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Frank quoted from my mailing-

"I know of only one successful small-boat lunar
observation that a listmember has made in ocean conditions, compared with
the many land observations that have been reported here. Neither I nor (as
far as I know) Frank has ever even attempted it. Perhaps he will tell us
about the sea-experience that he relies on, to allow us to decide whether to
take such pronouncements seriously."

That request, about the sea-experience, has not been answered.

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

I had written-

"Indeed, it's so tricky to observe useful lunars from the unstable deck of a
small craft, that it calls for sea conditions that are so infrequent as to
make the method impractical, most of the time. "

He, and I, were discussing Slocum's navigation, in Spray. That's the sort
of small craft that we were considering. However, Frank seems
to have other ideas, writing-

"PRIMARY SOURCE EVIDENCE, GEORGE. Go to the logbooks. Go to the accounts
written in the era. I've written about all of this on NavList before. For
example, there's the logbook of the schooner "Weymouth" from 1823. There's
also Crowninshield's yacht "Cleopatra's Barge" in 1817 which was visited by
that famous lunarian expert Baron von Zach who was astounded to discover
that the whole crew knew and worked lunars. Then there's the Baltimore
clipper "Erin" in 1807 which was captured and navigated by Basil Hall. Or
how about the "Hero" out of Stonington (right next door to Mystic) which was
a little sloop just over forty feet length and forty tons which was famously
captained by Nathaniel B. Palmer when he independently discovered Antarctica
in 1820. It's not hard to find historical examples of small ocean-going
vessels where lunars were used."

Spray, at 32 ft length, displaced 15 tons. I understand  that Cleopatra's
barge was something like 82 feet long, and 192 tons. Frank mentions a sloop
of 40 ft and 40 tons, but doesn't disclose the size of the other vessels he
refers to. Not very relevant, then, to the problems of taking lunars on a
craft the size of Spray, and not providing the primary evidence that Frank
claims. He will have to do better than that.

===========================
Frank wrote-
"It also explains why Victor mistakenly suggests that the circum-navigation 
was accomplished because his father used lunars. That was a huge error."

Big deal. I will quote, below, the relevant passage from "Capt. Joshua 
Slocum", by Victor Slocum, 1950, and readers can make up their own minds 
about the hugeness of the error.

Here are his full words on the topic, from page 356 onward-

"Dr David Gill, Royal Astronomer, invited the Captain next day to the famous
Cape Observatory. An hour with Dr Gill was an hour among the stars.He showed
the Captain the great astronomical clock of the observatory and he in turn 
was shown the tin clock on the Spray. They went over the subject of standard 
time at sea and how it was found from the deck of a little sloop at sea 
without the aid of a clock of any kind.

 At the time my father went to sea, lunar observation was not an unusual
method of obtaining Greenwich Time for longitude. It was very interesting as
it gave the observer a complete independence of extraneous circumstances,
like putting those cups on your ears and listening for electronic advice
from the ether -all very well as long as it works. There is still, on
cloudless days and nights, the moon going around like a hand on a clock to
give you your Greenwich Time. And a sailor who cannot make use of it is not
an altogether competent navigator, for independence is a rule of the sea.
The clock up aloft was the moon, which together with the sun and several
stars whose coordinates were tabulated, furnished the required data. Local
time was computed by the usual means of getting the hour angle.

Three observers were generally employed in a '1unar." The senior observer
took the angular distance, say, between the moon and the sun. Observers No.
1 and 2 would simultane�ously take the altitudes of the sun and the moon. To
facilitate the lunar method, her true angular distance from anyone of the
other available heavenly bodies was given in the Nautical Almanac for the
beginning of every third hour, Greenwich Mean Time; the time answering to
any intermediate angular distance being arrived at by inspection of a table
of propor�tional parts. The difference between the computed times was the
longitude required.

The degree of accuracy to be found in this method depended entirely on the
observer's skill in measuring the lunar distance with the sextant. If the
observer was sharp enough to make a contact of the two bodies coming within
hall a minute of arc of the truth, he could compute his longitude thereby
within fifteen minutes of a degree. Additional accuracy could be had,
however, by taking the mean of a series of angles, both east and west of the
moon, being sure to always use the same sextant; in this way, errors caused
by both mechanical and personal equation could be made to compensate each
other. Results warranted the continuance of this laborious method until the
present-day perfection of the chronometer had been reached, together with
its invaluable adjunct, the daily Marconi time signal.

  The Captain's astronomical conversation with Dr Gill throws a light on the
navigation of the Spray which has never been very well understood, owing
perhaps to the Captain's purposeful vagueness on this point. Even
professional navigators have taken his tin clock joke seriosly. My father
meant that he employed the same methods in navigating the sloop as he had on
all of his former vessels."

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

It's clear that here Victor, a master mariner himself, was giving his 
readers a little lesson in how lunars worked. That wasn't necessarily 
related to Slocum's voyage; for example, the reference to three observers 
couldn't apply to a single-hander. And  the "present-day perfection" that he 
describes, relates to Victor's own era, and that of his readers, and not 
Slocum's, before the days of worldwide Marconi time signals.

Remember, Slocum was competent in obtaining logitude from lunar distance, 
and indeed did so, if only once on the circumnavigation (which Frank, to his 
credit, pointed out, as he seldom ceases to remind us). Neither Victor, nor 
Frank, nor any of us, were privy to that conversation with the astronomer, 
but it's easy to imagine lunar distances being a matter of mutual interest, 
if only a historical interest. Indeed, Victor might usefully have explained 
to us the dead-reckoning on which Slocum mostly relied for his longitudes. 
But he was reporting on a conversation in an observatory, where the details 
of his towed log would not have occasioned much interest.

An undue emphasis on lunars for longitude, then? Agreed. A "huge error"? 
Come off it, Frank.

George.

contact George Huxtable, at  george@hux.me.uk
or at +44 1865 820222 (from UK, 01865 820222)
or at 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK.


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