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    Re: Manual calculation of compass variation
    From: Henry Halboth
    Date: 2004 Oct 16, 10:22 -0400

    George - the discrepancy you describe is the compass error to which
    deviation must be applied to obtain variation. Perhaps Brother Cook was
    not too much troubled with deviation, but it's there nevertheless; even
    the metal ring in an officer's hat while taking an azimuth can deviate a
    standard magnetic compass as much as 3-degrees, as I have proven by
    experiment. Please don't construe that I am suggesting Cook's hat,
    whatever that may have been, as the start of another thread, but I am
    saying that, without a knowledge of the deviation the variation recorded
    must be suspect.
    
    On Sat, 16 Oct 2004 11:00:31 +0100 George Huxtable
     writes:
    > Perhaps I can add a bit to Herbert Prinz's reply to Lisa Fiene's
    > question..
    >
    > You find variation by taking a compass bearing of any object of
    > which the
    > azimuth is known. Simple as that. The discrepancy between them is the
    > variation.
    >
    > For example, at apparent Noon, the azimuth of the Sun is known to be
    > due
    > North or South, depending on your hemisphere, and its.
    >
    > But (especially in the tropics) it's hard to take an accurate compass
    > bearing of the midday Sun, because it's so high in the sky. From
    > where I
    > live (51d N), in Winter, the midday Sun is so low, it's easy.
    >
    > It's best to use an object that's at, or near, the horizon. The
    > amplitude
    > tables give the predicted azimuth at Sunrise and Sunset. Trouble is,
    > the
    > uncertainties in refraction at very low altitudes make the moment of
    > Sunrise and Sunset rather indeterminate. In the tropics, that doesn't
    > matter too much, because the Sun is rising from, and falling to, the
    > horizon nearly vertically, so its azimuth changes only slowly. At
    > high
    > latitudes, it emerges at a shallow angle, so uncertainties in
    > refraction
    > make for big changes in azimuth. A good compromise is to calculate
    > the
    > azimuth when the Sun's altitude is, say, 10degrees, using some form
    > of
    > altitude-azimuth tables or calculation.
    >
    > Lisa quotes Cook-
    >
    > >"Found the Variation of the Compass by the Even'g and Morning
    > Amplitudes
    > >and by 2 Azths to be West 20� 59'"
    >
    > That raises another interesting question. How did Cook manage to
    > take a
    > compass bearing of the Sun (or even the average of several) to
    > sufficient
    > accuracy that he could quote the result to a minute of arc; to be
    > 20deg
    > 59', rather than 21deg? Compass bearings can't be taken to that
    > accuracy
    > now, nor could they then. Cook was simply quoting the answer that his
    > calculations gave him. We have to take the 59' part of his measured
    > variation with a large pinch of salt.
    >
    > George.
    >
    > ================================================================
    > contact George Huxtable by email at george---.u-net.com, by
    > phone at
    > 01865 820222 (from outside UK, +44 1865 820222), or by mail at 1
    > Sandy
    > Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK.
    > ================================================================
    >
    
    
    

       
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