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    Mercator Sailing
    From: Bill Murdoch
    Date: 1998 Jun 08, 6:22 PM

    The best source I have found for information on calculating sailings is the
    Admiralty Manual of Navigation Volume 1 by HMSO.  It does a better job than
    Bowditch in explaining the methods.  It contains formulas for great circle
    sailing on a sphere and on a spheroid and for mercator sailing on a sphere and
    a spheroid.  Bowditch does its great circle sailing on a sphere and its
    mercator on a spheroid.
    
    After reading the Admiralty Manual of Navigation, I have laughed few times
    over articles (and a USPS JN example) which compared a distance sailed in the
    tropics calculated by great circle sailing on a sphere to the same distance
    calculated by mercator sailing on a sphereoid and pronouncing the spherical
    sailing which is smaller to be greatly better.  In reality the earth is fatter
    at the equator, a minute of arc there is more than a nautical mile, and the
    spheroidal mercator method recognizes it.  The geat circle sailing on a sphere
    assumes the earth is a sphere and that a minute of arc is always a nautical
    mile.  The mercator sailing gives a larger result primarily because it is
    sailed on a different surface not because the rumb line course is longer.  If
    both were calculated on the same surface, the results would be much closer.
    
    Of course, it makes little practical difference, but I find it interesting.
    
    Bill Murdoch
    
    
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