Friday, December 14, 2007


The star finder shows the positions of the common navigational stars listed in the Nautical Almanac. The star finder consists of a flat disk with two sides:
On one side is a star base of the northem latitudes.
The other side has a star base of the southern latitudes.

This identification device has nine transparent blue templates with grids by which latitude in tens of degrees from 5 degree's to 85 degree's may be selected. A 10th red template is available for plotting the planets on the Starfinder. Each of the nine blue templates has a series of altitude and azimuth curves. Reversing the templates makes possible their use in either
northern or southern latitudes.
The circumference of the star base is graduated in degrees to the east from the first point of Aries from 0 through 359. These numbers represent, in effect, the right ascension of each of the bodies on the template.

The star finder may be used either to:
1. Identify an unknown body whose altitude and azimuth have been observed.
2. Make a list of stars and planets available for observation at morning or evening twilight for a fix.
To use the star finder, first determine GHA of Aries for your time of observation from the Nautical Almanac. Next, determine LHA of Aries by subtracting your longitude from GHA of Aries if in west longitude or by adding GHA of Aries to your longitude if in east longitude. Select the template nearest your DR latitude and place it on the northern or southern base, depending on whether you are north or south of the equator. Ensure that the proper side of the template is up, hence, north to north - south to south.

Rotate the blue template until the 0 to 180 arrow on the template is over the LHA Aries on the base plate. The stars or planets available to you at that time, will be under the grid system of your blue template. DIRECTLY OVER­HEAD (Zenith) then is represented by the cross at the center of the open space on the template.

The sky overhead or dome is now shown in the part of the base covered by the curves on the template. The approximate azimuth and altitude of any navigational star within these curves can be found by following the lines on the template.

Finding an Unknown Star or Planet
After a long period of heavy weather, you may see the navigator out on the bridge wing scanning the heavens, his sextant at hand. He is undoubtedly hoping that the overcast will break long enough for him to have a shot at even a single star. If the navigator should manage to pull a star down, the star's identity may not be known. This is where one of the uses of the star finder comes into play. An azimuth (bearing) of the star should be taken at the instant of observation. When the correct template is oriented properly on the star base, the name of the star can be read at the intersection of the azimuth and altitude lines on the grid.