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Building a Foucault Pendulum — Proving Earth Rotates with a Swinging Weight
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Astro

Created by

Astro

30. May 2026IS
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Building a Foucault Pendulum — Proving Earth Rotates with a Swinging Weight

On 3 February 1851, Léon Foucault hung a 28-kilogram brass bob from a 67-metre wire beneath the dome of the Panthéon in Paris and set it swinging. Over the course of hours, the plane of the pendulum's swing slowly rotated clockwise — not because any force was pushing it sideways, but because the Earth itself was rotating beneath it. The pendulum's plane of oscillation remains fixed in space (by Newton's first law), while the floor turns underneath. At the North Pole, the floor would complete a full 360-degree rotation in 24 hours; at lower latitudes, the rate is slower, proportional to the sine of the latitude. Foucault's demonstration was the first direct, visual proof that the Earth rotates — something long accepted theoretically but never before observed in a laboratory. The experiment caused a sensation: Napoleon III ordered a repeat at the Panthéon, and within months Foucault pendulums were swinging in public buildings across Europe. This blueprint builds a functional Foucault pendulum with a heavy bob on a long wire, a low-friction pivot, and a graduated reference ring, capable of demonstrating Earth's rotation over the course of several hours.

Intermediate
5-10 hours

Instructions

1

Understand the Foucault pendulum principle

A pendulum swinging freely maintains its plane of oscillation in space — this is a consequence of Newton's first law of motion. If the pendulum is suspended above a rotating surface (the Earth), the surface turns beneath the pendulum while the swing plane stays fixed. To an observer standing on the rotating Earth, it appears that the pendulum's swing direction slowly rotates. At the North Pole, the pendulum's plane appears to rotate 360 degrees clockwise in 23 hours 56 minutes (one sidereal day). At other latitudes, the rotation rate is 360 × sin(latitude) degrees per day.
2

Select the location and anchor point

The Foucault pendulum requires the longest possible wire for reliable results — the longer the pendulum, the longer its period of swing, the slower it loses energy, and the more clearly the rotation is visible. A minimum practical length is about 5 metres; 10-20 metres is much better. Find a high anchor point: a stairwell, attic beam, barn rafter, tall tree, or the ceiling of a multi-storey building. The anchor must be completely rigid — any swaying or vibration in the support will overwhelm the subtle Foucault rotation.
3

Build the pivot mount

The pivot is the most critical component. A Foucault pendulum must swing freely in any direction, so the pivot must not constrain the plane of oscillation. A simple universal joint (two perpendicular hinges) or a single-point pivot works well. The simplest effective pivot is a thin steel wire (piano wire, about 0.5-1 mm diameter) clamped firmly at the top — thin wire bends freely in all directions without introducing a preferred plane. The wire should be clamped in a rigid bracket bolted to the ceiling beam.
4

Prepare the pendulum bob

The bob should be as heavy as practical — Foucault used 28 kg. A heavier bob resists air drag better and swings longer. Use a dense material: a large lead or brass weight, a sand-filled container, or a concrete-filled can. The bob must be symmetrical about the wire axis — any asymmetry causes the pendulum to precess in ways that mimic or mask the Foucault rotation. A sphere or symmetrical cylinder works best. Attach the wire to the exact top centre of the bob.

Materials for this step:

Pendulum BobPendulum Bob1 piece
5

Attach the wire

Use steel piano wire or thin steel cable for the pendulum wire. The wire must be strong enough to support the bob's weight with a large safety margin (at least 10x), thin enough to flex freely at the pivot, and stiff enough not to twist. For a 5-10 kg bob, 0.5-1 mm piano wire is sufficient. Clamp the upper end securely at the pivot. Attach the lower end to the bob using a small eye-bolt or a drilled hole through the bob's top, ensuring the connection is perfectly centred and the wire hangs exactly vertical when the bob is at rest.

Materials for this step:

CordageCordage5 meters
6

Build the reference ring

On the floor beneath the pendulum, place a circular reference ring that shows the pendulum's direction of swing. A ring of about 1-2 metres diameter works well. Mark the ring with degree graduations every 5 or 10 degrees (0-360). You can use a large sheet of paper or card, a painted circle on the floor, or a ring of small markers. The centre of the ring must be directly below the pivot point. Mark true north on the ring using a compass.

Materials for this step:

Hardwood BlockHardwood Block1 piece

Tools needed:

ProtractorProtractor
Measuring RulerMeasuring Ruler
7

Launch the pendulum correctly

The launch technique is critical — the pendulum must swing in a single plane, not in an ellipse. Any sideways motion causes the bob to trace an elliptical path, and the ellipse precesses independently of the Foucault effect, confusing the measurement. To launch cleanly: pull the bob to one side using a taut cord tied to a fixed point, hold it still until all swinging has stopped, then burn through the cord with a match or candle (or cut it sharply). This releases the bob without any sideways push. Foucault himself used this burn-cord technique.
8

Observe and record the rotation

After launching, observe the direction of swing relative to the reference ring. Record the swing direction every 15-30 minutes. At a latitude of 50 degrees north, the swing plane rotates about 11.5 degrees per hour clockwise (360 × sin(50°) / 24 = 11.49 degrees per hour). After one hour, the swing direction should have shifted by about 11 degrees; after three hours, about 34 degrees. The rotation is slow and steady — this is the Earth turning beneath the pendulum.
9

Troubleshoot common problems

If the pendulum traces an ellipse instead of a straight line, the launch was impure — restart with the cord-burning technique. If the swing plane rotates much faster or slower than predicted, check for asymmetry in the bob (which causes mechanical precession), friction or constraint at the pivot (which forces a preferred swing plane), or air currents (which push the bob sideways). A drafty room or open door can produce apparent rotation far faster than the Foucault effect. Shield the pendulum from air currents and ensure the pivot is truly free.
10

Calculate and verify the rotation rate

The theoretical rotation rate is: degrees per hour = 15 × sin(latitude). At the equator (0°), the pendulum does not rotate at all. At 30° latitude, it rotates 7.5°/hour. At 45°, it rotates 10.6°/hour. At 90° (the pole), it rotates 15°/hour (one full rotation per sidereal day). Compare your observed rotation rate with the predicted rate for your latitude. If they match within a few percent, you have successfully demonstrated what Foucault proved in 1851 — the most elegant and direct proof of Earth's rotation ever devised, requiring nothing but a weight on a wire and the patience to watch it swing.

Materials

3

Tools Required

2

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