GLOSSARY OF MARINE NAVIGATION
abaft. , adv. In a direction farther aft in a ship than a specified reference position, such as abaft the mast. See also ABAFT THE BEAM, AFT, ASTERN. abaft the beam. . Any direction between broad on the beam and astern. See also FORWARD OF THE BEAM. abampere. , n. The unit of current in the centimeter gram-second electromagnetic system. The abampereis 10 amperes. abeam. , adv. In a line approximately at right angle to the ship’s keelopposite the waist or middle part of a ship. See also BROAD ON THE BEAM. aberration. , n. 1. The apparent displacement of a celestial body in the direction of motion of the earth in its orbit caused by the motion of the earth combined with the finite velocity of light. When, in addition to the combined effect ofthe velocity of light and the motion of the earth, account is taken of the motion of the celestial body in space during the interval that the light is traveling to the earth from the luminous body, as in the case of planets, the phenomenon is termed planetary aberration. The aberration due to the rotation of the earth on its axis is termed diurnal aberration or daily aberration. The aberration dueto the revolution of the earth about the sun is termed annual aberration. The aberration due to the motion of the center of mass of the solar system in space is termed secular aberration but is not taken into account in practical astronomy. See also CONSTANT OF ABERRATION. 2. The convergence to different foci, by a lens or mirror, of parallel rays of light. In a single lens having sphericalsurfaces, aberration may be caused by differences in the focal lengths of the various parts of the lens: rays passing through the outer part of the lens come to a focus nearer the lens than do rays passing through its central part. This is termed spherical aberration and, being due to the faulty figure of the lens, is eliminated by correcting that figure. A lens so corrected is called an aplanatic lens.Aberration may also result from differences in the wavelengths of light of different colors: light of the shorter wavelengths (violet end of the spectrum) comes to a focus nearer the lens than light of the longer wavelengths (red end of the spectrum). This is termed chromatic aberration, and is practically eliminated over a moderate range of wavelengths by using a composite lens, called anachromatic lens, composed of parts having different dispersive powers. aberration constant. . See CONSTANT OF ABERRATION. ablation. , n. Wasting of snow or ice by melting or evaporation. abnormal. , adj. Deviating from normal. abrasion. , n. Rubbing or wearing away, or the result of such action. abroholos. , n. A squall frequent from May through August between Cabo de Sao Tome and Cabo Frio on the coastof Brazil. abrupt. , adv. Steep, precipitous. See also BOLD. abscissa. , n. The horizontal coordinate of a set of rectangular coordinates. Also used in a similar sense in connection with oblique coordinates. absolute. . Pertaining to measurement relative to a universal constant or natural datum. absolute accuracy. . The ability of a navigation or positioning system to define an exact location inrelation to a coordinate system. absolute gain. . See ISOTROPIC GAIN (of an antenna). absolute humidity. . The mass of water vapor per unit volume of air. absolute motion. . Motion relative to a fixed point. If the earth were stationary in space, any change in the position of another body, relative to the earth, would be due only to the motion of that body. This would be absolute motion, or motionrelative to a fixed point. Actual motion is motion of an object relative to the earth. absolute temperature. . Temperature measured from absolute zero which is zero on the Kelvin scale, 273.16°C on the Celsius scale, and 459.69°F on the Fahrenheit scale. The sizes of the Kelvin and Celsius degree are equal. The size of a degree on the Fahrenheit scale equals that on the Rankine scale. absolute...
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