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    Re: Refraction near the Horizon ? Ob servation vs. Calculation
    From: Marcel Tschudin
    Date: 2013 Apr 6, 21:43 +0300

    Hi Brad,
    All your mentioned equations are approximations. It really depends on
    the accuracy one wants to achieve and whether this accuracy is really
    necessary or not. If one wants really to be exact, it is in this
    context actually not the radius of the earth which is of interest,
    it's rather the earth's radius of curvature at the location of the
    observer. Because the earth's shape is approximately an ellipsoid this
    radius of curvature depends on latitude and also on the azimuth,
    except at the poles. If you do not require this accuracy you can use
    any other less accurate mean earth radius which in the context of
    calculating dip are likely to be completely sufficient.
    Regarding the pressure and temperature dependency of refraction here
    some further explanations: The scaling factor (P/1010)/(T/283) adjusts
    the refactivity property of air, n, and as a consequence of it also
    the refraction resulting from it. However, the cause for light rays
    being bend in the atmosphere are temperature-GRADIENTS. The parameter
    k is introduced to describe in a simplified model the bended ray for
    terrestrial refraction and dip. From observations one obtained then
    typical values for it.

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