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The kind of accuracy that these units are spec'd to (12" or 20" of arc)
is rarely seen in practice on a rolling boat.  Errors of 1-3 miles are
common and are acceptable for many navigational situations (mid-ocean).

However, if you want to do almost surveying quality measurements, that's
where a high power scope and the best quality sextants are needed.

I continue to test my abilities by using an artificial horizon (the Davis
one)
in my backyard.  I set the sextant to an even angle, say 30 degrees or
something
near to where the sun is about to be.  I think swing the sextant until the
sun
gets right on the money, without adjusting the sextant.  The minute it is
perfect
I then glance at my GPS or accurate watch and record the time next to the
angle
which I had already written down.  I take a series of 10 of these
measurements over
as many minutes, and then I use Metcalf's nav.c to do the statistical
analysis of
my sights.

I have the artificial horizon for my Astra, but only used it once.  I prefer
the
Davis mainly because I can use it with any of my sextants and I don't need
to keep
taking on and off the scopes.

Separately I'll post cnav.c

Dan

-----Original Message-----
From  Navigation Mailing List
[mailto:NAVIGATION-L@LISTSERV.WEBKAHUNA.COM]On Behalf Of Yves Arrouye
Sent: Friday, April 20, 2001 6:41 PM
To: NAVIGATION-L@LISTSERV.WEBKAHUNA.COM
Subject: Re: Any happy/unhappy Astra IIIB owners here?


> I own quite a few metal sextants, and my measurements
> indicate that Tamaya
> and Plath are more accurate than the Astra.  I have seen used
> Tamaya Jupiter
> sextants sell for about the same as an Astra IIIb and I think
> I would prefer
> a Jupiter over an Astra given the choice, but you will be
> quite happy with a
> new Astra as well.

Celestaire's brochure gives 20" of accuracy for the Astra IIIB against 12"
for the Jupiter. I wish the Astra IIIB had a bronze arc, if that's the
secret in the reduction of precision.

I've always wondered: how accurate is a good sextant user? I mean, is 20"
going to be a limiting factor? That's an error of about 1/3 of a mile
compared to 1/5 mile for the Tamaya Jupiter, right?

YA

P.S.: Dan, while reading your thread about your sextant accuracy at
http://www.i-DEADLINK-com/lists/navigation/9903/0042.html you're mentionning Tom
Metcalf's nav.c. Where can I find that and the relevant litterature? Do I
need to get back issues of the journal of the institute for navigation?

Another question related to what you wrote there: how's the Davis artificial
horizon compared to the practice bubble horizon sold by Celestaire?

   

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