A pendulum that proves
the Earth turns.
Sixty-seven metres of wire, a twenty-eight-kilogram bob, and nothing to push it sideways — yet the plane of its swing wheels slowly across the floor, because the floor is turning beneath it.
Drag the latitude dial, or use arrow keys, to change how fast the plane turns.
The bob keeps a straight line. The room does not.
A free pendulum has no memory of the walls. Once set going it holds its plane of swing fixed against the distant stars — so as the building, the city and the continent rotate under it, the swing appears to walk around the floor.
Nothing pushes it sideways
Released cleanly, the bob traces one line back and forth. The long wire and heavy mass keep it swinging for hours with almost no damping.
The plane wheels round
Held fixed in space, the plane rotates relative to the ground. In the north it turns clockwise; the faint rosette in the sand is every pass laid over the last.
The proof falls over
A ring of pins stands just beyond the swing. As the plane comes round to each bearing, the bob reaches out and knocks it flat — the Earth's turn, made to topple.
How fast the floor turns depends on where you stand.
The precession rate is the Earth's own rotation scaled by the sine of your latitude. At the equator the plane never turns; at the pole it makes a full circuit every sidereal day; Paris sits neatly in between.
- Equator0°0.0°/hr — never
- Paris48.85° N11.3°/hr — 31.8 h
- Reykjavík64.1° N13.5°/hr — 26.6 h
- Pole90°15.0°/hr — 23.9 h
“You are invited to come and watch the Earth turn.” Léon Foucault, to the assembled savants
Foucault hung his wire from the dome of the Panthéon and let a brass bob swing across a bed of sand. There was no telescope, no calculation to follow — only a line drawn in sand that would not stay put, and a row of pegs the pendulum knocked over through the afternoon. The crowd watched the planet rotate with their own eyes, and understood.