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It seems the matter of beginner sights is being flogged to death and is
degenerating into a critique of plastic sextants generally - a subject
with which I am not overly conversant and quite frankly have no intention
of so becoming. In retrospect, I can vividly recall my first observations
at sea outside a classroom environment - they were nothing short of
disastrous, and I am sure it was a matter of months before the process
was comfortable and produced consistent results.

It is perhaps my erroneous recollection that the sextant utilized was
checked only for index error, without regard to perpendicularity of the
mirrors, line of collumation, centering error, shade errors, or perhaps
physical damage. These potential sources of error, plus observational
faults may well result in a cumulative error of considerable magnitude.
It should also be remembered that significant land mass distributions
eccentrically situated with respect to the artificial horizon, whether
liquid or leveled by liquid devices, may induce errors of observation,
although this error is generally quantified at less than 30-seconds.

There should be no problem in obtaining practical accuracy with the
marine sextant, plastic or otherwise, assuming a well adjusted
instrument, a suitable horizon (whether artificial or sea), proper
technique, correct time, and accurate data accumulation. For the beginner
it takes practice and more practice - whereafter the results will come;
there is seldom instantaneous gratification.

Henry

On Thu, 15 Sep 2005 16:25:18 +0100 george huxtable
 writes:
> Mike Hannibal wrote, about tests on certain sextants-
>
> >the outcomes were around
> >1-2 miles intercept for the C&P, 3-7 miles intercept
> >for the other metal sextants, and between 12 and 23
> >miles for the two Davis plastic sextants. Whilst there
> >may be small anomalies in my memory of the numbers I
> >think that I have been faithful in my recollection and
> >certainly the order of magnitude of the plastic
> >sextant errors is pretty right.
> >
> >Make what you will of that.
>
> =============================
>
> I think Davis have produced several widely-different grades of
> plastic
> sextant, though I am familiar with none of them. Unfortunately, Mike
> doesn't state which model was tested, to give those appalling
> results.
> Clearly, those were not proper altitude-measuring instruments at
> all, but
> toys, simulating sextants.
>
> But on the basis of tests (which I have no reason to question) on
> those two
> unnamed sextants, he tars Davis sextants in general with the same
> brush, by
> failing to specify which models were being tried. That may be fair;
> but I
> suspect it isn't.
>
> Then Mike says "make what you will of that." What one can NOT make
> from the
> evidence he quotes is the deduction that plastic sextants as a class
> are in
> general as defective as the ones in that test. And my experience
> with Ebbco
> sextants is that for those instruments, at least, the sextant
> contributes
> no more than a VERY few minutes to the errors in a celestial
> position line.
> Such observations are so imprecise anyway, when taken from a small
> craft,
> in anything but the most perfect weather, that a plastic sextant
> such as
> mine contributes little extra to the overall uncertainty.
>
> I am not claiming that such a plastic sextant as the Ebbco is as
> good as an
> expensive metal one. It would certainly not be appropriate for
> taking
> lunars; but then only an incurable optimist would expect to get
> decent
> lunars from a small boat. For anyone that has a big-ship underfoot,
> or is
> measuring from on-land, such a stable platform allows the precision
> of a
> fancy sextant to be exploited. What I will claim, though, as I have
> several
> times in the past on this list, is that a plastic instrument, of
> reasonable
> quality, is perfectly APPROPRIATE to the knockabout nature of
> measurements
> from a small craft, though the user may have to accept a bit of
> awkwardness
> in the optics and the shades.
>
> George.
> ===============================================================
> Contact George at george@huxtable.u-net.com ,or by phone +44 1865
> 820222,
> or from within UK 01865 820222.
> Or by post- George Huxtable, 1 Sandy Lane, Southmoor, Abingdon, Oxon
> OX13
> 5HX, UK.
>

   

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