An Etymological Dictionary of Astronomy and Astrophysics

A → pendulum consisting of a heavy weight on a very long wire attached to a support, that shows the rotation of Earth. The support must be nearly frictionless
in order that the pendulum can continue to swing freely for long periods of time. The pendulum will swing in the same plane as it started. The → Earth’s rotation is reflected in the slow turning of the plane of the pendulum’s motion,
which appears to rotate through 360° in T hours. The rotation time is
given by the expression: T = T₀/sin φ, where T₀ = 23.9344 hours is the → sidereal day and φ the → latitude of the place. At the poles the rotation period is 23h 56m 04s, and at the equator is ∞, i.e. the swing plane does not move. For regions near the equator it is very long; for example at Quito, the capital city of Ecuador, with φ = 00°15’S, it is 5485 days or more than 15 years! This phenomenon shows that the Earth is a → non-inertial frame.

See also: The experiment was performed for the first time by the French physicist Léon Foucault (1819-1868) in 1851, who set up, in the Pantheon in Paris, a simple pendulum consisting of a lead ball weighing 28 kg, suspended by a fine steel wire 67m long. At the latitude of Paris, the pendulum takes 31h 47m 38s to complete a precession cycle;
→ pendulum.