An Etymological Dictionary of Astronomy and Astrophysics
English-French-Persian

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.

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.

In its simple form, a device consisting of a body suspended from a fixed point on the end of a string to move to and fro by the action of gravity and acquired momentum. The period of oscillation for small amplitudes of swing is determined by the formula T = 2π √(l/g).

From Mod.L. pendulum, noun use of neuter of L. pendulus "hanging down," from pendere "to hang."

Âvang, related to âvixtan, âviz- "to hang" (akin to bixtan, biz- "to shake, to sort out, to sift"); Mid.Pers. âwixtan "to hang" (Sogdian wyc "to move, shake;" Chorasmian wc- "to tremble, shake;" Ossetic wigyn "to shake," awynzyn "to hang"), from prefixed (â-) Proto-Iranian base *uij-, *uic- "to shake, swing;" cf. Av. vij- "to shake, swing," vaējant- "swinging;" cf. Skt. vej- "to tremble, wince."

The time required for the plane of a freely suspended → Foucault pendulum to complete an apparent rotation about the local vertical. It is given by T = 23.9344 / sin φ in hours, where φ represents the latitude of the place. For Paris it is 31h 47m 38s; for the poles it 23.9344 h and for the equator it is ∞ since the plane of pendulum does not turn.

→ pendulum; → day.