill or a mountain, and you'll see a little farther, but you will not see to the other side of the earth, places we know to exist. For example, I've been to London. I can tell you other people have. I've been to Vancouver, British Columbia, and you cannot see Vancouver, British Columbia, from Boise, Idaho, let alone from New York City or Toronto or what have you. Just start there.

Then, if you'd like, look at pictures from space where you see the earth as a sphere. Those pictures are not faked. And I'll tell you, if nothing else, here is why you can tell they're not faked. Just to create the paperwork that NASA has created, NASA in this one case, just the paperwork to send anything out in space, to send people into orbit or to send them to the moon, that amount of paperwork would make faking it prohibitively expensive. No one could afford to generate that much documentation.

Then the other thing, if you want to get into this—if your friends are really serious—have them get on a boat or a ship and go out at sea, and you'll notice you can't see infinitely far. Furthermore, if you get into it enough, pick up a book about navigation or go online and learn about navigation. A very, very important thing you have to take into account when you try to navigate the ocean from a ship or a boat is how high you are off the sea surface. The higher you are off the sea surface, the farther you can see, the farther away the horizon is.

Now, this discovery that the world is round was made—I'm going to say it was published in 1540. Now, I'm concerned—it was Copernicus. I'm concerned it might have been 1530, but it was about then. It was in the 16th century that the earth was shown to be a ball. But it was known to be a ball by a lot of other people before Copernicus in the Western World, what we now call the Western World—the western hemisphere.

The ancient Greeks noticed that from time to time the earth casts a shadow on the moon. This is a lunar eclipse, where the moon gets in the shadow of the earth, the sunlight is being blocked by the earth, the moon is on the far side of the earth. And that shadow is always curved. And the only shape that always produces a curve, no matter what the orientation—that is to say whether you're standing in Greece at night or if you're standing then 12 hours later in Buenos Aires at night—the shadow is always curved.

So the ancient Greeks knew that the earth was a ball. They didn't know the size of it to the degree of precision we know now. And you guys, come on, everybody watches newscasts. You all use mobile phones. You all see airplanes fly around. You all go to see Ed Sheeran in concert one day in London, another day in Melbourne, Australia. This all depends on our fundamental understanding of the size of the earth and its shape with extraordinary precision.

And if you want to get into it, the earth isn't quite a sphere; it's a little—it’s [NOISE], its spin is a [NOISE] stretched, it made it slightly oblate, as the saying goes. This is all susceptible to analysis, but spend some time learning about navigation. Tell your friends to spend some time learning about navigation. Navigation has changed the world, by the way. What are you wearing? You're wearing stuff that came from another part of the world on a ship. It didn't get here by magic; it got here through science...