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    Re: Refraction
    From: Marcel Tschudin
    Date: 2005 Aug 16, 22:08 +0300

    ----- Original Message -----
    From: "Frank Reed" 
    Sent: Tuesday, August 16, 2005 9:09 PM
    Subject: Re: Refraction
    Refering to
    > "Index with "dry part"  only =1.000277  -> Refraction = 30.27 minutes
    > Index with "dry part"  and "wet part" =1.000293  -> Refraction = 32.17"
    Frank wrote
    > 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.
    Sorry, but I am not in a position to comment the values for the refractive
    indices. In my previous mail I indicated where they were taken from. The
    only thing I did was feeding those data in a program published by Sky &
    Telescope in 1989 which calculates the refraction by numeric integration.
    Unfortunately I do not have a copy of the article describing the program. I
    can however say that test calculations in the altitude range above 0deg
    correspond generally well with those calculated with the formula from
    Bennett when using a refractive index of 1.0003.

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