A Foucault pendulum is a simple but elegant experiment that demonstrates that the Earth rotates.
It consists of a very long pendulum (often 10–70 m long) with a heavy bob. Once set swinging, it continues to oscillate in almost the same plane relative to the distant stars because of its inertia. As the Earth rotates beneath it, the floor appears to turn, so the pendulum’s plane of swing seems to rotate.
The rate at which the swing plane rotates depends on latitude:
- North Pole (90° N): one complete rotation every 24 hours (clockwise when viewed from above).
- Equator (0°): no apparent rotation.
- Any other latitude (φ): rotation rate = 360° × sin(φ) per sidereal day (approximately 23 h 56 min).
For Lichfield (latitude about 52.7° N):
- sin(52.7°) ≈ 0.795
- The pendulum rotates by about 286° per sidereal day, or roughly 12° per hour.
- It takes about 30 hours for the swing plane to complete one full rotation relative to the floor.
The experiment was first demonstrated in 1851 by Léon Foucault in the Panthéon. It provided one of the first direct, visual demonstrations that the Earth rotates, without requiring observations of the Sun or stars.
As an amateur scientist and radio astronomer, you’d probably appreciate that the Foucault pendulum is analogous to a giant inertial reference frame: the pendulum keeps its direction in space while the Earth rotates underneath it.