HARMONIC EXPRESSIONS

Trigonometric terms of an infinite series used to approximate irregular curves in two or three dimensions.

Related Terms

NYQUIST CONTOUR

A directed closed path in the complex frequency plane used in constructing a Nyquist diagram, which runs upward, parallel to the whole length of the imaginary axis at an infinitesimal distance to the right of it, and returns from jn to jn along a semicircle of infinite radius in the right half-plane.

IMPULSE TRAIN

An input consisting of an infinite series of unit impulses, equally separated in time.

AMPERE

The base unit of electric current in the International System of Units; it is that constant current which, if maintained in two straight parallel conductors of infinite length, of negligible circular cross section

ASYMPTOTE

A straight line or curve which a curve of infinite length approaches but never quite reaches

CELESTIAL SPHERE

An imaginary sphere of infinite radius concentric with the earth, on which all celestial bodies except the earth are imagined to be projected

INFINITE

Without limits. The opposite is FINITE

ESCAPE VELOCITY

The minimum velocity required of a body at a given point in a gravitational field which will permit the body to escape from the field. The orbit followed is a parabola and the body arrives at an infinite distance from the center of the field with zero velocity. With respect to escape velocities characteristic of th

GEOMETRICAL HORIZON

The intersection of the celestial sphere and an infinite number of straight lines tangent to the earth's surface, and radiating from the eye of the observer. If there were no terrestrial refraction, GEOMETRICAL and VISIBLE HORIZONS would coincide.

SPHERICAL HARMONICS

Trigonometric terms of an infinite series used to approximate a two- or threedimensional function of locations on or above the earth.

SPHERE

1. A curved surface all points of which are equidistant from a fixed point within, called the center. The celestial sphere is an imaginary sphere of infinite radius concentric with the earth, on which all celestial bodies except the earth are imagined to be projected. The celestial sphere as it appears to an observer at the equator, where celestial bodies appear to rise vertically above the horizon, is called a right sphere; at the pole, where bodies appear to move parallel to the horizon, it is called a parallel sphere; between the equator and pole, where bodies appear to rise obliquely to the horizon, it is called an oblique sphere. Half a sphere is called a HEMISPHERE. 2. A body or the space bounded by a spherical surface. For most practical problems of navigation, the earth is considered a sphere, called the terrestrial sphere.

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