How Do We Know the Earth Rotates?

Lesson Plan - Get It!

Audio:

In the not-too-distant past, people did not think the earth moved.

The earth was assumed to be the center of the universe, sitting motionless while the stars, our sun, and the other planets moved around us.

• Have you ever laid on your back outside to watch the sky?

If it's been a while since you have, watch this video.

• What do you notice?

When watching the sky, it probably feels like you are staying still while the objects in the sky move around you.

Now, watch this timelapse of the galaxy video.

• How does this video differ from the experience of actually watching the night sky?

You probably noticed that in this video, the stars were shown as stationary instead of the earth (and the camera stationed on earth).

• Did the video feel weird? Maybe a bit off?

Now, imagine being alive before the idea of the earth's rotation was introduced to the public by Nicolaus Copernicus in the 1500s.

You would likely demand evidence before you believed this new theory that seemed to contradict observations you had made yourself.

• So, what is the evidence for earth's rotation?

Let's examine two observations that prove the earth is rotating.

Coriolis Effect

One phenomenon that shows the earth is rotating is called the Coriolis effect. The Coriolis effect causes objects moving in the Northern Hemisphere to veer right and objects moving in the Southern Hemisphere to veer left.

• Why does this happen?

Look at the wind as an example.

Air moves from high-pressure to low-pressure areas, causing wind. (Check out our lesson under Additional Resources in the right-hand sidebar to learn more.)

However, in reality, the air doesn't move in a straight line but appears to move to the right (from the perspective of the air).

What is happening is that the air IS moving in a straight line. However, the earth rotates underneath the moving air while that is happening. This causes the air to miss its target.

Picture yourself riding a merry-go-round while your friends toss a ball over you. The ball travels in a straight line when they throw directly at each other. However, from your perspective on the merry-go-round, the ball appears to curve to the right.

See this happen in the video below.

The Coriolis effect is most noticeable when examining air pressure systems. Air in a high-pressure system rotates clockwise, while the air in a low-pressure system rotates counter-clockwise and vice versa in the Southern Hemisphere.

This rotation is caused by the Coriolis effect.

To learn more about this phenomenon, watch the following video.

Foucault's Pendulum

The Coriolis effect is not the only evidence that shows the earth is rotating.

Another supporting piece of evidence is observed using a weight on a string. When attached to a high ceiling, this gadget creates a pendulum, a weight hung from a fixed point.

Unlike the pendulum in a clock that can only swing in one line back and forth, this pendulum can move freely.

French physicist Lé on Foucault developed the Foucault pendulum to demonstrate the effects of the earth's rotation.

He first set up his pendulum 1851 at the Paris Observatory. He touted his invention as a simple model proving the earth constantly rotated.

Foucault pendulums are now found in almost every large museum. You may have even seen one yourself.

They consist of a large weight, usually a ball, attached to a high ceiling by a long, sturdy rope. The weight swings slowly from side to side.

We would expect that, once moving, the pendulum would continue to swing back and forth in a straight line. This is exactly what happens. Sort of.

However, this isn't observed when watching the pendulum. Around the Foucault pendulum is occasionally a circle or semi-circle of objects like blocks.

Although at first glance the pendulum appears to travel in a straight path, the blocks are slowly knocked over, one by one.

Museums place the blocks around the pendulum so that this incremental change in movement is easier to observe.

Watch this excruciatingly slow yet fascinating process in a portion of the video below.

After several hours, most of the blocks that created the circle will be knocked over.

• Why?
• If it sways back and forth in a straight line, how can the pendulum move in a full circle?

The answer is it doesn't.

The pendulum sways back and forth in a straight line. The earth, however, rotates under the pendulum. As this happens, the pendulum appears to change course, eventually knocking over all the blocks slowly.

As you watch the following demonstration of this process, pay close attention to the explanation at the beginning. This pendulum IS swinging back and forth, but this video uses time-lapse photography that only captures the weight at one point in each swing. This makes it easier to see the overall movement of the pendulum but causes the weight to appear to rotate while not swinging weirdly. Don't be fooled!

• Confused?

Watch one more video to understand how this proves the earth rotates.

Later, you'll make your own Foucault pendulum, but for now, move on to the Got It? section to test what you've learned.