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Ken Gebhart wrote-

> I have been paying occasional attention to your posts (due to travels),
> and
> it seems to me that you are neglecting the effect of drum eccentricity.
> The
> readings of the micrometer drum itself can be in error by as much as 20 or
> 30" of arc all by itself. This is caused by any microscopic deformation of
> the drum shaft, or errors in machining (or damage to) the worm gear. This
> can be added (plus or minus) to any errors determined on the arc.  This
> will
> pretty much make a mess of any attempts to calibrate the arc unless all
> measurements are made at the same drum reading (not likely when measuring
> stellar distances).
>
> It seems to me that a way to get a handle on your own drum eccentricity
> might be to measure a moon- star distance over a period of about 2 hours.
> This should give a series of readings over a complete revolution of the
> drum, that could be plotted of graph paper to see what is happening.

And to me that seems a plausible explanation for some measurement errors. It
would result in a cyclic error repeating at one-degree intervals. Ken's
suggested test makes sense too, as the movement between Moon and star is
slow enough to explore the interval between one degree-mark and the next,
with multiple observations over a coupleof hours. Measuring altitudes would
suffer from all the problems of precise horizon observation.

Repeated star-to-star distances would also provide a good test, as the same
star-pair would always occupy roughly the same position on the drum, just
varying by a few minutes due to refraction (which needs correcting for).
Repeated tests on different nights around the same sidereal time should
always give exactly the same angle. Perhaps an even better, and simpler,
test, would be of horizontal sextant angle between two distant landmarks, as
long as the sextant can be reliably put into exactly the same location each
time.

======================

And Alex wrote, ten days ago or so-

"Disclaimer for George:
I understand that the issues discussed here are far from the
practical needs of a small-boat navigator:-) Still I think these are
proper questions for discussion on this list. My concern is whether
I can take reliable Lunars under good conditions from land to 0.2'
precision,
and whether it is possible at all, and whether it is possible with
my SNO-T."

Response from George-

Alex doesn't really need to defend himself here. I may chuckle a bit, about
such attempts at extreme precision, wondering how the experts will fare when
faced with real-life out at sea. But I don't scoff at such tests, to
determine the ultimate precision of an instrument and discover the origin of
any errors. That can only be done from on land. I agree that the questions
are proper ones, and Alex has my full permission to continue his
investigations, and indeed, for what it's worth, my blessing.

George.

contact George Huxtable at george@huxtable.u-net.com
or at +44 1865 820222 (from UK, 01865 820222)
or at 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK.

   

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