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From: George Huxtable (no email)
Date: Mon May 01 2006 - 06:32:05 EDT
This refers to my problems in trying to buy a Vernier sextant on eBay UK.
Recent correspondence about my attempted sextant purchase has shown up some confusions, between two mailing lists, which I will
tackle under another threadname "Two lists with a common topic".
I was owner of that sextant for a very short time, until the seller refunded my cash without a quibble. The reason for its rejection
was as follows. The fine arc divisions, at one-sixth of a degree, at the edge of the arc where it meets the Vernier, were entirely
missing between +5 deg and -5deg (ie off the arc), and faint and very hard to read between +5 and +10 . Elsewhere, where most
celestial measurements are made, they were fine. These are the divisions that have to be read (for coincidence) against the Vernier
scale, so are essential for any accurate work.
The coarser divisions, at 1 deg spacings, just back from the arc-edge, were also missing, between +5 and -5. What could be seen,
presumably because they had been incised a bit deeper, were a figure "0" and a corresponding dot, further back still from the
arc-edge, at the zero-point of the arc. These were so far from the edge of the Vernier that it would be difficult to estimate where
they lay, in relation to the zero-end of the Vernier, to (say) the nearest sixth of a degree.
The Vernier span (interpolating over a range of 10 arc-minutes) had the width of 20 degrees of arc, as I remember, so when measuring
small angles the lower quarter of the Vernier scale was unavailable, because there was no fine arc-scale to read it against. The
next quarter was difficult and nearly inpossible to use. When measuring angles greater than +10 deg, the whole length of the Vernier
could be used, with no problems.
So, when measuring small angles(such as a zer-check) the curious situation would arise that the Vernier was usable and could measure
precisely, the excess over a multiple of 10 arc-minutes, if that excess was greater than, say, 5 arc-minutes, but not if it was
less. But there seemed to be no way of being sure which of those fine-divisions, at 10-minute intervals, that excess would relate
to.
I considered the notion of offsetting the index error slightly, using the mirror screws, so as to put it always at the upper part of
the Vernier scale (to correspond to, say, 2 minutes off-the-arc). This presumed that once it had been established, from other
evidence, that the index-check position was correctly near to zero degrees on the scale, within 10 minutes, it would never wander so
far as to put that out, and the Vernier reading would tell me of any small changes. And then I wondered how the sextant could be
used for measuring small angles, such as the height of a lighthouse for distance-off, as sextants are occasionally required to do,
or a Moon diameter. That seemed impossible.
So it didn't take long to decide that the defects in the arc made the sextant unusable, and I was pleased to be able to back out of
the deal without hassle.
Those problems are now, for me, hypothetical, pleased to say, now that the instrument has been returned. However, Frank appears to
have up his sleeve a scheme for allowing such a defective instrument to be useful, and it would be interesting to learn how. It
eluded me.
By the way, the sextant has reappeared on eBay, at-
so you can check the information provided, for yourself. There is still no indication of any defect in the arc divisions, except
that in answer to a question, the seller has admitted that the markings around the zero point are "faint". A picture has been added,
but this shows another part of the arc, where no problem occurs. I have suggested to the seller, since, that he should include a
photo of the problem area.
There seems to be little in common between the problem that that Vernier sextant showed up, in making measurements of small angles
and index checking so impossible, and Alex's experience with a micrometer sextant, which appears to be significantly IN ERROR over a
small region of the arc.
George.
=================
contact George Huxtable at
or at +44 1865 820222 (from UK, 01865 820222)
or at 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK.
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