Some gyros, for instance, become erratic after the ship
makes a series of sharp turns at high speeds. The
possibility of a gyro malfunction does not mean,
however, that great confidence cannot be placed in the
gyro. When running properly, it can be depended upon
to point faithfully and steadily to true north. But the
magnetic compass, being more reliable, is used
constantly to check the gyro's performance.
Typical shipboard installations of gyrocompasses
consist of one or more master gyros, whose indications
are transmitted electrically to repeaters located in
conning stations, on bridge wings, and at other
necessary points.
BEARING AND AZIMUTH CIRCLES
Strictly speaking, azimuth and bearing are the
same in meaning: the horizontal angle that a line
drawn from your position to the object sighted makes
with a line drawn from your position to true north. The
word azimuth, however, applies only to bearings of
heavenly bodies. For example, it is not the bearing, but
the azimuth of the Sun; and not the azimuth, but the
bearing of Brenton Reef Tower.
A bearing circle is a nonmagnetic metal ring
equipped with sighting devices that is fitted over a
gyro repeater or magnetic compass. The bearing circle
is used to take bearings of objects on Earth's surface.
The azimuth circle (fig. 9-1) is a bearing circle
equipped with additional attachments for measuring
azimuths of celestial bodies. Either bearings or
azimuths may be taken with the azimuth circle.
Taking a Bearing
Assume that you are getting a bearing on a
lighthouse. Install either a bearing or azimuth circle
on the gyro repeater, and make sure that the circle
rotates freely. Train the vanes on the lighthouse so the
lighthouse appears behind the vertical wire in the far
vane. Drop your gaze to the prism at the base of the
far vane, then read the bearing indicated by a hairline
in the prism.
Taking an Azimuth
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. At the upper
center in figure 9-2, you see a concave mirror; and at the
lower center, a prism attachment. Sight with the mirror
nearest you, and the prism toward the observed body.
Light from that body is reflected from the concave mirror
into the prism. The prism, in turn, throws a thin beam 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 insufficient to throw such a
distinct beam. Behind the far vane on the azimuth
circle is a dark glass that may be pivoted so as 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 may be read under
the hairline in the prism.
Figure 9-l.—Azimuth circle.
The inner lip of the azimuth circle, in figure 9-2,
is graduated counterclockwise in degrees. It is
Figure 9-2.—Taking an azimuth.
9-2
