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    Re: Refraction
    From: Frank Reed CT
    Date: 2005 Aug 16, 14:09 EDT

    Marcel, you wrote:
    "Index with "dry part"  only =1.000277  -> Refraction = 30.27 minutes
    Index with "dry part"  and "wet part" =1.000293  -> Refraction = 32.17"
    I don't think  the effect of humidity is this large. Going from one end of
    the humidity scale  to the other (20% to 85%) changes the refractivity by only
    about 0.2% so that  the change in refraction even at the horizon is less than
    0.1 minutes of arc. By  comparison, a 1 degree Celsius change in temperature
    yields 0.3% change in the  refraction. Accurate knowledge of the exact
    temperature is far more important  than the humidity.
    "At this point it really would be interesting  to know to what values Bennett
    fitted the approxiation  formula."
    That's easy. They're the values in the tables in the front of  the Nautical
    Almanac --which have been the same for decades (apart from an  absurdly trivial
    and pointless modification in 2004).
    42.0N  87.7W, or 41.4N 72.1W.

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