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    Re: Refraction at the horizon. was: Re: Celestial Navigation without a sextant.
    From: Marcel Tschudin
    Date: 2008 Mar 15, 12:09 +0200

    An additional comment regarding the sunset observations made by
    Schaefer and Liller:
    Unfortunately they don't mention in their paper the meteorological
    conditions (temperature, pressure) at the time of the observations.
    The variations of the published refraction values contain therefore
    also the variations in meteorological conditions.
    For those of you who have an interest in these observations:
    I tried to retrieve this missing information from balloon soundings
    which have been made in those days at Qunitero. (In the mean time this
    sounding site has been abandoned.) This place is near the coast about
    20 km north of Vina del Mar and about 250 km south of Cerro Tololo.
    The sounding station has the codes 85543 and SCER. The data can be
    obtained e.g. from this Web-page
    http://weather.uwyo.edu/upperair/sounding.html The sunset observations
    were made at around 22h/23h UTC; this is close to the 00Z soundings of
    the following day. The temperature and pressure values have been
    obtained by interpolation between the nearest sounding data. The
    height information in the sounding data are geopotential heights; they
    differ slightly from geometrical heights. For Vina del Mar this
    difference is negligible; for Cerro Tololo the geometrical height of
    2215m was found to correspond to 2240m geopotential height. A few
    sounding data are missing; in those cases the values were obtained by
    further interpolation between the nearest available ones. The results
    have been compiled in the attached two Excel sheets. The values for
    Vina del Mar should again be verified since I don't remember if the
    interpolation were made for the correct height.
    Navigation List archive: www.fer3.com/arc
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    File: 104691.meteo-vina-del-mar.xls
    File: 104691.meteo-cerro-tololo.xls
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