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    Re: Winter Sextant Sight Accuracy?
    From: Brian Whatcott
    Date: 2002 Jan 10, 6:52 PM

    At 06:57 PM 1/10/02, George Huxtable wrote:
    >Jared Sherman asks-
    > >How tight a fix are other folks getting with a sextant in winter conditions?
    > >My last cluster of sights were within 1 mile (+-.5m) of each other, but
    > >still off by nearly two miles in absolute position.
    >Even in my latitude in the UK of 51 degrees-odd, we get a rather pale noon
    >sun at about 16 degrees altitude in midwinter. Even at that altitude, the
    >refraction correction is only 3 minute 30 sec and is rather well-known and
    >predictable. At that altitude, it's inconceivable for temperature or
    >pressure variation to be enough to account for the errors that Jared refers
    >to. The refraction is rather insensitive to overall pressure or temperature
    >variations. A change of 14 degrees Celsius in the temperature of the air
    >changes the refraction by only 0.2 minutes of arc. see Norie's tables, for
    >George Huxtable.
    Probably teaching grannie how to suck eggs, but a UK base can be
    a misleading basis for the effects of latitude on climatic temperature
    /war story follows/
         Playing with the kids many years ago, on a tobaggan down a snowy
      Montreal slope (5 degrees more southerly) with a brisk 12 kt breeze
    and an ambient of minus 40 is not the temperature George is likely
      to see often.
      That gulf stream does a lot for the NE Atlantic coast.
    Brian Whatcott
       Altus OK                      Eureka!

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