Taking a bearing of a terrestrial object
Assume that you are taking a bearing of a lighthouse. Place the azimuth circle on the gyro repeater, and make sure that the circle rotates freely. Train the vanes on the lighthouse so that the lighthouse appears behind the vertical wire in the far vane. Drop your gaze to the prism at the base of the far vane, and then read the bearing indicated by a hairline in the prism.
Taking an azimuth of a celestial body
The azimuth circle may be used in two ways to measure the azimuth of a celestial body. The first method is used with a brilliant body such as the sun. Other than the vanes, there is a concave mirror and a prism attachment on the circle. Sight the azimuth with the mirror on your side and the prism towards the observed body. Light from that body is reflected from the concave mirror into the prism. The prism, in turn, throws a thin beam of light on the compass card. This beam strikes the graduation that indicates the azimuth. The second method is used for azimuths of bodies whose brightness is not sufficient to throw such a distinct beam. There is a pivoted dark glass behind the far vane on the azimuth circle which can be adjusted to pick up celestial bodies at various altitudes. When a body is sighted, its reflection appears behind the vertical wire in the far vane, and its azimuth can be read under the hairline in the prism. Bearings taken when the azimuth circle is not exactly horizontal are inaccurate, and hence alignment of the bubble gauges is important.
Various methods for obtaining compass error
Three important methods described in detail in this module for obtaining compass error are:
a) Azimuth
b) Amplitude
c) Transit bearings
