Tuesday, March 31, 2009

Azimuths Circles

When taking an azimuth of a celestial body, the azimuth circle is used. An azimuth circle is a nonmagnetic metal ring sized to fit on a 7 inch compass bowl or on a gyro repeater. The inner lip is graduated in degrees from 000° to 360° in a counterclockwise direction for the purpose of taking relative bearings. Two sighting vanes the forward or far van containing a vertical wire, and the after or near vane containing a peep sight will help when making observations of bearings and azimuths. 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. Below the forward vane there is a reflecting mirror and a extended vertical wire are mounted, this helps the navigator to read the bearing or azimuth from the reflected portion of the compass card.

For observing 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° reflecting prism and is projected on the compass card from which the azimuth is directly read. In observing both bearings and azimuths there are two spirit levels, which are attached must be used to level the instrument. 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 applies only to bearings of heavenly bodies. Lets say for example, it is not the bearing, but the azimuth of the Sun, and not the azimuth, but the bearing of a lighthouse. 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 is a bearing circle equipped with additional attachments for measuring azimuths of celestial bodies. You can either take bearings or azimuths with a azimuth circle.

When taking a bearing lets say 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 a Azimuth
The azimuth circle can 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 you will 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 can 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.

The inner lip of the azimuth circle is graduated counterclockwise in degrees making it possible, to obtain relative bearings of objects by training the vanes on an object, then reading the graduation on the inner circle alongside the lubber’s line on the pelorus or repeater. Each of the far vanes contains a spirit level to indicate when the circle is level. Bearings taken when the azimuth or bearing circle is not on an even keel are inaccurate.

Telescopic Alidades
Another means of taking bearings is by using an alidade, which, like the bearing circle, is mounted on a repeater. The telescopic alidade is a bearing circle with a small telescope attached to it. The image is magnified, making distant objects appear larger to the observer. A series of prisms inside the low power telescope enables the bearing taker to read the bearing directly from the compass card without removing the eye from the eye piece. Bearings and azimuths can be true, per gyrocompass (PGC), magnetic, or per steering compass