Hang a heavy weight from a very long wire, set it swinging, and come back hours later: it is still swinging, but along a different line. Nothing touched it - the Earth turned underneath. That is the Foucault pendulum, the first experiment to prove the planet spins without looking at the sky. Slide the latitude and watch the floor track turn fast near the poles and freeze at the equator.
Published literacy: a Foucault pendulum swing plane appears to turn at a rate of one sidereal day divided by sin(latitude) - about 24 hours at the poles, 31.8 hours at Paris, and never at the equator; Leon Foucault proved the Earth spins this way in 1851.
Drag to orbit and scroll or pinch to zoom. Slide the latitude, hide the floor track, or pause the swing.
Foucault Pendulum 3D Explorer
In 1851 the French physicist Leon Foucault hung a heavy bob from a wire 67 metres long inside the dome of the Panthéon in Paris, set it swinging, and invited the city to see the Earth turn. A free pendulum keeps swinging in the same plane relative to the distant stars; but the floor beneath it is carried around by the spinning Earth, so to anyone standing there the swing plane appears to rotate. This explorer lets you watch that slow turn and change the latitude to see how it speeds up or stops.
The rate is set by a beautifully simple rule: the swing plane makes one full turn in one sidereal day divided by the sine of the latitude. At the North Pole the floor turns a full circle under the pendulum in about 24 hours, so the plane appears to sweep all the way around in a day. At Paris (about 49 degrees north) it takes 31.8 hours, drifting 11.3 degrees every hour - fast enough to see in an afternoon. At the equator the sine is zero and the plane does not turn at all, no matter how long you wait. South of the equator it turns the opposite way. Foucault demonstration needed no telescope and no astronomy - just a wire, a weight, and patience - which is why it remains one of the most convincing proofs that our planet spins.
- A long pendulum swinging in a fixed plane while the floor turns beneath it
- An orange floor track that sweeps around as the swing plane precesses
- A latitude slider - fast turn near the poles, frozen at the equator
- The full-turn time shown live for the chosen latitude
- Play, pause, and a floor-track toggle
- Runs fully in the browser with the vendored three.js engine - no account, no upload
Students see the Earth spin without leaving the room; teachers link the sine-of-latitude rule to real installations; museum-goers learn what the great swinging ball in the lobby is really showing.
| Latitude | Full turn of the swing plane | Note |
|---|---|---|
| North Pole (90 deg) | ~24 hours | One sidereal day |
| Paris (48.85 deg) | 31.8 hours | 11.3 degrees per hour |
| Equator (0 deg) | never | Swing plane stays fixed |
| Rule | sidereal day / sin(latitude) | Foucault, 1851 |
Everything renders on your device with WebGL. The 3D engine loads once (about 0.7 MB) and is cached; no scene data is sent to a server.
This is an educational visualization - the turn rate is hugely sped up so it shows in seconds, and the scene is not to scale.
For a step-by-step walkthrough, read the Foucault Pendulum 3D Explorer step-by-step guide. The Space 3D collection also includes Coriolis Effect 3D and Seasons Earth 3D.
Frequently Asked Questions
What is a Foucault pendulum?
A long, heavy pendulum free to swing in any direction. As the Earth rotates beneath it, its swing plane appears to turn, which is a direct proof that the planet spins.
Why does the swing plane turn?
It does not, really. The pendulum keeps swinging in the same plane relative to the stars; the floor under it is carried around by the rotating Earth, so the plane only appears to rotate.
How fast does it turn?
One full turn takes a sidereal day divided by the sine of the latitude - about 24 hours at the poles, 31.8 hours at Paris, and no turn at all at the equator.
Who built the first one?
Leon Foucault, in Paris in 1851. His famous version hung a 28-kilogram bob on a 67-metre wire from the dome of the Panthéon, and it turned 11.3 degrees every hour.
Does it turn the same way everywhere?
No. In the Northern Hemisphere the swing plane turns clockwise as seen from above, and in the Southern Hemisphere it turns counter-clockwise. At the equator it does not turn.
Is this scene to scale?
No. The turn is hugely sped up so it is visible within seconds instead of hours, and the sizes are not to scale. The latitude rule it follows is accurate.