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Appendix I, Continued
SEMIDIAMETER.—The radius of a closed
figure. Half the angle at the observer
subtended by the visible disk of a celestial
body.
SET.—The direction toward which a
current flows.
SEXTANT.—A double-reflecting
instrument for measuring angles, primarily
altitudes of celestial bodies.
SHORAN.—A precision electronic position
fixing system using a pulse transmitter and
receiver and two transponder beacons at
fixed points.
SIDEREAL HOUR ANGLE.—Angular
distance west of the vernal equinox; the arc
of the celestial equator or the angle at the
celestial pole between the hour circle of
the vernal equinox and the hour circle of a
point on the celestial sphere, measured
westward from the hour circle of the vernal
equinox through 360°.
SIDEREAL TIME.—Time based upon the
rotation of Earth relative to the vernal
equinox.
SKY WAVE.—An indirect radio wave that
travels from the transmitting antenna into
the sky, where the ionosphere bends it
back toward the Earth.
SLACK WATER.—The condition when
the speed of a tidal current is zero,
especially the momentary condition zero
speed when a reversing current changes
direction.
SOLAR TIME.—Time based upon the
rotation of Earth relative to the Sun.
SOUNDING.—Measured or charted depth
of water or the measurement of such depth.
SPEED OF RELATIVE
MOVEMENT.—Speed relative to a
reference point itself usually in motion.
SPRING TIDES.—The tides occurring near
the times of full moon and new moon
when the range of tide tends to increase.
STADIMETER.—An instrument for
determining the distance to an object of
known height by measuring the angle
subtended at the observer by the object.
STAND.—The condition at high or low
tide when there is no change in the height
of the water.
STAR FINDER.—A device to facilitate the
identification of stars.
STATUTE MILE.—A unit of distance
equal to 5,280 feet.
STRATOCUMULUS.—Low clouds (mean
upper level below 6,550 feet), composed of
a layer or patches of globular masses or
rolls.
STRATUS.—A low cloud (mean upper
level below 6,550 feet) in a uniform layer.
TANGENT.—The ratio of the side
opposite an acute angle of a plane right
triangle to the shorter side adjacent to the
same angle. A straight line, curve, or
surface touching a curve or surface at one
point.
TELESCOPIC ALIDADE.—A device used
with a gyro repeater for taking bearings.
AI-11
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