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    Re: Precision of lunars
    From: Frank Reed CT
    Date: 2007 Apr 21, 22:47 -0700

    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.
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