A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
From: Henry Halboth
Date: 2007 Sep 25, 18:29 GMT
Thanks for your interesting reply. I am of the opinion that Lunars generally received a bad name as far as accuracy is concerned primarily for two reasons.
1. The relative inaccuracy of early instruments. Let's remember that the octant, as originally produced was not fitted with a tangent screw and gear for easy and positve measurement of small arc increments. Although the vernier appeared rather early on, it was the incorporation of the tangent screw that allowed more positive fine adjustment. I also certainly cannot believe that some of the earlier instruments made of wood with ivory arcs, although works of art, could be relied on as respects accuracy over a wide range of condition to be met at sea.
2. The mathematical methods of clearing the distance, especially the so called approximate methods were a wonderment of complication, in some instances requiring the application of numerous corrections which in themselves sometimes defied reason. Most of the epitomes I have read did not apply to reason, but simply listed rules to follow. In my view,at least, a relatively simple explanation of the problem related to a trigonometrical format would have better served the mariner. The entire subject of spherical trigonometry does not appear as complicated as some of the approximate methods.
With an accurate instrument and a little practice, I see no problem in, under favorable conditions at least, attaining an accuracy of less than 0.5 arc minutes in measurement of the distance. Although unfamiliar positioning of the instrument, as well as the body, sometimes becomes necessary, there is actually less difficulting involved than atitude measurement on a sea horizon and certainly the potential of greater accuracy.Thereafter, the solution becomes one of nitpicking care in tabular interpolation, errors in which can induce particularly large deviations, and infinite care in simple addition and subtraction. There can be and should not be any rounding off if good results are desired.
I am not of the opinion that sea state has a significant effect on the observer, unless a vibratory effect is present, such as when a vessel be laboring or pounding, or when vibration is enduced by operating machinery. It does seem usual for a vessel to have been eased off or hove-to so as to provide more favorable conditions for observation.
You referred me to your Lunar Tables, with which I am well familiar.
My observation produced as follows ..
Date = July 27, 2007
Watch time = 00-44-05
Cleared distance = 17-44-04.4 by Borda's method
GMT deduced = 00-44-02
Watch fast on GMT = 03 seconds
True distance by FER's table = 17-44-06 for GMT = 00-44-02
Your table must be off a little. Ha ha!
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