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    Re: Precision of lunars
    From: Alexandre Eremenko
    Date: 2007 Apr 22, 13:33 -0700

    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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