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Hi George, you wrote:
"Nevertheless, his observation arrived at a cleared lunar distance that

differed by 1.8' from the predicted value. What surprised my was that
the lunar calculator then deduced a longitude error of 52.6', roughly
30 times greater; just the sort of number one would expect if the
skew-angle of the observation had been small.
So I wondered whether a skew-angle between the sight direction and the
Moon's travel is taken into account at all, when assessing longitude
error. "

No. It's come up before on the list, and a few minutes of
experimentation would verify the fact. That "longitude error" is
strictly 30x the error in the observed distance, and that's all it is.
It's just letting the user know the approximate longitude error (within
10% in most cases). Most people who shoot lunars today want to know one
number first and foremost: how far off was my observation? That's the
1.8 arcminute error in the observation you described. This, by the way,
is pretty darn awful, but with instruments that are either not
well-adjusted or simply have significant arc error, it's not uncommon.

I have considered various enhancements for those tools on my web site,
including an exact longitude error, but every time I look at the work
involved, I end up shelving the project. There is very little interest
today in "nautical astronomy" or "traditional marine navigation"
(celestial navigation is already losing its meaning as a phrase), let
alone navigation by lunar distances. It's only a small exaggeration to
say that it's "just us".

-FER
www.HistoricalAtlas.com/lunars
(please try not to store or reference the underlying address with
"clockwk.com"! That one may disappear any day which is why I always
include the "historicalatlas.com" reference in my signature)


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