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    Re: Low Alt Refraction
    From: Marcel Tschudin
    Date: 2013 Apr 29, 15:03 +0300

    Thank you, Kermit, for providing all these different text book values
    and thank you Paul for your explanation and for the refraction formula
    from the Explanatory Supplement.
    Kermit, you wrote: "When appropriate I leave upon you the task of
    reworking these different results into the very same environmental
    reference (e.g. 1010 mb and +10°C) so that you can get a good feeling
    of the Refraction Values dispersion at 0° apparent altitude. My guess
    is that they all should match the US NA -34.5' value to within +/- 1'.
    Let me know your results."
    Yes, for what it is worth I did compile these values and converted
    them to the same condition. I added also a few more to your list. You
    find the results in the attached screen shot. The extreme values
    scatter slightly more than you were expecting. One must however be
    aware that the "real" values, those resulting from daily and even
    hourly variations may possibly lead to even larger differences.
    Just a few comments:
    The French references differ by almost 3 arcmin.
    The value from reference #2 agrees close with reference #14, which was
     obtained by ray tracing using the US Standard Atmosphere (1976)
    Reference #3: The air pressure is given. I do not know what the water
    pressure relates to.
    Reference #6: Could this have been a typo and the value been in
    degrees, still requiring to add the sun diameter?
    References #7 and #8: Bennett derived his formula from previous
    (analytical) work done by Garfinkel.
    Reference #9: This relates to Paul's comments. Because it refers to
    the Explanatory Supplement it is assumed that the value relates to the
    nautical standard conditions (1010 hPa, 10 C).
    References #10 and #11: It looks like being the same formula. There
    seems however to be a slight difference. Was #11 not copied correctly?
    (I do not have the original reference.)
    Reference #12 and #13: Relate to
    http://astro.ukho.gov.uk/data/tn/naotn63.pdf Reference#13 uses a
    simplified standard atmosphere consisting of 2 layers, the troposphere
    and the stratosphere.
    Best regards,


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