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    Re: True North and Precession
    From: Jim Rives
    Date: 2021 Feb 16, 12:42 -0800

    Thank you, Bill, Ian, and Frank.   

    As soon as I started looking into precession I soon came upon a NASA website discussing Milankovitch Orbital Cycles, of which precession is just one.  I got the idea to use the navigational triangle to see if I could calculate an azimuth using the cosine formula but found I needed one piece of information that I can't find.  The idea was to establish a triangle using the north pole, the location of P0 being Beijing at 39.92N and Long of 116.4E, and a third at P1.  In 1400, looking straight north, a flag for P1 would be established on the P0 longitude at a latitude nearer the pole corresponding to the intersection of that longitude with a lattitude representing the section of a circle made by the pole over the 26,000 year precession cycle.  But.. I couldn't figure out what latitude that should be at.  In any event, having decided on an appropriate P1 in 1400, let the precession do it's thing for 600 years and call that my LHA.   600/26,000 * 360 = 5.54 degrees.   Then solve using the azimuth formula in the Law of Cosines...  

    Fortunately, Frank found the wiki info that lets me off the hook...  they meant to do it!   Which seemed so unlikely to me that I thought surely it could not be true.  But... it appears it will remain a mystery.   Bill, I'm not sure if 600 years is noticable in geologic time but worth a look... as soon as my head stops spinning.  Coordinate systems seem to be a bit beyond my reach!   

    Thanks again.


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