In 1851, during the Victorian era (1800 – 1899), a period defined by rapid scientific advancement and public fascination with discovery, French physicist Léon Foucault introduced an experiment that fundamentally changed how people understood the Earth. Instead of relying on astronomy or complex calculations, he demonstrated something remarkably simple. The Earth’s rotation could be observed directly, right where you stand.

Foucault suspended a long, heavy pendulum and set it into motion. As it swung back and forth, the path of the swing appeared to slowly rotate over time. The pendulum itself was not changing direction. Instead, the Earth beneath it was rotating. This provided one of the first clear, visual proofs that the Earth is constantly spinning.

What made this experiment so powerful was its simplicity. It did not require advanced instruments or distant observations. Anyone watching could see the effect unfold in real time. Because of this, Foucault pendulums became iconic demonstrations in scientific institutions and museums.

In this project, you will recreate that Victorian-era experiment in a garage using simple materials. More importantly, you will go beyond just observing the motion. You will measure it.

By carefully controlling the release, recording the motion, and analyzing the change in swing direction, you can compare your results to the expected rotation of the Earth.

At about 33 degrees North latitude, the pendulum should rotate approximately 8.17 degrees per hour. This project shows how to measure that directly.

This is not just a demonstration. It is a hands-on reconstruction of a historic scientific experiment that allows you to measure Earth’s rotation yourself.

How It Works

A pendulum naturally swings in a fixed plane due to inertia. However, the Earth rotates beneath it. From our perspective, the swing direction slowly rotates over time.

The rotation rate depends on latitude:

ω=360∘⋅sin⁡(ϕ) per 24 hours\omega = 360^\circ \cdot \sin(\phi) \text{ per 24 hours}ω=360∘⋅sin(ϕ) per 24 hours

At 33 degrees North:

1. About 8.17 degrees per hour

10 lb anchor or any heavy plumb bob

Ball bearing fishing swivel

30 lb SpiderWire Stealth braid fishing line

Sony Handycam or similar camera

Thin thread

Lighter

Photoshop

Eye hook mounted into a ceiling stud

Tape

Measuring tape

Start by attaching the fishing line to your anchor.

Tie a strong knot and pull on it to confirm it will not slip. This connection must be secure because it will carry the full weight of the pendulum.

Now go to the top of the line and attach the ball bearing swivel. Make sure it moves freely.

Lay the entire assembly on the ground and check for tangles or twists. You want the line to hang clean and straight.

Locate a solid ceiling joist. Do not install into drywall alone.

Drill a pilot hole and install the eye hook into the wood. Tighten it fully so it does not move.

Attach the swivel to the eye hook. Pull down gently to test the strength of the setup.

⚠️ This is a 10 pound suspended object. Make sure everything is secure before proceeding.

Allow the pendulum to hang freely without movement.

Wait until it comes to a complete stop. The point directly below it is your center reference.

This step is important because all measurements depend on this alignment.

Mount your camera near the pivot point and aim it downward.

Make sure the pendulum path is clearly visible.

Zoom in enough to clearly see the swing direction.

Lock the camera in place so it does not move during recording.

Your pendulum length is about 7.5 feet, so you must keep the starting angle small.

Measure out a pull-back distance of about 5-6 inches.

Do not exceed about 8 inches.

Pulling the pendulum back too far introduces sideways motion and causes the swing to become unstable. This will reduce accuracy and make measurement difficult.

Take your time here and measure it carefully.

Hold the pendulum at your measured pull-back distance.

Tie a thin thread so it holds the pendulum in place.

Do not release it by hand. This thread will allow a clean release without adding sideways force.

Start recording.

Carefully burn the thread with the lighter. Step back and allow the pendulum to begin swinging on its own.

⚠️ Keep the flame away from the fishing line and paper.

Do not touch the pendulum after release.

Allow the pendulum to swing for at least one hour.

Keep the environment still. Avoid airflow and movement near the pendulum.

The longer you record, the easier it will be to measure the angle change.

From your recording, select one frame near the beginning and one frame about one hour later.

Choose frames where the swing direction is clearly visible.

Open both frames.

Draw a line along the swing path in the first image.

Draw a second line along the swing path in the later image.

Make sure both lines share the same center point.

Use the ruler tool in Photoshop.

Draw the first line, then hold Alt or Option and draw the second line.

Photoshop will display the angle between them.

At 33 degrees North, the expected rotation is about 8.17 degrees per hour.

Compare your measured angle to this value.

You can calculate percent error if desired.

At a latitude of about 33 degrees North, the expected precession rate for a Foucault pendulum is: 8.17∘ per hour

After analyzing my footage in Photoshop, I measured a precession of approximately: 13.2∘ per hour.

This is noticeably larger than the expected value, but that does not mean the experiment failed. In fact, this is a great example of how sensitive Foucault pendulums are to setup conditions and why large museum pendulums are designed so carefully.

Even though the measured value was higher than expected, the pendulum still clearly demonstrated rotational precession over time, which is the core principle of the experiment.

Foucault pendulums are surprisingly difficult to do accurately on a smaller scale. A lot of factors can influence the measured precession rate.

One of the biggest limitations in this build was pendulum length. My garage ceiling is only about 7 feet tall, limiting the pendulum length to roughly 6.5 feet. Shorter pendulums lose energy faster and are more sensitive to small disturbances.

Another major factor is the starting release.

Even though I used a burn-thread release to avoid pushing the pendulum by hand, very small imperfections can still introduce sideways motion. If the pendulum develops even a slight elliptical swing instead of a perfectly straight one, the apparent swing direction can drift faster than true Foucault precession.

Possible sources of error include:

1. Pulling the pendulum back too far
2. Slight sideways motion during release
3. Elliptical swinging
4. Small twisting forces in the line
5. Camera alignment errors
6. Perspective distortion from the camera angle
7. Air movement in the garage
8. Pivot friction
9. Imperfect frame selection in Photoshop
10. Limited experiment duration

One important lesson from this experiment is that measuring Earth’s rotation with a pendulum is absolutely possible, but achieving high accuracy requires careful control of many small variables.

This is one reason why famous Foucault pendulums in museums are often:

1. Much longer
2. Much heavier
3. Mounted in extremely controlled environments

Even with these limitations, the experiment still successfully demonstrated measurable precession over time.

I would have liked to repeat this experiment multiple times to average the results and reduce error, but I was limited on time for this build.

A proper repeated trial process would ideally include:

1. Multiple one-hour recordings
2. Different release attempts
3. Comparing several measured angles
4. Averaging the results together

That would likely improve the accuracy significantly.

However, even with a single primary run, the experiment still clearly showed measurable rotational precession, which was the main goal of recreating this historic Victorian-era demonstration.

In some ways, I think the imperfect result actually adds educational value because it highlights how sensitive real-world physics experiments can be to setup conditions.

The final result clearly showed the pendulum’s swing direction changing over time.

By overlaying multiple swing directions from different points in the recording, the gradual precession becomes easy to see visually.

The final measured value was larger than the theoretical prediction, but the experiment still successfully demonstrated the fundamental principle behind the Foucault pendulum:

the Earth rotates beneath the swinging pendulum.

This was an incredibly interesting Victorian-era experiment to recreate because it combines:

1. Physics
2. Engineering
3. Observation
4. Measurement
5. Historical scientific discovery

Using only simple materials and careful setup, it is possible to directly observe one of the most fundamental motions of our planet.