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

The precise curve required on the surface of an optical element, especially the mirror of a reflecting telescope.

From O.Fr. figure, from L. figura "a shape, form."

Šekl from Ar. šakl "figure."

A curve traced out by a point which is oscillating simultaneously in two mutually perpendicular directions. In general, the amplitude and frequency may be different in the two directions, and the two oscillations may have an arbitrary initial phase difference. The simplest pattern is a straight line, being obtained from two oscillations of equal frequency in phase with each other. The patterns can become very involved if the ratio of the frequencies is not a simple one.

After the French physicist Jules Antoine Lissajous (1822-1880), who first demonstrated such curves (Comptes-Rendus, 1857, 44, 727); → figure.

Xam, → curve.

A two-dimensional geometric figure. The points of the figure lie entirely in a plane.

→ plane; → figure.