Friday, December 26, 2008

How to Determine the Deviation or Gyro Error by a Azimuth of a Celestial Body

I will explain how to use the azimuth circle to observe the azimuth of a celestial body and how to compute the 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. 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 on the navigator's knowledge of position and the correct time.

In taking a azimuth of a celestial body, the azimuth circle is used. A 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 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 althe 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° 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. A azimuth is similar to a amplitude but it is taken at anytime, not at sunrise or sunset. When taking a azimuth it 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° overlap occurring between volumes. When taking the Coast Guard exam you will be using Volume 2 - Latitudes 15° - 30°.

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 and north contrary page. The normal practice of navigation at sea is that the ship's compasses be checked frequently, it has been a custom to check for compass error at least once a day. There are two main celestial navigation methods of determining compass error, which are azimuths and amplitudes. Azimuth observations are simply bearings taken of celestial bodies using one of the ship's compasses. Normally, it is best to take an azimuth when the body's altitude is less than 20 degrees. Azimuths may be taken of any celestial body but the sun is preferred because it is the easiest to observe. The sight reduction calculations for solving azimuths are very similar to determining computed altitude (Hc) and azimuth (Zn) when solving a line of position sight.