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

Same as → eddy current.

→ Foucault; → current.

A method used to test the → image quality of → mirrors and → lenses. The test is performed by moving a knife edge laterally into the → image of a small → point source. The → eye, or a → camera, is placed immediately behind the knife edge, and the → exit pupil of the system is observed.

Named after the French physicist Léon Foucault (1819-1868), who invented the method; → knife; → edge; → test.

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.

The first functioning → reflecting telescope with a silvered glass mirror. It was built by Léon Foucault in 1826 for the Marseille Observatory. The mirror of 80-cm in diameter (f/d = 5) had an excellent quality. The telescope was used for a century as a visual instrument. Edouard Stéphan (1837-1923) used it from 1871 to 1884 to find 800 high-brightness galaxies, among which the → Stephan's Quintet. From 1906 to 1962 the telescope was used by Robert Jonckheere (1888-1927) to discover 3,350 new binary stars. In 1873, following an idea of Hippolyte Fizeau (1819-1896), Stéphan attempted to use it as an → interferometer to measure the diameter of a number of stars. In 1914 Charles Fabry (1867-1945) and Henri Buisson (1873-1944) used the telescope to obtain the first astronomical Fabry-Pérot interferogram, on the → Orion Nebula.

After the French physicist and optician Léon Foucault (1819-1868); Marseille (Observatory), the second largest city of France, located on the south east coast of the Mediterranean Sea, from L. Massalia, from Gk. Massalia; → reflector.