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    Re: Refraction
    From: George Huxtable
    Date: 2005 Aug 7, 16:52 +0100

    In reply to Marcel Tscudin, who said-
    >George wrote:
    >>My best answer to Robert's enquiry is that he is likely to see unexpected
    >>values of dip whenever there are mirage-type effects near the horizon;
    >>distant ships appear to have expanded upperworks or float above the
    >>horizon. What weather conditions give rise to such effects, I don't know.
    >>But if there are no nearby vessels to show up such effects, just a clear
    >>horizon, how can you tell?
    >This refers to terrestrial refraction.
    It referred to the dip of the horizon, which is the biggest factor to upset
    measured sextant altitudes. That was what I took Robert Eno's question to
    be about. It relates to refraction of light in its path between the horizon
    and the observer's eye.
    To be sure, refraction of the light from the object being observed in the
    sky also gives rise to observational errors, but these are, in general,
    rather less than the uncertainties in dip, as long as altitudes less than 5
    or 10 degrees are avoided. The distortions that can often be seen in the
    disc of a setting Sun are evidence of such effects occurring, and of layers
    of density-change in the atmosphere, of the type that Marcel was referring
    to. The possibility of such effects is all too real. Perhaps I was too
    ready to dismiss his contribution (copied below).
    >>Marcel Tschudin wrote-
    >>>I think you might have more success to ask this question some
    >>>As already mentioned temperature inversions are very much disturbing. A
    >>>meteorologist  may tell you all the signs of this like e.g. type of clouds
    >>>etc. May be you can start here
    >These thoughts referred to astronomical refraction
    Contact George at george@huxtable.u-net.com ,or by phone +44 1865 820222,
    or from within UK 01865 820222.
    Or by post- George Huxtable, 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13
    5HX, UK.

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