Navigation and Seamanship

2.2.6. Bearing Circle and Azimuth Circle or Azimuth Ring

                                                                                                              

An azimuth circle is an instrument for determining both bearings of terrestrial objects and azimuths of celestial body. This is a nonmagnetic metal ring. It is an auxiliary instrument of the compass. It consists of a prism mounted on a horizontal axis above and inclined to in which there is a magnifying lens. Shades are provided to enable bearing of bright objects being taken. It is mounted on a compass and is capable of being rotated over it. This arrangements permits bearings of celestial and terrestrial objects being taken. It is sized to fit a 7 1/2-inch compass bowl or a gyro repeater. The inner lip is marked in degrees from 0° to 360°counterclockwise for measuring relative bearings. The azimuth circle is fitted with two sighting vanes. The forward or far vane has a vertical wire and the after or near vane has a peep sight. Two finger lugs are used to position the instrument while aligning the vanes. A hinged reflector vane mounted at the base and beyond the forward vane is used for reflecting stars and planets when observing azimuths. Beneath the forward vane are mounted a reflecting mirror and the extended vertical wire. This lets the mate read the bearing or azimuth from the reflected portion of the compass card. For taking azimuths of the sun,an additional reflecting mirror and housing are mounted on the ring, each midway between the forward and after vanes. The sun’s rays are reflected by the mirror to the housing, where a vertical slit admits a line of light. This admitted light passes through a 45^o reflecting prism and is projected on the compass card from which the azimuth is directly read. In observing both bearings and azimuths, two attached spirit levels are used to level the instrument. An azimuth circle without the housing and spare mirror is called a bearing circle