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    Re: Astronomical Refraction: Computational Method for All Zenith Angles
    From: Marcel Tschudin
    Date: 2005 Aug 18, 15:15 +0300

    Refering to
    
    > Marcel you wrote:
    > "Looking at table 2 with  the results showing the two parameters hw and
    > ho.
    > According to the text they  both indicate the height of the observer. But
    > what does then mean e.g. the  column with hw=0m and ho=2000m? "
    >
    > As stated in the article, that "hw" is  the height at which the
    > temperature
    > and pressure were "measured" (it's a  calibration altitude for the model
    > atmosphere) while h0 is the observer's  altitude. I can't think of any
    > reason in
    > your application to choose anything but  zero for hw.
    
    The text in the paper was - at least for me - misleading. "The subscript o
    denotes the values at the observer's station" and "..according to the
    observed values of the temperature, Tw, and the pressure, pw, at an altitude
    hw" can easily been understood as being what the observer observes. A
    formulation as you mentioned or as e.g. "..according to the values of a
    reference point with the temperature, Tw, and the pressure, pw, at an
    altitude hw" would have been more clear, at least to me.
    
    
    On the other hand, I also must admit that the expression "standard
    atmospheric condition" I was using in my foregoing mail is also not a clear
    formulation since different values are used for it, such as e.g. for the
    standard atmosphere T=15degC and P=1013.25hPa, for the formulae of Bennett
    and Saemundson, according to Meeus,  T=10degC and P=1010hPa and, if I
    remember right, some other authors normalise also to a standard temperature
    of T=0degC.
    
    > And you  wrote:
    > "Unfortunately the paper does not mention why this value was chosen or  to
    > what it corresponds to. "
    >
    > Ah, but that's the great thing. It's up to  you. YOU may choose any value
    > you
    > want for the index of refraction corresponding  to whatever frequency of
    > (visible) light you need for your model. If you intend  to use the
    > standard "zero"
    > points for temperature and pressure that are used in  this article, then,
    > naturally, you should choose a refraction index for zero  degrees Celsius
    > and 760
    > mm (Hg) pressure but the variation with frequency is an  input to the
    > problem. The specific value of the refraction index chosen for the  sample
    > runs in
    > the article is just an example. Notice also that this  calculational
    > approach
    > can be extended easily to atmospheres with completely  different structure
    > and
    > composition which might have radically different  refraction indices
    > (realistic
    > calculations for Mars perhaps?? the night sky as  seen from the cloudtops
    > of
    > Jupiter??).
    
    OK, my thoughts were more related to our good old planet earth, for which
    the paper shows results of sample calculations. From our atmosphere we know
    that blue and green wavelengths are, with decreasing altitude,  more and
    more absorbed, so that at low altitudes only some orange/red part of the
    light source can be seen.
    
    Refering to the rest of your mail:
    Please, Fred,  receive my sincere thankyou for what you added in your mail.
    Since I am programming at the moment in Delphi/Pascal, I will transcribe it
    into this language. The detailed comments you added will be of great help.
    Once more, thank you very much for this piece of code!
    
    Marcel
    
    
    

       
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