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I had written, in [5644]
 "Measuring a lunar distance is a means to an end; the end being to
establish Greenwich time"

And Frank replied-

"A small correction: measuring lunars WAS a means to an end (establishing
GMT), a long time ago. But nobody loses GMT anymore. Today most people use
lunars to test their sextants and their skills."

Well, that might make the basis of a fruitful argument.

Frank and I are discussing two quite different meanings of "lunar distance".
Frank is referring to what I would describe (and don't intend it to be taken
pejoratively) as the "sextant hobbyist's" approach. It's hard enough for
anyone, living far from the sea, to test or hone their sextant skills, for
lack of a horizon. I live about as far from the sea as it's possible to get
in Britain; in the US, there are far more, living much further inland. Lunar
distance is a measurement that anyone can make, from anywhere, whenever the
Moon shines, and provides a demanding test of (some) observational skills.
And Frank has provided a useful tool to make it easy to assess those skills;
all you need to do is to put in some position coordinates and GMT, and you
can get a predicted lunar distance, to compare with the angle you measure.

Well, that's fine by me. One might even think of it as a new hobby, created
largely with Frank's guidance. As he says, "most people use lunars to test
their sextants and their skills".

But let nobody think, no matter how good those observations may be, and how
precisely they approach the predictions, that he has got very far towards
understanding how lunar distance NAVIGATION was done. That's a much more
complex business, of which measuring the subtended angle between the two
bodies is no more than a subset (though an important and demanding one).
Yes, indeed you can call such an exercise "measuring a lunar distance";
after all, that's literally what it is. But don't confuse it with solving
the problem of discovering where you are when GMT is as yet unknown, which
is what lunar distance navigation was all about..

Solving that problem calls for a different approach, because when the exact
time is as yet unknown, neither are the exact positions of the bodies in the
sky. That's why additional observations of the altitudes of the bodies were
called for; or another technique, that of iteration, might be possible.
Scheduling of those observations, with respect to the lunar distances, had
to be done with care. Calculating the result, calling for extreme precision,
presented many pitfalls, as can be seen by the difficulties a recent example
has presented to this list.

It's also worth mentioning the additional difficulties that measurement from
an unstable platform underfoot, in real sea conditions, presented to the
sextant observer: difficulties that have sometimes been made light of in
list-postings.

To several list members, certainly including me, those intellectual aspects
of lunar distance navigation are of considerable interest, and have given
rise to much discussion. Yes, they all have a historical focus: nobody is
measuring lunars to navigate with any more. In fact, if we admit it, hardly
anyone is really doing celestial navigation any more. Almost all our
discussions concern a dead subject. So what?

I reckon that it's worth preserving the distinction between lunar distance
navigation, that starts with observations and ends up with GMT and thence
longitude, and sextant exercises, that start from GMT and predict lunar
distance.

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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