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Hello all,

I have been lurking for a long time here and will finally participate. I do
a fair amount of celestial nav stuff, almost all from land. Use an Astra
IIIb, pocket sextant, liquid and mirror artificial horizons and theodolites
(Wild T-2, old K&E Paragon and Kern DKM-1). At my high elevation barometric
pressure can have a significant influence on refraction but another large
influence here is deflection of the vertical. I understand this can be as
large as about 1' and where I live causes an E-W error of 29" and a north
south error of 1". I live right on the edge of the rocky mountains thus the
large deflection. As I understand it, if I did everything perfectly,
observation, refraction, parallax (if needed), augmentation (if needed),
calculations, perfect tables,... I would still be off by 29" (about 2900
feet) due to deflection of the vertical. With the Wild T-2 theodolite
(measures to 1") the readings I get are consistently about 1/2 mile in error
to the east but quite a bit better north and south. With the T-2 the big
deal is trying to get a really accurate time. There are free programs on the
internet that will allow you to calculate the deflection at any location.

Doug Stephen




----- Original Message -----
From: "Fred Hebard" 
To: 
Sent: Monday, March 17, 2003 9:12 PM
Subject: Re: Sextant Accuracy


> I don't know that I'm not too upset by recent war-precipitating events
> to respond well, but here goes.  Perhaps we can find rational refuge in
> our abstruse meanderings about an _almost_ obsolete technology.
>
> It was very gratifying to hear from our professional sea officer, Doug
> Royer.  I might imagine that my attempt to get as accurate as possible
> is related to his comments about real-world conditions.  I also wonder
> how close the winner of one of those jackpots generally was to the GPS
> position?  I recall our correspondent from the arctic saying his shots
> were within 0.2', and Bowditch saying that an experienced observer
> could shoot to 0.1'.  I doubt I'll ever get to the level of experience
> to which Bowditch refers, which I would expect comes from taking rounds
> of shots five times a day or more.
>

   

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