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People have suspected that Earth rotates for thousands of years, but how did we first prove it?

Hosted by: Stefan Chin

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[♪ INTRO] Right now, the Earth’s surface is spinning at several hundred meters per second, and we’re all along for the ride.

It doesn’t actually feel like you’re moving, though. Which is nice, because being super dizzy all the time doesn’t sound like a fun way to live, but that also means that for a long time, we didn’t know for sure that the Earth was spinning at all.

People have suspected the Earth rotates for thousands of years, but it wasn’t like they could just go to space and see for themselves, so it was hard to prove in a simple, clear way. Until 1851, when French physicist Leon Foucault hung a pendulum from one of the highest ceilings in Paris and let it swing. Before this, some astronomers as far back as the ancient Greeks thought the Earth rotated, because that would explain why the Sun and other stars move across the sky.

But not everyone accepted that. Most people believed that stars were attached to some kind of celestial sphere that spun around us. And it probably didn’t help that some of the ideas about what caused the Earth to rotate were a bit odd.

Take Philolaus of Croton. About 2300 years ago, he suggested the Earth rotated because it had a twin planet that no one in Greece ever saw. Supposedly, Earth and this mysterious counter-Earth spun around each other, always facing opposite directions, at least, from the Greeks’ perspective.

Even back then, it wasn’t exactly a popular idea. Over the next few thousand years, astronomers in Europe, India, and the Middle East also looked at the moving stars and suggested the Earth rotated. But none of them could prove it.

Finally, in 1651, an Italian physicist named Giovanni Battista Riccioli had an idea. He wrote that if the Earth rotated, a cannonball shot due north should curve to the right. It sounds like a weird way to explain the Earth’s rotation, but Riccioli was on the right track.

If you did fire a cannonball north, it would curve to the right because of what we now call the Coriolis effect, although it wasn’t named that until 200 years later. The Coriolis effect says that something moving in the northern hemisphere will look like it’s being pushed to the right, and something moving in the southern hemisphere will look like it’s being pushed to the left. Part of the reason for the Coriolis effect comes from the fact that the closer you get to the equator, the faster the Earth’s surface rotates.

That’s because the planet’s is widest at the equator, so you have to travel farther to get all the way around it than you do closer to the poles. But it takes one day for the Earth to finish one rotation no matter where you are, which means the surface closer to the equator has to travel farther in the same amount of time. So it moves faster.

Say you’re standing at the equator and fire a cannonball north. The ball zooms north, but at the same time, it's still moving east with the speed of the Earth’s rotation at the equator, where it left the cannon. That means that as it moves north, it’s moving east faster than the Earth’s surface.

From your perspective standing next to the cannon, it looks like it’s moving to the right. Unfortunately, or maybe luckily, physicists at the time didn’t have much access to cannons. Besides, it would be tough to track the path of the cannonball and see if it curved or not.

So scientists decided to try something easier. They dropped balls off the top of the tallest towers they could find. The top of the tower would be moving faster than the bottom for the same reason that the Earth’s surface at the equator moves faster than it does near the poles: to rotate once, the top of the tower has to travel in a bigger circle.

So if the Earth didn’t rotate, the balls should drop straight down. But if it did, the balls should land off to the side because the top of the tower is spinning faster than the ground. One physicist, Giovanni Battista Guglielmini, found that, if you dropped a ball off a tower about 74 meters tall, it would land around a centimeter to the side.

It was just a small difference, and it was hard to know if the measurements were accurate. But it was encouraging evidence that the Earth rotated. Now, by the mid-1800s, most people did believe the Earth rotated because they’d finally accepted that Earth orbits the Sun.

A rotating Earth was the best explanation for why the Sun and stars move across the sky, and they had some evidence for it, like from watching how the stars moved relative to each other. But it was still hard to prove in a clear, easy-to-see way, until Leon Foucault. In 1851, Foucault found a proof that could be easily repeated and didn’t require any tall towers: he hung a pendulum from a string and let it swing.

Since the pendulum is just hanging from the ceiling on a string, Earth’s rotation can’t force it to swivel. The ceiling it’s attached to is rotating, but the string stays put while the ceiling turns around it. So, once Foucault started the pendulum swinging, it just kept moving back and forth.

But in the meantime, Earth kept rotating underneath it. From the perspective of someone standing on the ground, it looked like the pendulum’s swing was turning clockwise. That was the Coriolis effect in action.

The pendulum’s swing didn’t turn by much, but as time passed and the Earth kept rotating, its orientation changed more and more. Foucault first tested his pendulum in his basement, and then in the Paris Observatory for fellow scientists, and it worked! Then he hung a 67 meter pendulum from the dome of Paris’ Panthéon, a mausoleum for the greatest French thinkers, and showed it off to the public.

For the first time, people could see a simple demonstration of Earth’s rotation with their own eyes. Still, watching a swinging pendulum isn’t something you want to do for hours, so to help them out, Foucault placed the pendulum over a bed of sand. As the pendulum swung, the tip traced out a line in the sand, and over time, the line showed the pendulum was no longer swinging in the same direction.

You can find Foucault’s pendulums in science museums all over the world. And even though they might seem like just a cool party trick now that we can see Earth rotating from space, they were really important for helping us understand how our planet works. The only people who weren’t excited about Foucault’s discovery were the scientific elite, who were annoyed by his lack of traditional scientific training.

Unfortunately for them, that didn’t change the fact that Foucault created the first simple, clear proof that the Earth rotated. But maybe they were just mad that they didn’t think of it first. Thanks for watching this episode of SciShow!

If you’d like to learn more about the Coriolis Effect, you can watch our episode about how it might not mean what you think it means. [♪ OUTRO]