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    Re: Calibrating a sextant scale
    From: Alexandre Eremenko
    Date: 2007 Nov 21, 13:52 -0800

    Dear Fred,
    On Nov 21, 1:33 pm, Fred Hebard  wrote:
    > Nice post Alex
    Thank you. It is encouraging to know that someone reads
    my postings. The "ultimate accuracy" of sextant observations
    to be too exotic subject, even on this list:-)
    I can share some experience.
    For several years, I had systematic
    overshots with my sextant (roughly by 0.3').
    I thought a lot about this, I had my arc
    checked by Freiberger and Cassens-Plath,
    I tried to investigate all
    imaginable causes of this error, I complaned to the list:-)
    Finally I concluded that this is either
    some misterious "personal error",
    or even more misterious "irradiation".
    After making several 100-s of shots
    I learned JUST HOW should the picture in the telescope look,
    so that
    there is no error.
    So now I can measure very precisely Sun- and Venus/Jupiter-
    Moon distances.
    What I wanted to comment on is something about Sun-Moon case.
    It matters very much how CONTRAST the Moon
    is against the sky. The best
    results are obtained when the sky is deep blue,
    and the moon is bright and
    slightly yellow. Normally it is not so.
    The Moon is usually white and the sky
    is sort of white-blue.
    Under these (normal) nonditions, what I think is the
    Moon's edge is not really the edge.
    When I really touch the edges of
    the sun and the Moon, the Sun's edge near the touching point
    looks distorted (indented).
    And this seems to be the real source of the
    about +0.3' error.
    If I adjust the sextant angle so that the Sun
    edge is PERFECTLY ROUND,
    then a visible gap remains between the Sun and Moon edge.
    Trying to liquidate this gap, distorts the
    Sun edge image near the touching point.
    After many observations, I learned how exactly
    the gap should look under different
    sky-Moon relative brightness conditions.
    So I can obtain nearly perfect Sun-Moon
    results in two ways in two ways: by leaving this gap
    when I shoot, or
    by actual touching the limbs, disregarding the
    resulting distortion of the Sun's edge
    and then subtracting additional -0.3' from my observation.
    One can call this -0.3' whatever one wishes,
    but I am inclined to believe that this
    is not a "personal error".
    This is why I post this observation on the list:-)
    "toNavList" 62 lines, 2199 characters
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