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    Re: Possible limitaion for lunar distance measurement
    From: George Huxtable
    Date: 2009 Feb 28, 18:09 -0000

    Good to hear from Kent Nordstrom again, who has raised teasing questions 
    before. He wrote-
    
    ....some weeks ago I was approached by an old collegue of mine raising a
    question related to lunar distance calculation, which I have not considered
    so far. My fellow made a reference to the German Manual of Navigation issued
    1906 ( title: Lehrbuch in Navigation). On page 386 in this manual it is
    stated that it is only possible (or feasible) to use the lunar distance
    method if the distance is between 70 and 110 degrees. The formula used is
    the Dunthorne formula. There is no explaination in the manual why it should
    be like that. After solving quite many lunar cases I have not found any
    limitation as indicated above. Is there anyone who have any answer or
    explanantion or thought about this.
    
    ===========================
    
    reply from George.
    
    It doesn't make sense to me. Of course, it's hard to be sure without reading
    the detailed text, and being in German, that would be no use to me. It's
    certainly the case, as Frank has frequently pointed out, that when the lunar
    distance is near 90�, it's very insensitive to the details of the
    correction. If the Lehrbuch was taking advantage of that insensitivity to
    propose some short-cut in the clearing procedure, which would only apply in
    that limited angular range, fair enough. But there's no such inherent
    restriction in the Dunthorne procedure. I see no reason not to apply lunar
    distance over the whole range of angles, say 40� to 120�, if the Moon can be
    clearly seen.
    
    But that's often the difficulty; that it's hard to pick out the Moon clearly
    in daylight, when it gets close to the Sun, and only a sliver can be seen.
    
    Perhaps the Lehrbuch is recommending switching to Moon-star lunars, when
    Moon-Sun lunar distance departs from optimum. There would be sense in that.
    If you restrict yourself to Sun lunars, there is only one Sun, and it can
    hamper viewing of the Moon, so that lunars available only for about half the
    days in the month. But if you have star lunars in your repertoire, then as
    long as you have dark nights when the Moon appears, there's lots of choice
    among the stars, and you can always find a star that gives you an easy angle
    to measure. Depending on the conditions, star lunars can be taken any time
    of the month except within a couple of days either side of New Moon, and
    (unlike Sun lunars) without a break around full Moon.
    
    And there can be a point in avoiding small angles between star and Moon, as
    Kent himself reminded us some time ago, when you're using traditional lunar
    distance prediction tables at 3-hour intervals. Depending on how far the
    star is from the Moon's path, at small angular distance its change of lunar
    distance can become non-linear enough so that second-order corrections can
    be called for when interpolating.
    
    But if the Lehrbuch is indeed stating, straight out without qualification,
    that "it is only possible (or feasible) to use the lunar distance method if
    the distance is between 70 and 110 degrees", then it is wrong.
    
    George.
    
    contact George Huxtable, at  george{at}hux.me.uk
    or at +44 1865 820222 (from UK, 01865 820222)
    or at 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK.
    
    
    
    
    
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