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    Re: Terrestrial refraction
    From: Richard B. Langley
    Date: 1999 Feb 05, 16:03 EST

    For a good discussion of astronomical refraction, including horizontal
    refraction, see Chapter 15 of Jean Meeus's Astronomical Algorithms.  He gives
    as one reference a paper by Bennett: "The Calculation of Astronomical
    Refraction in Marine Navigation" in the Journal of the (U.K.) Institute of
    Navigation, Vol. 35, 1982, pp. 255-259.
    -- Richard Langley
       Professor of Geodesy and Precision Navigation
    On Fri, 5 Feb 1999, Tom McHugh wrote:
    >R.H. van Gent wrote: (and others as well have stated substantially the
    >same thing regarding dip)
    >>
    >> The small angle between both horizons is known as the 'dip', and can be
    >> approximated by the following relation found in almost any astronomical
    >> or navigational handbook:
    >>
    >>   dip (minutes of arc) = 0.97 sqrt(h[ft])
    >>
    >> with 'h' denoting the height of the observer?s eye above sea level in
    >> feet.
    >
    >I think, that for beginners on the list, to avoid confusion, it
    >would be well to state clearly that the above dip formula refers to
    >the sea level as being one's local horizon of reference. and relates to
    >one's
    >vertical elevation of eyepoint above sea level. It must be pointed out
    >that this formula will not be correct if one is on a horizontal plane
    >at some considerable distance above sea level where the local horizon is
    >also
    >well above sea level. Put another way, someone living in Denver or
    >other high plateau regions would have to calculate dip based upon
    >height above the local horizontal plane, not referred to sea level.
    >I am of course, referring to that portion of dip which is related to
    >atmospheric refraction. Naturally, the eye level position above the
    >horizontal
    >will be the same.
    >
    >Even at "sea level" there would be differences in dip, as it has been
    >determined that there are areas of the ocean's surface which are below
    >mean sea level because of mass concentrations within the earth's crust
    >or mantle.
    >
    >Tom McHugh
    >
    >tbmchugh{at}XXX.XXX
    >
    
    ===============================================================================
     Richard B. Langley                            E-mail: lang{at}XXX.XXX
     Geodetic Research Laboratory                  Web: http://www.unb.ca/GGE/
     Dept. of Geodesy and Geomatics Engineering    Phone:    +1 506 453-5142
     University of New Brunswick                   Fax:      +1 506 453-4943
     Fredericton, N.B., Canada  E3B 5A3
         Fredericton?  Where's that?  See: http://www.city.fredericton.nb.ca/
    ===============================================================================
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