| Section A | A index | 631-639 of 917 terms |
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aperiodic—1. Not periodic. 2. Applied to an instrument in which the indicator moves to a new position without oscillating.
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aperture—1. In a unidirectional antenna, that portion of the plane surface that is perpendicular to the direction of maximum radiation and through which the major part of the radiation passes. The physical aperture of a horn or parabolic antenna is identical with the actual area of the antenna face. Other types of apertures may be defined that are related to the effectiveness with which an antenna can remove energy from an incident radio wave (usually called effective area), and to the extent to which the intercepted energy is lost in heat or is reradiated. 2. In an opaque disk, the hole or window placed on either side of a lens to control the amount of light passing through.
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aperwind—(Also called aberwind, alpach.) A warm wind of the Alps that thaws the snow.
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aphelion—The point on the orbit of the earth (or any other body in orbit about the sun) that is farthest from the sun; the opposite of perihelion. At present, aphelion occurs about July 1, when the earth is about three million miles farther from the sun than at perihelion, but the seasons in which aphelion and perihelion fall undergo a cyclic variation with a period of twenty-one thousand years. The date of aphelion passage is advancing slowly (toward dates later in the calendar year) at a rate of about one-half hour each year. This is a consequence of the eastward rotation of the line of apsides at a rate of about 11 sec of arc per year and the precession of the equinoxes of about 50 sec of arc per year, for a total of 61.9 sec of arc per year. See apocenter, apogee.
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Apheliotes—The Greek name for the east wind (“blowing from the sun”). According to legend, it brings light showers. On the Tower of the Winds at Athens it is represented by a young man carrying fruit and grain.
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apogean tide—The tide of reduced range when the moon is near apogee, its farthest point from the earth in its elliptical orbit.
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