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Alex E, you wrote:
"Let me first cite a great authority, Lord Kelvin"

Although he was a great authority on late 19th century navigation,
Lord Kelvin was absolutely NOT an authority on lunars. The lecture
you're quoting from was delivered in 1875. This is forty or fifty
years after lunars ceased being used even as a backup measure aboard
British ships and twenty to thirty years after they ceased that role
aboard American vessels. It's clear from the lecture that Kelvin was
trying to warn young navigators not to be seduced by the stuffy old
advocates of lunars who still enforced their teaching in navigation
schools. In fact, his principal astronomical suggestion to navigators
has nothing to do with lunars: his suggestion is that they should all
be using Sumner's method instead of the methods that are usually used
aboard ship in 1875. As we've discussed previously on the list, even
decades after Sumner's method was published, the great majority of
navigators were still shooting separate sights for latitude and
longitude --they just didn't see the merit of Sumner's method over the
common meridian sights for latitude and time sights for longitude.
There's a moment in the lecture where you can almost see him standing
there: Kelvin announces that he is publishing some tables (indeed he
did) to help facilitate Sumner's method and he says "I hold in my hand
copies of these tables which are soon to be published" (or words to
that effect).

Kelvin is dismissive of lunars for the same reason that Lecky was
dismissive of them at about the same time. Everybody with common sense
knew very well that the best backup for the chronometer was another
chronometer. It was rather silly that all those poor students were
still studying lunar distance calculations so many decades after they
had fallen out of use. Kelvin is simply repeating the "common
perception" of the accuracy of lunars decades after they were commonly
used. He is not describing his own research or saying anything about
the fundamental accuracy of sextants.

Somewhere along this thread you speculated that Lord Kelvin had access
to better ephemerides for the Moon than those available in the
official almanacs. No way... Exceedingly unlikely. There were very few
people on Earth who dealt with modelling the Moon's motion, and Kelvin
would have had no reason to hunt down that research since it was
irrelevant to his practical advice to navigators. I would add that the
nautical almanacs were improved just a few short years after Kelvin's
lecture and the inaccuracy due to the almanac data went away. Lecky
notes this improvement in his book, but of course, it was too little,
too late.

-FER


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