DETERMINING THE DEVIATION OR GYRO ERROR BY AZIMUTH OF A CELESTIAL BODY
AZIMUTH OF THE SUN
Computation of compass error at sea depends upon the observation of the azimuth of celestial bodies. The Sun is the most commonly used for this purpose. Upon observation, the observed azimuth is recorded. The time (to the nearest second) and the DR position are also noted. With DR position and time, the navigator computes Zn by using the Nautical Almanac and PUB 229 Sight Reduction Tables. The difference between pgc bearing and Zn (true bearing) is the gyro error (G.E.), and the difference between psc bearing and the magnetic bearing is the deviation. It should be appropriately labeled. Keep in mind that accuracy depends upon the navigator's knowledge of position and the correct time.
In taking an azimuth of a celestial body, the azimuth circle is used. An azimuth circle, is a nonmagnetic metal ring sized to fit upon a 7-inch compass bowl or upon a gyro repeater. The inner lip is graduated in degrees from 0 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) facilitate the observation of bearings and azimuths. Two finger lugs are used to position the instrument exactly 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 a reflecting mirror and the extended vertical wire are mounted, enabling 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 degree reflecting prism and is projected on the compass card from which the azimuth is directly read. In observing both bearings and azimuths, two spirit levels, which are attached must be used to level the instrument.
Azimuth is similar to an Amplitude, but it is taken at anytime -not at sunrise or sunset. Thus an azimuth requires the use of PUB 229, Sight Reduction Tables for Marine Navigation to obtain the Zn (true bearing).
PUB 229 Sight Reduction Tables for Marine Navigation
Pub. 229 Sight Reduction Tables for Marine Navigation is a set of six volumes of precalculated solutions for the computed altitude (Hc) and the azimuth angle (Z) of the navigational triangle. Each volume covers a 15 degree band of latitude with a 1 degree overlap occurring between volumes. We will be using Volume 2 - Latitudes 15° - 30° for this problem when needed.
Entering arguments for the tables are local hour angle (LHA), latitude. and declination expressed in whole degrees. Values of Hc and Z are tabulated for each whole degree of each of the entering arguments. Tables inside the front and back covers of each volume allow for interpolation.
Each volume contains two sets of tabulation for whole degrees of LHA between 0 and 360. The front half is for the first eight degrees of latitude (15°-22°) covered by that volume, and the second half is for the remaining eight degrees of latitude (22° - 30°). The values of LHA are at the top and bottom of each page. The eight degrees of latitude form the horizontal argument and the declination is the vertical argument.
Instructions at the top and bottom of each page indicate whether the tabulations on that page are for the latitude which is the same or contrary to the declination:
If both latitude and declination are north or both south - same name page.
If they are of opposite names - north and south or vice versa south to north contrary page.
