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    Re: Principles and Being Practical
    From: Peter Fogg
    Date: 2003 Sep 7, 19:32 +1000

    ----- Original Message -----
    From: "George Huxtable"
    
    > >However, I ask Peter if he has checked out the two examples that I
    > >provided, and compared the azimuth result from Bennett's table with that
    > >given by his nav. calculator, and if not, to do so, please. And I ask him
    > >to REPORT HIS RESULTS BACK TO US. If he finds a discrepancy between the
    > >Bennett azimuth and his own calculator (which he will), I hope he will
    then
    > >tell us where he thinks the fault lies.
    > >
    > >Example 1. dec = 55deg 29', LHA = 54deg 31', alt = 61deg 31'.
    > >
    > >Example 2. dec = 55deg 31', LHA = 54deg 29', alt = 61deg 29' .
    > >
    > >I chose those examples to show up where the faults in the method are near
    > >their worst, but have no reason to think that they are in any way unique.
    
    Well of course they are unique, they have been carefully chosen to show the
    same worst case scenario. Look at how the 2 examples are almost identical,
    and in each case the minutes are the hardest to round up or down to the
    nearest whole degree. They fall, no surprise, in the very circumstance of
    body near the prime vertical (the east/west line of the observer) that leads
    to an ambiguity in the result.
    
     In the book ("The Complete On-Board Celestial Navigator" by George G
    Bennett, 2003-2007 edition, if anyone has just tuned in)
    the user is shown how to resolve this rarely occuring ambiguity, and there
    are 2 methods shown. For both the latitude of the observer is required.
    
    So what is the latitude of this (essentially the same) Huxtable example?
    Without it the ambiguity cannot be resolved. The first method is simpler
    (notice a pattern here?), the resulting azimuth to adopt then becomes
    obvious. In even more remote circumstances 'the declination has the same
    name as the latitude but is numerically smaller' there is another process to
    be followed. I can't take this example along these routes without the
    latitude. Now I have to wonder whether George Huxtable ever got this far?
    
    'and compared the azimuth result from Bennett's table with that
    > >given by his nav. calculator'
    
    Same problem of incomplete data. The nav. calculator, silly machine, is
    designed to be used in the real world. I would need the DR, time and date,
    sextant altitude, body, etc for it to do its magic.
    
    But how often will a navigator run into this particular set of dec, LHA and
    alt? And when he does the Weir diagrams are available, as is his compass, as
    is his common sense, hopefully.
    
    There has been no effort made to cover up this potential problem in the
    book; indeed it is discussed and, once again, the methods to resolve it are
    explained - with examples! Adding an additional warning note to the latest
    edition is like MacDonalds adding a warning to their cups of coffee advising
    you might burn your lips on the hot coffee.
    
    There is, perhaps, nothing new under the sun. These azimuth tables are a
    clever adaptation of an old navigator's tool, known as the Rust Diagram,
    which used a visual or graphic method for a similar result. Those old-time
    navigators liked the simplicity of the method and were prepared to trade off
    the simplicity against greater accuracy against the ambiguity occuring
    around the prime vertical.
    
    Those of you with access to Bowditch (potentially all as it is available
    online) can research it there, and also possibly elsewhere.
    
    
    

       
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