NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: Variation of compass
From: Trevor Kenchington
Date: 2002 Oct 2, 22:37 -0300
From: Trevor Kenchington
Date: 2002 Oct 2, 22:37 -0300
Back when men were men, women stayed ashore and nobody had thought of electronic navigation, the last generation of big (steel) sailing ships were typically fitted with a "standard compass" on a raised (about 6ft above deck) platform -- built of teak and brass, in at least one case, presumably to minimize local magnetic influences. I can't immediately find confirmation but I think it was standard routine to check variation daily, in clear weather, using this compass and celestial sights. Presumably, the standard compass had small and known deviations, while it could be checked against the steering compass at frequent intervals to determine any irregular component of the latter's deviation which resulted from proximity to movable various bits of steel. Does anyone know what celestial sights were used to find the variation in the standard compass? Much as Peter Fogg noted, the azimuth of a rising Sun, Moon or Venus observed across the compass could be used, at the cost of first determining what the True azimuth should be from the vessel's EP. Alternatively, if the navigator could recognize a few stars with near-zero declination, he could save the calculations at the cost of more difficult observations and those made at night (rather than twilight). I assume that the difficulty of determining the exact moment of local noon would prevent the observation of an accurate compass azimuth when the Sun crossed the true meridian. But what was actually done? Trevor Kenchington -- Trevor J. Kenchington PhD Gadus@iStar.ca Gadus Associates, Office(902) 889-9250 R.R.#1, Musquodoboit Harbour, Fax (902) 889-9251 Nova Scotia B0J 2L0, CANADA Home (902) 889-3555 Science Serving the Fisheries http://home.istar.ca/~gadus