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In my posting earlier today, I was somewhat dismissive about many of the 
sources of error in lunar distance, that were suggested by Kent Nordstrom in 
navilst 5380.

But I wish, now, to retract what I said about errors in interpolation at 
times between the lunar distance predictions. These predictions were 
provided in the almanac at three-hourly intervals through the day (and still 
are, in Steven Wepster's lunar website). As you might expect, the biggest 
errors can occur at times half-way between the predictions, for example 
around 1:30 am. Under some circumstances, the rate of change of lunar 
distance is altering quite rapidly, and that curvature in its graph needs to 
be allowed for, and the necessary adjustment can be substantial. This 
happens, particulaly, when the body is getting close to the Moon (less than 
30�, say) and happens to be some way off the line of the Moon's path through 
the stars.

A table is provided (in my 1864 Nautical Almanac, it's on page 496) to make 
that correction, and presumably, having made the correction, little or no 
error remans from that cause. But the correction can be important, up to 42 
seconds in the deduced time of GMT, so up to 10.5' in the resulting 
longitude, and becomes well worth making, unless one is being very selective 
in choosing which body to "shoot". I was wrong, then, to suggest that it was 
an insubstantial matter.

Although it's an easy businessto make the correction (or at least it was 
then, because it could be deduced from the "proportional log", which was 
printed with each lunar distance), it highlights an advantage of using 
modern technology, and predicting lunar distance for the moment that it's 
needed, not just at 3-hour intervals.

George.

contact George Huxtable at george@huxtable.u-net.com
or at +44 1865 820222 (from UK, 01865 820222)
or at 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK. 


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