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    Re: Refraction
    From: Henry Halboth
    Date: 2005 Aug 6, 15:50 -0400

    Experiences in the Red Sea during daylight hours frequently produced
    errors in astronomical positions amounting to as much as 12-miles,
    perhaps more, due to heat induced refraction. The area was notorious for
    these conditions, necessitating reliance on evening star sights in
    establishing anywhere near accurate positions. It was never a good place
    for the unwary celestial navigator, utilizing the sea horizon,
    refinements in calculation notwithstanding. The only method generally
    practiced to averge out such errors is observations on reciprocal
    azimith, if available - at very least such observations will alert to the
    existance of the condition, if not contribute to the accuracy of position
    determined. As previously noted, sea horizon placement is opined the
    greates bugaboo of the ocean navigator.
    On Sat, 6 Aug 2005 13:17:59 -0400 Robert Eno 
    > George wrote:
    > > Refraction works in the opposite direction to geometrical dip,
    > reducing it
    > > by about 8% when seen from a boat. It can be roughly predicted,
    > for
    > > standardised atmospheric conditions. However, the lower few feet
    > of the
    > > air, being strongly influenced by the temperature of the sea
    > surface just
    > > below it, can suffer from unpredictable temperature gradients,
    > which can
    > > upset the predicted dip by several arc-minutes, on a bad day. This
    > can be
    > > the biggest source of error in marine sextant observations.
    > Robert responds:
    > Again, this gets back to a handful of questions I earlier asked but
    > to which
    > no one replied, so I will ask you directly George:
    > In your experience, are there certain weather conditions and/or
    > other
    > observable factors during which these unpredictable errors are
    > likely to
    > occur?
    > Robert

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