The Rude Star Finder

[HenryC]

They call it the the Rude Star Finder not because it's impertinent or impolite, but because it was invented by Captain Rude.  Actually, You rarely see the original Rude any more, I use a Weems and Plath Model 2102-D, but the principle is the same.

The heart of the star finder is an opaque white plastic disc with stars and their names plotted on both sides of it.  One side represents the Northern Hemisphere, the other the South, and they are marked appropriately with an N or S.  This disk represents a map of the entire sky, and all the named navigational stars for which data is published in the Nautical Almanac are plotted. Along the outer edge of the disk are printed degrees, from 0 to 360. These numbers represent the Local Hour Angle of Aries, or the number of degrees the zero point of the astronomical coordinate system is from your location. LHA Aries depends on the Almanac value for the Greenwich Hour Angle (GHA) of Aries, where you are on earth as well as the date and time of day.

Your DR position already gives you a rough idea of where you are, give or take a degree, and presumably you have a timepiece aboard which reads or can be corrected to Universal Time (UT, AKA as Greenwich Mean Time or GMT).  Use your DR longitude and add it to the value in the Almanac for GHA Aries for that time and date (remember, West longitudes are negative).  If the result is outside the range of 0-360, add or subtract 360 degrees until it falls within that range.  That is your LHA of Aries. Write it down before you forget it.

In your Star Finder kit are a stack of 9 clear plastic templates, one for each 10 degrees of latitude between equator and pole, marked with a blue coordinate grid.  They are printed in such a way that they can be used for either north or (by flipping them) south latitudes, just place them so that the N or S Latitude label printed on the template reads correctly depending on which hemisphere you are sailing in.  A little white plastic bead fits in a hole in both the map and latitude template so that you can rotate the latter about the former until the blue arrow points to the appropriate LHA on  the white disk's edge. You can now read the azimuth and elevation from the latitude template of the star on the opaque disk.  Be really careful with that white plastic bead, it's easy to drop, and if you do it will be lost forever and you will have to rely on a jury rig, like a bent paper clip, to fasten the disk to the template..

There is also a red-ruled transparent template used to locate the planets (I never use it because I can always ID the planets, I just look for a bright star where none should be). I know a couple of dozen stars by heart, but there are almost 60 navigational stars, and I am unfamiliar with the southern sky altogether, so the Star Finder comes in handy.  I suggest you use it the night or day  before to get a list of evenly spaced stars ready for observation at twilight, with their names, magnitudes, altitudes and azimuths listed for quick location and ID.  Even if it's cloudy, you might be able to shoot one through a hole in the overcast and salvage an LOP from the session.

There are excellent instructions with the Star Finder, but I wanted to show you that it isn't necessary to memorize the constellations in order to find stars to shoot with a sextant.

[Bob23]

Thank you , Henry:
   When I see a navigation-related post of yours, I read 'em. I've printed some of them for reference in my pile of cel nav books. Recently, my wife bought me "Celestial Navigation for the Clueless". (I assume she means that I'm clueless about cel nav, but she may have a more expanded definition.) Have you seen that book and what is your opinion of it? It seems quite layman friendly.
Bob23, not quite lost...yet.