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    Re: The Noon Fix
    From: James N Wilson
    Date: 2009 Apr 11, 09:53 -0700
    George:
     
    I think I finally understand your valid criticism. We were at cross purposes. I'm proposing this as a backup to GPS, and expect its use to be in emergency conditions. So, I tried to make it as simple as possible. To be exact, Dh should be calculated at LAN, whose exact time is unknown as yet. I calculated Dh at the time of highest altitude for simplicity. For my example the change in Dh  is quite slow. DWT is almost an hour, and the resultant difference in time is 95 seconds. That gives a change in Dh of 0.2', less than the expected sextant reading scatter.
     
    Should anyone really try to just set the sextant to the adjusted altitude, Dh can be calculated based on the best approximation of the time of highest altitude, thus theoretically achieving the same accuracy as my example.
     
    If the moon is chosen, determining Dh at two times bracketing meridian transit is required, since its declination can change quite rapidly.
     
    Again, thanks for making me think.
     
    Jim Wilson

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