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    Re: Manual calculation of compass variation
    From: George Huxtable
    Date: 2004 Oct 16, 11:00 +0100

    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
    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.
    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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