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    Re: Calibrating a sextant scale
    From: Fred Hebard
    Date: 2007 Nov 21, 13:33 -0500

    I just wrote Alex off list, then decided to write on list to say:
    Nice post Alex
    On Nov 21, 2007, at 1:00 PM, Alexandre E Eremenko wrote:
    > 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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