Saturday, December 20, 2008

Rude Starfinder 2102-D

The objective of this is to describe the procedure to use the Rude Starfinder and to be able to identify an unknown body, plot the planets on the Starfinder, and to select the best stars or planets for a fix.

To solve the navigational triangle for a computed altitude and true azimuth, the navigator must know beforehand or be able to determine afterwards the name of the celestial body observed, so that you can obtain its GHA and declination from the Nautical Almanac. Several aids are available to the navigator to assist in identifying and locating celestial bodies, among which is the starfinder. Starfinders are intended to furnish the approximate altitude and true azimuth of celestial bodies either before or after navigational observations. One of the best and most common is the Rude Starfinder.

Star Identification
As a navigator, you may be required to obtain a fix from two or more stars. Actually, only a few of the of stars are used regularly for celestial navigation, and they are not too difficult to locate and identify. No matter where you may be navigating, you can manage very well if you are able to recognize 20 or so. The Nautical Almanac consists of 57 principal stars as well as tables for finding latitude by the North Star (Polaris).

Relative brightness of stars is called magnitudes the lower the magnitude, the brighter the star. Sirius, brightest of them all, has a magnitude of - 1.6, Acamar, dimmest of the navigational stars, is listed at + 3.1 magnitude.

First magnitude stars range from magnitude - 1.6 to magnitude +1.50

Second magnitude stars are those from +1.51 to +2.50

Stars of third magnitude range from +2.51 to +3.50, and so on.

Stars of the sixth magnitude are barely visible to the unaided eye.The magnitudes given here of principal stars are only a fraction of the navigational celestial bodies. Selected navigational planet magnitudes vary due to atmospheric conditions. Mars magnitude, for example, varies from + 1.6 to -2.8. The moon usually has a magnitude of 12.6, however, its "phase" must be considered prior to use. The king of celestial bodies, with a magnitude of -26.7, is the sun, limited only by nighttime and atmospheric conditions. The magnitude of the planets is listed at the top of daily pages and stars at the end of the white pages in the Nautical Almanac.

One or more of the stars in a constellation may be navigational stars. Obviously, if you can recognize a constellation and know which of its stars may be used, you can identify them whenever the group is visible in the sky. The stars and constellations that might be familiar to you are not always visible from where you may happen to be. For this reason, you must have some means of identifying navigational bodies when nothing you know by sight can be seen overhead. One method by which you can identify those celestial bodies is to use the Star Finder.

The Rude Starfinder consists of the star base, an opaque white plastic circular base plate fitted with a peg in the center, and ten circular transparent templates. On one side of the star base the north celestial pole appears at the center, and on the other side the south celestial pole.

Description of the Rude Starfinder
The Rude Starfinder, 2102-D, is designed to permit the determination of the approximate apparent altitude and azimuth of any of the 57 selected navigational stars tabulated in the Nautical Almanac. All of the 57 navigational stars are shown on each side at their positions

relative to the appropriate pole in a type of projection called an azimuthal equidistant projection. In this projection, the positions of the stars relative to one another are distorted, but their true declinations and azimuths relative to the pole are correct, the pattern of the stars on the star base does not correspond to their apparent positions as seen in the sky. Each star on the base is labeled, and its magnitude is indicated by its symbol, a large heavy ring indicates first magnitude, an intermediate sized ring second magnitude, and a small thin ring third magnitude. The celestial equator appears as a solid circle about four inches in diameter on each side of the star base, and the boundary of each side is graduated to a half-degree of LHA of Aries.

There are 10 templates included for use with the star base. Nine of these are printed with blue ink and are designed for apparent altitude and azimuth determinations, while the tenth, printed in red ink, is intended for the plotting of bodies other than the 57 selected stars on the base plate. There is one blue template for every 10° of latitude between 5° and 85°, one side of each template is for use in north latitudes, the other for south latitudes. Each of these "latitude" templates is printed with a set of oval blue altitude curves at 5° intervals, with the outermost curve representing the observer's celestial horizon, and a second set of radial azimuth curves, also at 5° intervals. The red template is printed with a set of concentric declination circles, one for each 10° of declination, and a set of radial meridian angle lines. The appropriate template is snapped in place on the star base.

Using the Star Finder
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 the 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 your longitude of GHA Aries 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, north to north, south to south. Rotate the blue template until the 000° 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 in hand. He is 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 uses the star finder. 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. The Star Finder is designed to help locate and identify, by altitude and azimuth, the 57 stars listed in the Nautical Almanac or any other celestial bodies that may be plotted on the star base. Because the unit uses an Azimuthal Equidistant Projection, it can not be compared directly with the heavens due to distortion. The complete unit consists of one star base, ten templates, and instructions.

To Find or Identify Celestial Bodies
1. From the Nautical Almanac determine the GHA of Aries for GMT of the observation.
2. Convert GHA Aries to LHA Aries by subtracting DR longitude if west, or adding DR longitude if east. When this answer is negative add 360°, or if the answer is over 360° subtract 360°.
3. Select blue-line template for latitude nearest your DR position. Center selected template over star base so that template and star base both conform to hemisphere (N or S) of observer. Rotate the template until arrow is over LHA Aries. The approximate altitudes and azimuths of celestial bodies above the horizon are then indicated by the curves.

To plot the Sun, Moon, Planets, or additional Stars From the Nautical Almanac, determine the body's declination and right ascension (RA). The body's RA is obtained by:
When GHA body is zero, GHA Aries equals RA. Center red-line template over star base, use correct hemisphere on both, then rotate until arrow (0°) points to RA body. If the body's declination is the same as the hemisphere in center of base, then position will be plotted towards center from celestial equator. If declination is opposite, then position will be plotted away from celestial equator towards edge of base. With a pencil through the cut-out slot, mark the body's declination.

Identifying Unknown Bodies
Using the appropriate blue-line template and base side, align index arrow to LHA Aries for the time of sighting. Locate intersection of altitude and azimuth of shot. If no star is near intersection, the body may be a planet or unmarked star. Keeping blue-line template in place, put red-line template on top and rotate until the cut-out slot is over the altitude / azimuth intersection of sight. Determine declination and SHA of body, then refer to the Nautical Almanac for identification.