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    Re: U.S. Standard Atmosphere Supplements - Moonrise/set
    From: Marcel Tschudin
    Date: 2005 Aug 27, 14:18 +0300

    Frank I wrote
    > So if a single day's difference in atmospheric conditions at  a site
    > chosen
    > at random can yield a six arcminute difference in refraction right  at the
    > horizon and one arcminute at half a degree altitude, why don't we notice
    > it? I
    > think the simple answer is that we never measure any astronomical  objects
    > down
    > that low except the Sun and the Moon and them very rarely. The
    > variability in
    > refraction will change the exact time of sunrise/set and  moonrise/set by
    > something like a half a minute (depending on latitude) for a six
    > arcminute
    > change in refraction. I think the time of sunset is as  variable as that
    > on a
    > regular basis.
    Some years ago I made some coarse comparisons on light (surface) densities
    of the moon and the sky at different altitudes near the horizon. The results
    indicated that due to the extinction a moonrise or moonset seems not to be
    visible at sea level. The moon disappears some where around 1 to 2 degrees
    above the visible horizon. At my present temporary domicile near the sea I
    could verify this now for the first time. This indicates that the "real"
    moonrise or moonset would not be available for navigational purposes since
    the time can not be measured. It would be of interest if someone made
    different experiences and under which conditions.

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