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    Re: Precision of lunars
    From: George Huxtable
    Date: 2007 Apr 28, 20:40 +0100

    Frank wrote, in Navlist 2661,
    "Although he was a great authority on late 19th century navigation,
    Lord Kelvin was absolutely NOT an authority on lunars."
    
    so I asked, in 2655-
    | "Well, how is Frank able to state that, with such certainty? Has he
    | found some error, in Kelvin's text about lunars, that the rest of us
    | have missed? "
    
    And this is Frank's reply-
    | Well, let's see... He says that lunars were the only option available
    | before 1765 --so right off the bat he's got the history wrong.
    
    What Kelvin wrote, that I assume Frank is paraphrasing, is this, on page 98-
    "...till Harrison's invention of the first useful artificial marine
    chronometer ... in 1765 ...the only chronometer generally available for
    finding longitude at sea was that great natural chronometer presented by the
    moon in her orbital motion round the earth."
    
    And I don't see anything to question in that statement, or in Frank's
    paraphrase of it either. So in what way, exactly, does Frank claim that
    Kelvin "got the history wrong"? It seems absolutely correct.
    
    And Frank goes on to deride Kelvin on the grounds that in 1895 he predicted
    that heavier-than-air machines were an impossibility. What on earth is the
    connection between that and his knowledge of lunars? Or his other
    predictions, that Frank mocks, for that matter?
    
    Frank wrote-
    | But I don't "belittle"
    | his authority on lunars.
    
    (he did, though....)
    
    | I'm saying that he, an authority on
    | navigation, was MOCKING lunars because in his era they were long over
    | and done with, and he exaggerated their inaccuracy because he didn't
    | know any better.
    
    Sometimes I think that Frank and I have read two different papers. True,
    Kelvin pointed to the weaknesses of lunar-distance navigation, in a balanced
    way. Did he "mock" lunars? No. Did he "exaggerate their inaccuracy"? Not
    that I can see. "because he didn't know any better"? What on Earth is that
    meant to imply?
    
    No end of poison in that posting, then, about Kelvin, and for what?
    
    ======================
    
    About the demise of lunars, Frank's actual words, to which I objected in
    Navlist 2665 as being an unduly  sweeping statement, were as follows-
    
    | 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.
    
    and in reply Frank quoted the following words, from Kelvin's lecture, as
    though they backed up his original statement-
    
    | Let's start with Kelvin himself. In the lecture we've been discussing,
    | Lord Kelvin wrote:
    | "Just two kinds of observations are used in astronomical navigation
    | which are shortly designated as 'altitudes' and 'lunars.' I shall say
    | nothing of lunars at present, EXCEPT THAT THEY ARE BUT RARELY USED IN
    | MODERN NAVIGATION, as their object is to determine Greenwich time, and
    | this object, except in rare cases, is nowadays more correctly attained
    | by the use of chronometers than it can be by the astronomical
    | method." [emphasis added]
    |
    | So do you hear Kelvin's words? Lunars are BUT RARELY USED in 1875
    | according to him.
    
    Well, if Frank had written that "in 1875, lunars were but rarely used", I
    would not have disagreed with that, and I doubt whether anyone else would..
    But he went much further, with his sweeping statement. In no way do Kelvin's
    words support that statement.
    
    What other evidence can Frank offer, then? His strongest suit is his study
    of vessels' logbooks held at Mystic. But we need to know a bit more. Because
    these are American vessels, we learn nothing from them about British
    practice, which concerned half of his sweeping statement. And we should
    discount from his reckoning any Navy vessels, which presumably were
    equipped, from early on, with multiple chronometers, and had no further need
    of lunars. Similarly, we should exclude the well-found clipper-ships, which
    would have been similarly supplied with nothing-but-the best, just as East
    India Company vessels were in England. And how many of the others, from
    Mystic, would have been whalers, I wonder? Many pelagic whalers wandered the
    oceans, for years at a time, without needing to bother much about
    navigation, as long as they knew which ocean they were in. In general,
    whalers were unsophisticated navigators, rough-and-ready seamen without
    mathematical skills, few of whom could have handled a lunar distance even at
    the height of the "lunar era".
    
     No, the remnants of the "lunar brigade" would have been the small trading
    vessels, making port-to-port passages under sail across the oceans. Brigs
    and schooners of 100 tons or so, parish-rigged, which still existed in
    profusion in the second half of the 19th century, scraping a living in the
    slowly-dying days of sail. Vessels which kept going by avoiding any
    unnecessary expense; when one chronometer might have been afforded, but
    backups would be out of the question. Those are the vessels where a lunarian
    might still be found on board. And those are the vessels that tended to be
    neglected by museums and by history. I wonder how many, from that category,
    are represented by logs in the Mystic collection. Perhaps there are indeed
    many, in which case Frank's evidence from those logs carries some weight.
    
    As for the British aspect of Frank's sweeping statement, he justifies it as
    follows-
    
    How about something
    | from the British side: in a letter from Lt. E.D. Ashe of the Royal
    | Navy writing to the Royal Astronomical Society in 1849, he says that
    | in "twenty years' sea time, I have not seen the chronometers checked
    | by lunar observations except once." That's very strong evidence. An
    | officer in the Royal Navy who has seen lunars used only a single time
    | in the whole period from 1829 to 1849. Before that, especially during
    | the Napoleonic wars, lunars were actively used in the Royal Navy.
    
    It isn't evidence at all. By the first quarter of the century, Royal Navy
    vessels all had a chronometer, even in home waters, and shortly after,
    multiple chronometers were made available. So of course Naval navigation had
    no further need for lunars, neither for longitude directly, nor for
    chronometer checking. But that happy state of affairs didn't apply to all
    trading vessels.
    
    George.
    
    contact George Huxtable at george---.u-net.com
    or at +44 1865 820222 (from UK, 01865 820222)
    or at 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK.
    
    
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