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Dear Fred,

> My only
> quibble is that you might rephrase your conclusion to,
> "I could not
> get it to work."  Maybe a more skilled observer

Yes, of course. 4 years ago, when I joined the list, one of
the list members (I think it was George) wrote that he
challenges anyone to determine his (modern, metal)
sextant arc correction from
stars. I tried to do this. I made several hundred observations
of stars, and failed.
My MOST PRECISE observations are in fact lunars
(Sun-Moon, Venus-Moon and Jupiter-Moon).
To determine arc correction from such lunars, one has
to make observations for few weeks of good weather
as the distance from the Moon to the body changes over the whole
range of the scale.
I did it. My results are in reasonable agreement with the
arc correction determined by factory instruments (Freiberger
and Cassens-Plath).
So I can say that from such Lunars I can determine the
arc correction with 0.1-0.2 accuracy, by several months
of observations of Sun-Moon and Moon-Jupiter distances,
and a lot of averaging.

In principle ANY observation of known angles would give
you the arc correction.
I tried many other methods. But nothing else works in my
conditions. For the artificial horizon I cannot find a stable
enough platform. A dog running 100 feet away shakes
the artificial horizon! Not mentioning cars and trucks.
(Have not tried
mercury yet:-)
For the church-steeple
method discussed on this list (which sounds
very reasonable to me), I do not have enough church-steeples,
and anyway do not have a 360 degree panorama from my observation
place.

Alex.



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