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    Re: Table A4 + elevation?
    From: George Huxtable
    Date: 2003 May 4, 00:23 +0100

    Marc Bernstein said-
    >>The sight reduction tables for air navigation do have refraction tables. The
    >>altitudes are listed in 5000' increments up to 55,000' and there is a
    >>correction table for non-standard temperatures that goes from +47 to -95 C!
    >>For 2100' and 20C, a little interpolation shows you need to subtract about
    >>1.5' from Hs
    and I responded-
    >I don't understand this. What is Hs, in this context?
    >The amount of refraction from a celestial body varies VERY greatly with
    >its angular altitude in the sky, and so does the additional correction for
    >height-and-temperature. Marc quotes an interpolated correction of about
    >1.5', without specifying any angle for the incoming light. Such a large
    >correction could only apply to a very small apparent angle, of the order
    >of a couple of degrees, that is, a body just above the horizon, way below
    >what any self-respecting navigator would choose to observe.
    >Can Marc explain further, please?
    Marc then replied-
    >Sorry I did leave out a vital piece of information. This is for elevations
    >of 32-34 degrees in Doug's example.
    Ah! That explains it. The 1.5' that Marc is quoting is the TOTAL
    refraction. His table differs from table A4 in the almanac, which gives the
    CORRECTIONS to the refraction (a correction to a correction) caused by
    non-standard temperature and pressure.
    A refraction of 1.5' between 32 and 34 degrees, is almost exactly the same
    as the Nautical almanac gives for "standard" conditions. It quotes 1.5' as
    the refraction that applies between 32deg00' and 33deg 45'. So the table
    quoted by Marc has made no correction at all to the "standard" sea-level
    refraction, for the 2100 ft height and the 20 deg C temperature.
    contact George Huxtable by email at george@huxtable.u-net.com, by phone at
    01865 820222 (from outside UK, +44 1865 820222), or by mail at 1 Sandy
    Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK.

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