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Subject: Re: [Nml] Hello (+ a few questions)
From: Thomas Schmidt (schmidt@XXX.XXX)
Date: Mon Feb 08 1999 - 09:08:23 EST
R.H. van Gent wrote:
...
> 1) I am trying to locate an obscure reference cited in Bowditch’s
> _American Practical Navigator: An Epitome of Navigation_ (p. 423, 1958
> edition; vol. II, p. 546, 1977/81 edition) to a study performed by the
> Carnegie Institute of Washington on the application of the standard
> formula for the correction for dip:
...
Dear Mr. van Gent,
I have taken the liberty of forwarding your question on HASTRO-L to this
list last Friday, and I received this reply which I'll simply repeat
here
since you've joined us now:
Richard Langley wrote:
> 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
> >
>
>
> ===============================================================================
> 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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