Physics
Foucault pendulum
Mechanics · Rotating frames
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Can you prove the Earth is spinning without ever looking at the sky?
A very long, heavy pendulum keeps swinging back and forth in the same direction in space. But the floor under it is slowly turning with the Earth, so over a few hours the swing seems to rotate around the room. That slow turning is the Earth’s spin made visible.
A freely suspended pendulum keeps swinging in a plane fixed relative to the stars, so on a rotating Earth the plane appears to turn slowly across the floor. The apparent rotation rate is the vertical component of Earth’s angular velocity, Ω sin φ, giving a precession period of one sidereal day divided by |sin φ|.
Key facts
- Earth’s angular velocity
- Ω = 7.292 × 10⁻⁵ rad/s (15.04° per hour)
- Precession rate
- Ω sin φ; 11.3°/h in Paris (48.85° N)
- Precession period
- 23.93 h/|sin φ|: 31.8 h in Paris, ∞ at the equator
- Foucault’s Panthéon pendulum
- 67 m wire, 28 kg bob (1851)
- Swing period, 67 m
- T = 2π√(L/g) ≈ 16.4 s
- Direction
- clockwise (north), anticlockwise (south), seen from above
Parts you can explore (11)
- Fixed-star ring
- Floor dial
- Marker pegs
- Support frame
- Suspension
- Wire
- Bob
- Swing plane
- Earth globe
- Latitude marker
- Rotation vectors
In the 3D model
Rotate and take apart the model, follow a guided tour of what each part does and why it’s needed, change it with a live slider, and test yourself with a short quiz. Free, no sign-up.
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