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    Re: True North and Precession
    From: Jim Rives
    Date: 2021 Feb 18, 12:10 -0800

    Bill, I couldn't open the article you mentioned due to some privacy setting, but I believe you summarized as pointing out that in China it was common to align buildings with other, however distant they might be.  And both you and Frank had pointed me to the Wikipedia article... which I had also read... sorta.  Frank pointed out the part I'd missed...

    "This axis is not exactly aligned north–south, but is tilted by slightly more than two degrees. Researchers now believe that the axis was designed in the Yuan dynasty to be aligned with Xanadu, the other capital of their empire.[39] 

    I had been reading and fiddling around with John Karl's book "Celestial Navigation in the GPS Age" when I "flew over" the Forbidden City.  I made the assumption  that if the building was no longer aligned with North, then the earth or the coordinates must have shifted somehow.(wrong, as it turns out... a western prejudice for Cardinal Points, I suppose)  I got the confused notion that somehow the actual north pole, the actual physical spot around which the northern hemisphere turns, actually moved.  I must have imagined the wandering of the magnetic pole.  In any case... I was an accountant by trade, so what can you expect? On thinking it over, I believe the poles don't physically wander around but the axis wobbles, pointing at different points in the sky over very long time periods... right? With the information about aligning buildings I used the law of cosines and the formulas on page 36 of Mr. Karl's book and came up with the following;

    Point      Name                Lat                Long  

    AP        Forbidden City   +39.916      +116.392                                                                                                                                                                                                                GP        Xanadu              +42.450      +116.185

    Distance calculated....    152NM    Zn  356.6 

    Per GE measurement     147NM    ZN 356.3

    If you fly along that path from FB to X you will see many mountains between the coastal and high plains.  The difference between the 256.6 calculated vs my initial measurement of abt 357.5 is only .9 degrees, or 54 arc minutes.  Astounding.  And where else can you find such intriguing geographical points to use in a cel nav problem? 

    Well, this has been a very interesting little exercise.  Somehow I now know more about Beijing, Xanadu, Chinese history,Kublai Khan's intense homesickness for The Big Sky,  and fun/practical applications of spherical trig than I ever did!   Thanks for your patience, all.  It's great fun reading all the posts.                            

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