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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.

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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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