The pendulum
Foucault's pendulum at the Panthéon
A 28-kilogram bob on a 67-metre wire, hung under the dome in 1851 to make the Earth's rotation visible.
See current listings on Viator ↗The 1851 demonstration
In 1851, a few weeks after a first public demonstration at the Paris Observatory, the physicist Léon Foucault hung a 28-kilogram brass-coated lead bob from a 67-metre wire beneath the Panthéon's dome. It gave Paris simple, direct evidence that the Earth turns.
Why it actually works
The pendulum keeps swinging in the same plane; the floor beneath it is what turns, carried by the Earth. How fast the swing appears to rotate depends on latitude — at the Panthéon's, it takes roughly 32 hours to complete a full circle, drifting about 11 degrees an hour.
The original bob isn't there any more
It was moved to the Conservatoire des Arts et Métiers in 1855, and the Musée des Arts et Métiers keeps it today. The pendulum under the Panthéon's dome now is an exact copy, swinging there since 1995.
Seeing the rotation in one visit
Over the course of an hour, the swing's path visibly shifts against the markings on the floor beneath it — most noticeable in a quiet room, which makes the first hour after opening the best time to stand and watch it for a while.