How We Know the Earth Rotates

Written by  Ethan Siegel

"I feel the earth move under my feet
I feel the sky tumbling down
I feel my heart start to trembling
Whenever you're around" -Carole King


I had so much fun earlier this week telling you about  how we know that the heliocentric model is better than the geocentric one, that I thought I'd go a little farther down that rabbit-hole. You see, the first astronomical thing that any living creature likely notices is that, as seen from Earth, the Sun rises in the East each day and sets in the West. 

(I don't want to hear it from you kids at the poles, either)!

I had so much fun earlier this week telling you about how we know that the heliocentric model is better than the geocentric one, that I thought I'd go a little farther down that rabbit-hole. You see, the first astronomical thing that any living creature likely notices is that, as seen from Earth, the Sun rises in the East each day and sets in the West. 




The Sun, as it were, appears to move in half of a great circle throughout the sky. What's more, is that if you look up at night, you'll find that the night sky appears to move along a similar path, like the entire heavens rotates. 

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Why Does This Happen?

It turns out that there are two possible explanations that -- at first glance -- are equally good.

First off, the Earth could be stationary, and everything in the heavens, including the Sun, stars, Moon, and planets, could be revolving around it, once per day. 

The other option, though, is that these objects in the sky don't move so rapidly, around the Earth, once per day. Rather than having everything orbit the Earth, the Earth could simply be rotating. 

How would we know? What test could you perform on Earth to tell these two cases apart?

You'd hope that there'd be some easy way to see it from looking at the atmosphere, but when things with atmospheres rotate at constant rates, their atmospheres rotate at constant rates with them. It turns out that it wasn't until the 1850s that we figured out a simple, straightforward method to test it. The key device, believe it or not, is a simple pendulum.

If you have a perfect pendulum, it will simply swing back-and-forth in a straight line forever, until something like air resistance slows it down to a full stop. For a very long, very heavy pendulum, this could take days.

But if you have a perfect pendulum that lives on, say, a rotating planet, something very, very interesting happens. 

(Image credit: Cleon Teunissen.)

The pendulum, swinging back and forth, will be affected by the Earth's rotation! The effect won't be visible in just a few swings; in fact, if you were at 30 degrees North latitude (as the diagram above shows), it would take two full days for the pendulum to spin around once, or five minutes for it to rotate a mere one degree.

So what happens if you do this? 

Well, the first time this was demonstrated was in 1851, by the Frenchman Leon Foucault, and the device is now known as the Foucault Pendulum. In fact, there are many of these all over the world, including the world's heaviest one right by me in Portland, OR. The results are almost immediately striking.

The simplest way to do it is to build a long pendulum with a heavy mass, and tie it off at a small but significant (5-10 degree) angle. Leave it overnight. In the morning, come in and mark its position, and then burn the string you used to tie it off.

Why burn it? Because this way, you won't accidentally introduce any angular momentum; the pendulum will simply swing back-and-forth in a straight line. 

And then come back to it. Or, if you've got some time, just watch it go. Know what you'll see?

The pendulum indeed rotates at the exact rate you'd expect if the Earth rotated once per day, regardless of your latitude! Not only that, but there are some pendula out there that are situated with a spike on their bottoms and a sand-pit beneath them to draw in.

Over the course of a day, they trace out a very telling pattern. 

(Image credit: Cleon again. Note, this cuspy, updated image replaces an old one, which is less correct).

This is firsthand, easy nproof that the Earth, indeed, moves! Although we already knew it via other means, this is an experiment that anyone can do with a little effort, and many people all over the world already have!

In fact, some people even videotape it... 

And that's how you know the Earth moves, and how you can demonstrate to anyone who thinks otherwise what science actually has to say!