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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@XXX.XXX       *
> * 3515 AE Utrecht    * Home page (under construction):         *
> * The Netherlands    *           http://www.fys.ruu.nl/~vgent/ *
> ================================================================
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