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    Re: Table 8, Bowditch
    From: Frank Reed
    Date: 2009 Jan 12, 23:19 -0800

    "I'm trying to understand why for calculating the approximate radar range at 
    which any given target will return an echo-it is suggested that Bowditch 
    Table 8 can be used with the distances increased by 4-5%% to allow for the 
    slight curvature of radar waves over the geometric horizon but that provision 
    is apparently not used for the visible horizon calc?"
    This table already incorporates the refraction for visible light. The 
    refraction for radio waves is greater. So you can use the visible light table 
    by applying that 4-5% correction, if I am remembering correctly.
    All of the behavior of light with respect to terrestrial refraction for dip, 
    dip short, etc., but NOT astronomical refraction, can be calculated by 
    pretending that the radius of the Earth has increased (one can show using 
    some physics that this is actually rigorously correct under certain 
    simplifying assumptions about the atmosphere). For visible light it's 
    effectively about 15-20% bigger under normal atmospheric conditions and 
    that's already built into the tables. 
    By the way, if you've worked out the distance to the horizon by geometry, and 
    it appears to match nearly enough the values in the tables, beware that it's 
    a spot where it's easy to get nautical miles and statute miles working 
    against you beause, just by chance, the 15% difference in those types of 
    miles happens to be about the same as the 15% correction for refraction of 
    light. Do the whole calculation in nautical miles, and you will find that the 
    table values do not match a purely geometric calculation (for the actual size 
    of the Earth, that is).
    Note that these phenomena depend on the density gradient of the air in the 
    lower atmosphere which itself depends primarily on the temperature gradient. 
    Depending on the weather, these things can change by 5-10% easily and 
    sometimes quite a bit more.
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