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