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pole, the observer's zenith, and the body.
Azimuth Angle (Z)—The interior angle of the astronomical triangle at the zenith measured from the
observer's meridian to the vertical circle through the body.
Azimuth Stabilization—Orientation of the picture on a radarscope so as to place true north at the top of
the scope.
Azimuth, True (Zn)—The angle at the zenith measured clockwise from true north to the vertical circle
passing through the body.
Beacon—A light or electronic source that emits a distinctive signal used to determine bearings, courses,
or location.
Beam Width—The angle between the directions, on either side of the axis, at which the intensity of the
radio frequency field drops to one-half the value it has on the axis.
Beam-Width Error—The effective width in azimuth of radiation from an antenna.
Bearing—The horizontal angle at a given point measured clockwise from a specific datum to a second
point.
Magnetic Bearing (MB)—The direction to an object from a point, expressed as a horizontal angle,
measured clockwise from magnetic north.
Relative Bearing (RB)—The direction expressed as a horizontal angle, normally measured
clockwise from the true heading or forward point of the longitudinal axis of the aircraft to an object
or body.
376 AFPAM11-216 1 MARCH 2001
True Bearing (TB)—The direction to an object from a point, expressed as a horizontal angle
measured clockwise from true north.
Bellamy Drift—The net drift angle of the aircraft calculated between any two pressure readings.
Blip—The display of a received pulse on a cathode-ray tube; a spot of light representing a target.
Branch, Lower—Half of an hour circle opposite from upper branch, defined below.
Branch, Upper—That half of an hour circle or meridian that contains the celestial body or the
observer's position.
Celestial Equator—The great circle formed by the intersection of the plane of the earth's equator with
the celestial sphere. Also known as Equinoctial.
Celestial Meridian—A great circle on the celestial sphere formed by the intersection of the celestial
sphere and any plane passing through the North and South Poles. Any great circle on the celestial sphere
which passes through the celestial poles.
Celestial Navigation—See Navigation Aids.
Celestial Observation Errors (Sextant):
Acceleration Error—An error caused by the deflection of the bubble due to any change in
acceleration of the aircraft.
Coriolis Error—The error introduced in a celestial observation taken in flight resulting from the
deflective force on the liquid in the bubble chamber, as caused by the path of the aircraft in
counteracting the earth's rotation.
Index Error—An error caused by the misalignment of the sighting measurement mechanism of an
instrument.
Parallax Error—The difference between a body's altitude above an artificial or visible horizon and
above the celestial horizon. The error is present because the body is not at an infinite distance.
Personal Error—Differences in observations caused by sighting limitations of an observer.
Refraction Error—An error caused by the bending of light rays in passing through the various
layers of the atmosphere.
Rhumb Line Correction—The correction applied for the bubble-acceleration error caused by the
rhumb line path of the aircraft.
Wander Error—The bubble-acceleration error caused by a change of track during the celestial
shooting period.
Celestial Poles—The points where the earth's axis, if produced, would intersect the celestial sphere.
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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.
Chart, Aeronautical—A specialized representation of mapped features of the earth, produced to show
selected terrain, cultural and hydrographic features, and supplemental information required for air
navigation, pilotage or for planning air operations.
Checkpoint—A geographical reference point used for checking the position of an aircraft in flight.
Normally, well-defined and selected in preflight planning, a checkpoint can usually be easily identified
from the air.
Circles—Circle of Equal Altitude. A circle on the earth that is the focus of all points equidistant from
the substellar point of a celestial body. The altitude of a celestial body measured from any point on the
circle is the same. Also called circle of position.
Diurnal Circle—The daily apparent path of a body on the celestial sphere caused by the rotation of
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