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----- Original Message -----
From: "Frank Reed" 
To: 
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.

Marcel

   

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