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    Re: Lunars using Bennett
    From: George Huxtable
    Date: 2008 Jul 3, 21:27 +0100

    I had written, in [5644]
     "Measuring a lunar distance is a means to an end; the end being to
    establish Greenwich time"
    
    And Frank replied-
    
    "A small correction: measuring lunars WAS a means to an end (establishing
    GMT), a long time ago. But nobody loses GMT anymore. Today most people use
    lunars to test their sextants and their skills."
    
    Well, that might make the basis of a fruitful argument.
    
    Frank and I are discussing two quite different meanings of "lunar distance".
    Frank is referring to what I would describe (and don't intend it to be taken
    pejoratively) as the "sextant hobbyist's" approach. It's hard enough for
    anyone, living far from the sea, to test or hone their sextant skills, for
    lack of a horizon. I live about as far from the sea as it's possible to get
    in Britain; in the US, there are far more, living much further inland. Lunar
    distance is a measurement that anyone can make, from anywhere, whenever the
    Moon shines, and provides a demanding test of (some) observational skills.
    And Frank has provided a useful tool to make it easy to assess those skills;
    all you need to do is to put in some position coordinates and GMT, and you
    can get a predicted lunar distance, to compare with the angle you measure.
    
    Well, that's fine by me. One might even think of it as a new hobby, created
    largely with Frank's guidance. As he says, "most people use lunars to test
    their sextants and their skills".
    
    But let nobody think, no matter how good those observations may be, and how
    precisely they approach the predictions, that he has got very far towards
    understanding how lunar distance NAVIGATION was done. That's a much more
    complex business, of which measuring the subtended angle between the two
    bodies is no more than a subset (though an important and demanding one).
    Yes, indeed you can call such an exercise "measuring a lunar distance";
    after all, that's literally what it is. But don't confuse it with solving
    the problem of discovering where you are when GMT is as yet unknown, which
    is what lunar distance navigation was all about..
    
    Solving that problem calls for a different approach, because when the exact
    time is as yet unknown, neither are the exact positions of the bodies in the
    sky. That's why additional observations of the altitudes of the bodies were
    called for; or another technique, that of iteration, might be possible.
    Scheduling of those observations, with respect to the lunar distances, had
    to be done with care. Calculating the result, calling for extreme precision,
    presented many pitfalls, as can be seen by the difficulties a recent example
    has presented to this list.
    
    It's also worth mentioning the additional difficulties that measurement from
    an unstable platform underfoot, in real sea conditions, presented to the
    sextant observer: difficulties that have sometimes been made light of in
    list-postings.
    
    To several list members, certainly including me, those intellectual aspects
    of lunar distance navigation are of considerable interest, and have given
    rise to much discussion. Yes, they all have a historical focus: nobody is
    measuring lunars to navigate with any more. In fact, if we admit it, hardly
    anyone is really doing celestial navigation any more. Almost all our
    discussions concern a dead subject. So what?
    
    I reckon that it's worth preserving the distinction between lunar distance
    navigation, that starts with observations and ends up with GMT and thence
    longitude, and sextant exercises, that start from GMT and predict lunar
    distance.
    
    George.
    
    contact George Huxtable at george---.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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