NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: The Noon Fix
From: James N Wilson
Date: 2009 Apr 11, 09:53 -0700
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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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