Foucault Pendulum
National Museum of Nature and Science · Installed 1931; present pendulum later
In the centre of the Japan Gallery a heavy brass ball swings slowly at the end of a wire that hangs from the dome high above, nearly twenty metres up. Around the edge of its path a ring of small pegs stands on the floor, and every so often the swinging ball knocks one over. If you come back an hour later you will see that the direction of the swing has shifted, and more pegs have fallen. The pendulum is not turning; the building, and the Earth beneath it, are turning under the pendulum.
The experiment was first performed in public by the French physicist Léon Foucault in the Panthéon in Paris in 1851, and it was the first direct demonstration of the rotation of the Earth that anyone could see with their own eyes. At the poles the plane of swing would come full circle in twenty four hours; at the equator it would not shift at all. Here in Tokyo, at about thirty five degrees north, it takes roughly forty two hours to complete a turn, which is why the pegs are spaced as they are. Look at the way the wire is fixed at the top, in a mount that lets it swing freely in any direction, and at the small electromagnet that gives the ball a nudge to keep it moving.
A pendulum has hung in this stairwell since the building opened in 1931, and it was a sensation at the time; newspapers reported crowds coming to watch the Earth turn. The present one is a later replacement, but the demonstration is the same as Foucault's, and it makes the grand old hall feel like a scientific instrument in itself. Stand at the railing on one of the upper landings and watch the ball pass below.
Did You Know? At Tokyo's latitude the pendulum's swing takes about forty two hours to rotate once, compared with twenty four hours at the North Pole.
Unlock the Full Guide
Read every room and listen to every work in all museums with premium.