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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@XXX.XXX
>

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 Richard B. Langley                            E-mail: lang@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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