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    Re: Variation of compass
    From: George Huxtable
    Date: 2002 Oct 3, 10:40 +0100

    Some comments about compass variation.
    
    Trevor Kenchington said-
    
    >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.
    
    Yes, and steamers too, often on top of a deckhouse aft of the bridge, well
    away from the funnel and other steel gubbins that poked up through the
    deck. The standard compass was there for two reasons. First, as Trevor
    says, it was less likely to be disturbed up there by magnetic effects of
    changes to the vessel. Second, it would have a clear view all around
    (except for a small arc where the masts or funnel got in the way) so
    enabling compass azimuths to be taken of landmarks as well as of celestial
    bodies. For this purpose an azimuth mirror could be used, placed on top of
    the gimballed compass: alternately a simple shadow-pin for the magnetic
    bearing of the Sun. I don't know whether ships are so fitted nowadays.
    
    There would also be a pelorus. This is a gimballed device which enabled
    azimuths to be taken of landmarks and celestial bodies with respect to the
    centreline of the vessel.
    
    
    >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.
    
    Regularly, and particularly when there was a change of course, it would be
    the responsibility of the watch officer to ensure that the course indicated
    on the standard compass was correct, taking account of any known compass
    errors: if not, the course as indicated on the steering compass would be
    trimmed accordingly.
    
    >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?
    
    From long before the days of iron or steel vessels, a ship's master would
    always have a good idea of the moment of local apparent noon (noon by the
    Sun). It was available from the chronometer, if one was carried, after
    correcting for equation of time, but the longitude needed to be known to
    some degree of accuracy. But even without a chronometer, if skies were
    clear, local time was precisely available. As long as the latitude was well
    known (it usually was), and the Sun's declination also, then a single
    observation of Sun altitude, well away from Noon, would give a precise
    measure of local apparent time. This would be used for setting the ship's
    clock, and more important, for working out the longitude from a lunar
    distance. Azimuth tables and diagrams have existed for many years which
    avoid all the calculation and are quite accurate enough for determining the
    azimuth of a body, and hence the compass error, from a known lat and dec
    and the local hour angle of the body.
    
    But the azimuth of a body can be calculated directly from lat and dec and
    the altitude of the body, without reference to time, as the relevant
    spherical triangle is fully defined by these quantities. Details if
    requested.
    
    Usually a compass adjuster would operate in daylight and use the Sun, or a
    local landmark, as his azimuth reference, but at sea stars, planets, or
    even the Moon could be used. Usually, the Sun would have been chosen,
    however, and at a time of day when it was not inconveniently high.
    
    Most of the above I have learned from Lecky's "Wrinkles", which I recommend
    to any reader.
    
    =========================
    
    Cliff Sojourner said-
    
    
    >here's the polaris idea, without sights.
    >
    >align a reference object (such as mast of boat, or forestay) with polaris.
    >assuming the boat's compass is swung correctly (deviation known and possibly
    >corrected) and aligned correctly with the reference object, note the compass
    >reading, that's the local variation +/- 1o.  obviously the further away the
    >reference object and the compass are the better this will work.
    
    It is much easier and more accurate to use the azimuth of a body when it is
    near rising or setting, rather than when it is high in the sky, to
    determine the compass error, and a known star which sets near the West
    (i.e. small declination) is easiest of all.
    
    For those who sail in higher latitudes (even for me at 50 deg North)
    Polaris is inconveniently high to provide a good azimuth, in the absence of
    an accurately-levelled pelorus. And remember that Polaris diverges from the
    Pole by 45 minutes, which when it's high can give rise to azimuth errors
    that are significantly greater still. Not quite good enough for compass
    error, in my opinion.
    
    Cliff's suggestion relies on there being little roll of the vessel, and no
    heel at all. It also works only on a Northerly heading of the vessel, but
    the usual purpose of making such observations is to construct a table of
    the vessel's deviations, which vary with heading. Normally, variation, from
    a chart, is known more precisely than the deviations of the vessel.
    
    Vessels used in the past for magnetic survey work were very carefully
    constructed to avoid compass deviation. The research yacht "Carnegie",
    operated by CIT, even had a gasoline engine made from bronze! She perished,
    with her master, in a gasoline explosion while refuelling in harbour some
    time around 1930.
    
    ===========================
    
    Dave Weilacher said-
    
    >One thing I attempted was to compute compass variation by assuming a GP for
    >the mag north pole of 82N x 108W then doing a sight reduction to obtain
    >azimuth (Zn).
    >
    >Ended up deciding there were two problems with this; both biggies.
    >
    >First, it ignores the earths own magnetic anomolies. No help for this.
    >
    >Second, it gives me an initial great circle bearing when I suspect what I'm
    >really after is a rhumbline bearing.
    >
    >
    >BTW, it yielded a reasonable answer from Florida, but was whacko for Bermuda
    
    
    As I see it, it's the great circle bearing that he needs, and NOT a
    rhumbline. But he is right on the other question. The Earth diverges very
    significantly from a perfect bar-magnet.
    
    ========================
    
    George Huxtable.
    
    ------------------------------
    
    george---.u-net.com
    George Huxtable, 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK.
    Tel. 01865 820222 or (int.) +44 1865 820222.
    ------------------------------
    
    
    

       
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