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Subject: Re: Winter Sextant Sight Accuracy?
From: Brian Whatcott (inet@XXX.XXX)
Date: Thu Jan 10 2002 - 20:52:03 EST
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.
/snip/
>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
>example.
/snip/
>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
extremes.
/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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