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    Re: from a watcher
    From: Dan Allen
    Date: 2002 Oct 17, 21:25 -0700

    On Thursday, October 17, 2002, at 08:11 PM, David Weilacher wrote:
    > cos(c) = cos(a)*cos(b) + sin(a)*sin(b)*cos(ab)
    > Isn't this the flatlanders side-angle-side trig formula.
    No, it is the spherical law of cosines.  It is the most fundamental
    formula in celestial navigation.  One can use it to determine
    great circle distances and courses, do sight reduction, measure the
    length of twilight, and many other useful things.  The first chapter
    of W. M. Smart's classic "Textbook on Spherical Astronomy" is all
    about this formula and how it is derived and used.  If you understand
    this formula and how to use it, you can do much of sight reduction
    in your sleep.  It is not a difficult formula and it is worthy of
    everyone's study.
    Gee, that sounds rather pedantic, but it really is handy in any case.

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