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    Re: Biruni and the radius of the Earth by dip
    From: Marcel Tschudin
    Date: 2011 Jan 5, 17:42 +0200

    Uups, I only notice now that I made here a mistake:
    > John, you wrote:
    >> Both the dip and the refraction are in the 40 arc minute region for a 500 m
    >> peak, ....
    > Could there be a misunderstanding? From a 500m high peak the horizon
    > has a geometrical dip (without refraction) of about 40 moa. The
    > refraction increases this geometrical dip by e.g. 6 moa leading to an
    > observed dip of e.g. 46 moa. In this case we deal with the
    > "terrestrial" refraction which is considerably smaller than the
    > "astronomical" refraction since in the "terrestrial" case the length
    > of the ray path through the atmosphere is much shorter.
    > Marcel
    The refraction has to be deducted from the geometrical dip. Please
    read therefore: "... leading to an observed dip of e.g. 34 moa. "

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