"That was when I saw the Pendulum," records the hero of Umberto Eco's novel Foucault's Pendulum. "The copper sphere gave off pale shifting glints as it was struck by the last rays of the sun that came through the great stained-glass windows.
"Were it to graze, as it had in the past, a layer of damp sand spread on the floor of the choir, each swing would make a light furrow, and the furrows, changing direction imperceptibly, would widen to form a breach, a groove with radial symmetry - like the outline of a mandala or pentaculum, a star, a mystic rose." Pythagoras had asserted that the Earth was round. A sphere, however, is one thing; a rotating sphere is quite another. Some medieval philosophers pointed out that it was easier to imagine that the Earth itself might revolve, than that the whole host of heavenly bodies should move in concert around our stationary orb, but it was not until the gradual acceptance of the Copernican system in the 17th and 18th centuries that this was generally agreed.
No one could prove it, however, until Jean Bernard Leon Foucault came along. He proved it in 1851 by means of a much-publicised experiment, in which he suspended a vast pendulum from inside the dome of the Pantheon in Paris.
The idea behind the experiment is easier to understand if you imagine a pendulum suspended over the North Pole - a heavy weight swinging to and fro at the end of a long piece of string. As the pendulum swings back and forth, the earth rotates beneath it, anticlockwise. Since the plane of oscillation of the pendulum stays fixed in space, the path traced by the moving weight will appear to rotate in a clockwise direction, completing a full revolution -"around the clock", so to speak - every 24 hours.
The matter is more complex at lower latitudes. Theory predicts that the plane of oscillation of the pendulum should still rotate, but that it will take a much longer time to complete a full revolution. But in any event, the changing path of the moving weight is conclusive proof that the earth rotates beneath it.
This is precisely what Foucault set out to demonstrate in Paris. His pendulum consisted of a metal sphere weighing 55 lbs, suspended from the roof of the Pantheon on a metal thread 63 ft in length. Sand was placed on the floor, as described by Eco, so that changes in the path of the swinging weight could be observed - and we had our first conclusive proof that the world goes around.