English

A SMALL CIRCLE on the surface of the EARTH at every point of which, at the instant of OBSERVATION, the observed CELESTIAL BODY (SUN, STAR, or PLANET) has the same ALTITUDE and, therefore, the same ZENITH DISTANCE.

The SMALL CIRCLE on the surface of the EARTH on which an OBSERVATION shows the observer to be. The expression is most frequently used to indicate the CIRCLE OF EQUAL ALTITUDE. Also called position circle.

A circle having as its centre a POSITION, however determined, and as its radius the maximum PROBABLE ERROR of the POSITION; a circle within which a craft is considered to be located.

(U.S. terminology). A BOAT SHEET or a SMOOTH SHEET on which intersecting systems of curves are drawn, each curve corresponding to the LOCUS of some angle between two STATIONS. POSITIONS are plotted on circle sheets at the intersection of the LOCI of two observed angles, each LOCUS being found by INTERPOLATION between the curves drawn on the sheet. Circle sheets are prepared and employed in HYDROGRAPHIC SURVEYS for plotting FIXES without the use of PROTRACTOR. Also calledstandard circle sheet; formerly calledsextant chart. See also FIXED ANGLE PLOT, and LATTICE.

A RADIO STATION which needs not necessarily be manned, the emissions of which, radiated around the HORIZON, enable its BEARING to be determined by means of the RADIO DIRECTION FINDER of a ship.

(adj.). Revolving about the ELEVATED POLE without setting. A CELESTIAL BODY is circumpolar when its POLAR DISTANCE is approximately equal to or less than the LATITUDE of the observer. The actual limit is extended somewhat by the combined effect of REFRACTION, SEMIDIAMETER, PARALLAX, and the HEIGHT of the observer's eye above the HORIZON.