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    Re: refraction
    From: Marcel Tschudin
    Date: 2005 Aug 6, 16:35 +0300

    Hello Paul
    Thank you for your additional details.
    > The problem is similar to a rise/set calculation, isn't it?
    Yes, except that the reference altitude is not always the apparent horizon
    at sea level. I do however very much appreciate your coment on what else to
    consider relative to the apparent horizon at sea level. (I am actually
    verifying here my program with observed sunsets in the range of seconds.
    What you indicated is an additional correction which I have not yet
    considered in my data. Therefore: Thank you.)
    > That is, you
    > want to know the altitude the object would have if refraction were
    > turned off and you could see through the Earth. That equals dip of the
    > horizon plus the total curvature due to refraction of the light from the
    > object to the observer.
    I am interested in both, to convert
    a) physical (true) altitudes in apparent altitudes and
    b) apparent altitudes in physical (true) altitudes
    I am still confident to find one day either an approximation formula to
    calculate refraction in the altitude range from 90° down to the negative
    altitude at the apparent horizon, this for a given set of observation data,
    such as elevation (above sea level), temperature and air pressure, or, if an
    approximation formula does not exist, a programmed function, which would be
    able to calculate this. As for the moment the table from the Air Almanac is
    the only reference for negative altitudes. Thank you all for obtaining this!

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