How Do We Know the Earth Rotates?

Contributor: Lindsey Congalosi. Lesson ID: 13122

We are taught at an early age that the Earth is constantly rotating, and most people accept this as fact. But, how do we know? What's our evidence? Read more to find out!

categories

Physical Science, Space Science and Astronomy

subject
Science
learning style
Auditory, Kinesthetic, Visual
personality style
Beaver, Golden Retriever
Grade Level
Middle School (6-8), High School (9-12)
Lesson Type
Dig Deeper

Lesson Plan - Get It!

Audio:

Children are taught at an early age that our planet is moving.

The Earth is constantly rotating (spinning) and revolving (moving around the sun). This movement causes changes in everything from day and night to seasons to the constellations we see in the sky.

If you're like most students, you probably know this already.

  • But, can you explain how you know this?

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 The sky in movement from Eugene Komashkov:

  • 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 Earth's Rotation Visualized in a Timelapse of the Milky Way Galaxy - 4K from Aryeh Nirenberg:

  • 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.

Copernicus

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?

Let's 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.)

Yorkshire Terrier in the wind

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 actually happening is that the air IS moving in a straight line. However, while that is happening, the Earth is rotating underneath the moving air. This causes the air to miss its target.

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

See this happen in The Coriolis Effect, Part 1 from UWAtmosOutreach:

The Coriolis effect is most noticeable when we examine 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.

air pressure diagram

This rotation is caused by the Coriolis effect.

To learn more about this phenomenon, watch Coriolis Effect: IDTIMWYTIM from SciShow:

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 contraption creates a pendulum, a weight hung from a fixed point.

Foucault pendulum

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

clock pendulum

French physicist Léon Foucault developed the Foucault pendulum to demonstrate the effects of Earth's rotation.

Foucault

He first set up his pendulum in 1851 at the Paris Observatory. He touted his invention as a simple model that proved the Earth was constantly rotating.

Foucault experiment, 1902

Image, via Wikimedia Commons, is in the public domain.

Foucault pendulums are now found in almost every large museum, and 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 what is observed when watching the pendulum. Around the Foucault pendulum, there is occasionally a circle or semi-circle of objects such as blocks.

Foucault pendulum

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

Foucault pendulum at Cleveland Museum of Natural History

Image by Tim Evanson, via Wikimedia Commons, is licensed under the CC BY-SA 2.0 license.

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 Foucault Pendulum at the Chicago Museum of Science and Industry from SquiggleMom:

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, then how is the pendulum able to 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 slowly change course, eventually knocking over all the blocks.

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 weirdly rotate while not swinging. Don't be fooled!

Watch The World Turn from Kurtis Baute:

  • Confused?

Watch one more video from SciShow to understand How We Proved Earth Rotates Using a Giant Swinging Ball:

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

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