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    Re: Variation of compass
    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

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