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OK, here's synthetic data for George's challenge.

The sun's declination is 23S.  The red line is for observations from a vessel 
north bound at 10 knts.  The black line is for another vessel stationary at 
56N.  He sees LAN at 80 min WT.  At 20 min WT they are at the same 56N 
location.

The navigator on the moving vessel makes our linear 10-knt  correction, 
producing the adjusted data marked by the red stars.  You can see the 
excellent agreement with the data on the stationary ship in black.  Note that 
out at 140 min you just start to see a divergence between the corrected data 
and the black line.  This error becomes much larger for higher speeds and 
longer times, showing that, indeed, the correction is an approximation, 
albeit, a darn good one (for sailors).

It's always seemed to me that it's hard to determine the time of LAN much 
better than, say, five minutes -- not even one degree of longitude.  
Depending on real data scatter, maybe folding the paper like Frank says can 
improve this considerably.  I'd really be impressed with consistent 5-mile 
accuracy.

-- JK


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