Henry Halboth wrote in [8202], under threadname "How many chronometers?"
this paragraph, which I have separated under a new threadname.
"I do not question your often stated accuracy of LD
observations to only
within 1' of arc when using the instruments available at inception of the
method, i.e. the late 1700's, however, by the 1850's much improved
instruments were available and accuracies to within 10" of arc have been
opined by writers of that era. In my opinion, for what it may be worth, an
LD error of 1" of arc, by an experienced observer using a well-calibrated
modern instrument would be enormous - no so, however, to quickly state, in
an altitude measured on a questionable sea horizon. My last full, single
observer, LD, worked up on July 27, 2007 (Greenwich date), in a known
Longitude, cleared by Borda's Method, using 6-place Logarithms, with careful
attention to the 2nd Correction, produced results to within 3-seconds of the
true GMT; this result was posted on this List at the time."
======================
I suspect there's a typo in there, and that Henry intended to write-
"... an
LD error of 1' of arc, by an experienced observer using a
well-calibrated modern instrument would be enormous", rather than "1" of
arc". Perhaps he will confirm that (or otherwise).
Compared with Henry's vast experience, from his many years on the bridge in
all weathers, in the era when sextant observatios were still important, my
own experience of astronavigation is puny. Mine has been done in a small
craft, and usually with a plastic sextant, and only in Summer weather; fair
weather if I could manage it. In the sort of sailing I've done, which hasn't
included crossing an ocean, I've always known by other means reasonably well
where I was, so the safety of my craft has never depended on my astro. I've
never taken a lunar distance at sea, and know of only one small-craft
navigator who has. So there's no way I would pit my experience against
Henry's.
Have I stated that lunar distances with modern instruments
can be made only
to 1' of arc? I've often used phrases such as " each 1' of arc in a lunar
distance gives rise to a corresponding error of 30' in deduced longitude",
which is a rather different matter. I'm quite happy to concede that in the
right conditions, a lunar distance can indeed be measured quite a lot more
precisely than that.
When we're discussing precision, and scatter of observations, it is, I
suggest incumbent on us to be as clear as we can about what we are talking
about. We have to use a bit of statistical language. When we discuss an
accuracy of 1' (or whatever), is that in terms of one standard deviation (in
which roughly one-third of the cases, it will be exceeded) or two standard
deviations ( which will only be exceeded on one occasion in twenty)? There's
an immense difference. And we need to be clear, too, about the conditions
we're thinking of. Is a lunar distance being taken from on-land,
as is done
by many denizens of this list, or at-sea? There's an immense difference.
From a small-craft or a big-ship? An immense difference. In a mirror-sea
calm or in rough conditions? An immense difference. By a grizzled old salt
or a beginner?
So it's hard to put an all-embracing simple number on the precision of a
lunar distance. I doubt if I've ever tried to do so, or set a limit of 1',
on this list, but could be proved wrong...
We need to consider the instruments that were being used, throughout the
lunar era, which would all be Vernier sextants, not micrometer types. I have
such a sextant, which I haven't taken to sea. Using it on land, my old eyes
have great difficulty, even with a good magnifier, perceiving which of three
adjacent lines on the Vernier makes the best fit, even with really good
illumination. Bill Morris, an expert on sextants if himself no navigator,
has noted the same
thing.
I recall Henry's 2007 posting about those observations, but no longer have
it on-board, and it could be useful if he provided another copy or a link. I
remember that it showed a remarkably consistent series of observations,
which proved his hand and eye remain as precise as ever. But as I recall,
they were taken from on land, and though others may disagree (and have) I
regard that as a particularly benign environment for the taking of lunars.
George.
contact George Huxtable, at
george@hux.me.ukor at +44 1865 820222 (from UK, 01865 820222)
or at 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK.
--~--~---------~--~----~------------~-------~--~----~
Navigation List archive: www.fer3.com/arc
To post, email NavList@fer3.com
To , email NavList-@fer3.com
-~----------~----~----~----~------~----~------~--~---
