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Frank Reed refers to the paper from E J White, in the Proceedings of the
Royal Society of Victoria, Vol 1 June 1889, that we've seen a year ago.

It can be found at-
http://www.clockwk.com/lunars/ejwhite

And indeed, it's a well-informed article, describing some years of
painstaking observation, and raising several interesting questions.

White refers at one point to Dudley Observatory. Is that the local
observatory in Melbourne, from where he made his observations? Does it still
exist?

He records Sun lunars, well spaced in time, by days, weeks, or months, taken
from the same spot on land, to the highest precision he can achieve, looking
for consistency and conformance with his longitude from Greenwich, known
from telegraphic determinations.

White uses a Troughton "pillar-sextant" from the early 1800s. That term has
been used elsewhere in two different contexts.

Occasionally, it's been used for a handheld sextant in which, to save
weight, the frame was made from two thin brass sheets, spaced apart by short
brass pillars, which should more properly have been called a double-frame
sextant, and usually was.

Otherwise (and this is the meaning that seems to be relevant here) it's a
sextant firmly mounted on a pillar stand, for use on land, not at sea. It is
usually arranged so that the whole sextant can be easily rotated about a
polar axis, parallel with that of the Earth, just as a telescope can. In
this way the two views from the sextant can be aimed appropriately at the
two objects (such as Sun and Moon) and then the instrument can be slowly
rotated about the polar axis to keep them in view, for as long as desired.
That allows the observer to perfect the alignment of the two bodies taking
as much time as he wishes, and then time the result, without being
constrained by the lasting-power of his arm muscles. It aso allows the
sextant itself to be larger and therefore heavier that a handheld instrument
could be. It would have been intended for measuring precise longitudes from
on land (just as Evans was doing) in exploration, and perhaps also for
measuring star-to-star distances in sky surveys. One would expect the
ultimate in precision when using it. It would, of course, have been a
Vernier instrument, not micrometer, but will have been machine-divided, to
the highest standards, for which Troughton were renowned at that time. Arc
division of sextants did not get much better over the following century and
a half.

It's also interesting to note that he was taking lunar distances that
frequently exceeded 120 degrees, the greatest being just over 130. That has
some interesting implications. First, that his sextant could cope with that
excess angle, in its length of scale and in depth of view in its index
mirror (though that wasn't uncommon). Second, that as an observer he fould
it possible to get Sun and Moon aligned in the plane of his sextant when so
far apart. That's a matter that many observers find really difficult with
large angular distances, and perhaps his pillar-mounting would help,
allowing him to take his time over the job. And thirdly, that although the
early Nautical Almanac had restricted its Sun lunar distance predictions to
120 degrees, by White's date that limit had increased to over 130.

The telescope on his sextant was mounted on a "rising-piece", to alter its
spacing from the frame, while keeping it parallel, and White explains how he
used this to equalise the brightness of the images of the two bodies, so
that he never needed to change shades.

White compares his longitudes as being only 4.8s out from those determined
by the electric telegraph. I wonder how recently the telegraph had arrived
in Melbourne, and how precisely time from Greenwich could then be determined
in that way? The responses of early telegraph receivers were slow, and no
doubt there were several relay stages along the way, with perhaps human
hands required to pass the signals on. No doubt a lot could be done by
timing round-trip loops to decide a figure for the delay. But if Greenwich
Time could be transferred to Melbourne to within decimal parts of a second,
that was pretty good going, in 1889. Not that I am questioning it, just
remarking on the achievement.

Another point that White refers to is the errors in Moon predictions in the
Nautical Almanac, which "forty years ago" he says (i.e. 1849) could amount
to 22 miles in longitude. That was a disgraceful state of affairs, little
better than Mayer's predictions of a century before. Improvements came first
in the American ephemeris, followed later by Greenwich.

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

What I would like to question, however, is Frank Reed's treatment of White's
scatter in longitudes, in writing- "If we take his lunars in sets of four
and average them (which I consider the best approach with lunars), the
results are generally within 0.1 minutes of arc. I would note that these
results are very similar to my own experience."

Just take as an example the quoted longitude errors (in seconds of time) for
the first four of the 42 observations that White records, at four
widely-spaced dates in 1887. I have added a third column in which those
longitude errors are converted, roughly, to error in lunar distance.

Aug 27,   -55 sec , -0.46'
Sept 10,  +11 sec,  +0.09'
Oct 8,     +30 sec,  +0.25'
Nov 19,  -35 sec,  -0.29'

It seems to me that these are numbers that Alex would recognise as being
very similar in scatter to those that he reports from his own balcony. But
in grouping that set of four into one, and averaging, Frank has reduced them
to a mean error of 0.1', and in doing so he has discarded the baby with the
bathwater, and the relevant information on the real scatter has been quite
lost.

As evidence, those details support Alex's view of achievable precision. Here
is a professional astronomer, using gear with which he is thoroughly
familiar, the best of its kind that existed (from the early 19th century),
correcting carefully for his index error, and even the temperature. And yet
from one observation, to the next on another day, he records differences of
a large fraction of an arc-minute. In my view, it does not support Frank's
contention, which I paraphrase, perhaps unfairly, that he can pull in anyone
from the street, present them with a sextant to throw up, to achieve lunar
distances to a tenth of a minute.

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

Of course, these matters are relevant only to the question of the precision
achievable in measuring a lunar distance from on land, in perfect conditions
with precise instrumentation and a firm footing from an already-known
position. That's a rather academic question, of interest to sextant
enthusiasts who wish to test their prowess. But it bears little relation to
the difficulties that would have been faced by a real mariner, in a rolling,
pitching, vessel, often doused with salt spray, with much of the sky
obscured by square sails, and when his corrections depended on measured
altitudes, not those deduced from a known position. We need to keep those
differences in mind.

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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