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    Re: Nav exercise - ex-meridian
    From: George Huxtable
    Date: 2008 May 20, 20:59 +0100

    Jeremy's ex-meridian exercise turns out to be of great interest. It has
    taken me into unfamiliar waters, as I have never even attempted an
    ex-meridian correction before. And it's showing up some of the problems that
    are involved.
    
    Am I right in thinking that this exercise goes right up to, and perhaps even
    beyond, the viable limits for which the ex-meridian procedure can be
    expected to work? Perhaps that's the lesson that we are intended to draw.
    With such a high Sun, it doesn't allow much difference, between observer's
    long and Sun's GHA, before the Sun's direction gets way off the meridian.
    
    The difficulties of using ex-meridian tables really show up here. In the
    tables that I have, Bowditch and a modern(ish) Norie's, the quantity A, or
    alpha, is tabulated only to the nearest degree of lat or dec., and changes
    greatly for each such step, so getting the right number calls for a tricky
    double-interpolation. Just by in-the-head estimation, I would put A at about
    21 or so. And that leads me to an ex-meridian correction of about 14', but I
    can see that the correction could easily be half an arc-minute off, either
    way.
    
    Am I in the right ball-park, I wonder? Normally, I would have gone straight
    for a St Hilaire position-line. Being pushed by Jeremy into trying an
    ex-meridian method has only reinforced that view.
    
    George.
    
    contact George Huxtable at george@huxtable.u-net.com
    or at +44 1865 820222 (from UK, 01865 820222)
    or at 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK.
    
    
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