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    Terrestrial refraction
    From: Thomas Schmidt
    Date: 1999 Feb 05, 09:36 EST

    Hi all,
    this came up on the HASTRO-L list (history of astronomy).
    The dip is due to geometry rather than pure refraction, but what
    _is_ the effect of refraction on the constant? Does anyone know
    the source in question?
    Thanks,
      Thomas
    R.H. van Gent wrote:
    >
    > 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 refraction in the terrestrial atmosphere.
    >
    > As is well known the terrestrial atmosphere affects the paths of light
    > rays near the horizon, causing the observed horizon to be slightly lower
    > than the geometrical horizon.
    >
    > 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.
    >
    > The constant in the above relation depends on the local atmospheric
    > conditions and the quoted value applies to average weather conditions.
    > The studies by the Carnegie Institute appear to have been made to verify
    > how well this relation stands up to varying weather conditions and how
    > large the deviations can be in extreme cases.
    >
    > I would be most grateful if someone on this list can provide me with a
    > more complete reference.
    >
    > ================================================================
    > * Robert H. van Gent * Tel/Fax:  00-31-30-2720269              *
    > * Zaagmolenkade 50   * E-mail:   r.h.vangent{at}XXX.XXX       *
    > * 3515 AE Utrecht    * Home page (under construction):         *
    > * The Netherlands    *           http://www.fys.ruu.nl/~vgent/ *
    > ================================================================
    <PRE>
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      -------------------------------------------------------------------
      Thomas Schmidt                    e-mail  : schmidt{at}XXX.XXX
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