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    Re: sextant index error measurement
    From: Bill B
    Date: 2006 Nov 03, 22:12 -0500

    Bill wrote:
    > I would think refraction would be for all practical purposes identical for
    > both laser beams or rays from a distant celestial object.
    Peter responded:
    > The experiment you are working you way towards carrying out involves
    > observations made over a distance parallel to the ground, close to the ground.
    > Refraction will depend on your atmospheric conditions there, and could be
    > different to the refractive effect from sighting from eye height skywards.
    I agree, but I think negligible over the 2" to 3" given the
    precision/accuracy of the tools used.
    If I was unclear, I feel that the affect on two parallel beams from a laser
    2"-3" apart vertically a few feet above ground level (given a breeze) will
    not be affected by refraction to a significantly greater degree than one ray
    traveling millions or billions of miles that is reaches the scope's line of
    the sight and the index mirrors line of sight.
    To be certain, the ray(s) of a celestial body will be bent by the
    atmosphere, gravity etc.,  but that is what we have refraction tables for,
    which is a separate adjustment to the observed angle.
    I am not saying that a temperature/pressure difference large enough to
    individually affect two horizontal laser beams inches apart vertically is
    impossible, but I'm betting against it. ;-)
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