Signalman 1 & C - Historic Naval Ships Association

Some gyros, for instance, become erratic after the shipmakes a series of sharp turns at high speeds. Thepossibility of a gyro malfunction does not mean,however, that great confidence cannot be placed in thegyro. When running properly, it can be depended uponto point faithfully and steadily to true north. But themagnetic compass, being more reliable, is usedconstantly to check the gyro's performance.Typical shipboard installations of gyrocompassesconsist of one or more master gyros, whose indicationsare transmitted electrically to repeaters located inconning stations, on bridge wings, and at othernecessary points.BEARING AND AZIMUTH CIRCLESStrictly speaking, azimuth and bearing are thesame in meaning: the horizontal angle that a linedrawn from your position to the object sighted makeswith a line drawn from your position to true north. Theword azimuth, however, applies only to bearings ofheavenly bodies. For example, it is not the bearing, butthe azimuth of the Sun; and not the azimuth, but thebearing of Brenton Reef Tower.A bearing circle is a nonmagnetic metal ringequipped with sighting devices that is fitted over agyro repeater or magnetic compass. The bearing circleis used to take bearings of objects on Earth's surface.The azimuth circle (fig. 9-1) is a bearing circleequipped with additional attachments for measuringazimuths of celestial bodies. Either bearings orazimuths may be taken with the azimuth circle.Taking a BearingAssume that you are getting a bearing on alighthouse. Install either a bearing or azimuth circleon the gyro repeater, and make sure that the circlerotates freely. Train the vanes on the lighthouse so thelighthouse appears behind the vertical wire in the farvane. Drop your gaze to the prism at the base of thefar vane, then read the bearing indicated by a hairlinein the prism.Taking an AzimuthThe azimuth circle may be used in two ways tomeasure the azimuth of a celestial body. The first methodis used with a brilliant body such as the Sun. At the uppercenter in figure 9-2, you see a concave mirror; and at thelower center, a prism attachment. Sight with the mirrornearest you, and the prism toward the observed body.Light from that body is reflected from the concave mirrorinto the prism. The prism, in turn, throws a thin beam onthe compass card. This beam strikes the graduation thatindicates the azimuth.The second method is used for azimuths of bodieswhose brightness is insufficient to throw such adistinct beam. Behind the far vane on the azimuthcircle is a dark glass that may be pivoted so as to pickup celestial bodies at various altitudes. When a bodyis sighted, its reflection appears behind the verticalwire in the far vane, and its azimuth may be read underthe hairline in the prism.The inner lip of the azimuth circle, in figure 9-2,is graduated counterclockwise in degrees. It isFigure 9-l.—Azimuth circle.Figure 9-2.—Taking an azimuth.9-2