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Frank,
Putting aside for a moment your speculations about Kelvin's
motivation and about his expertise,
do you object his statements about the accuracy of the lunars
that I cited?

If yes, what are your corrections to these statements,
and what evidence can you give to support them?

Alex.

On Apr 22, 1:47 am, Frank Reed  wrote:
> 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
> are your  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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