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    Re: Precision of lunars
    From: Frank Reed CT
    Date: 2007 Apr 23, 23:27 -0700

    I wrote that Kelvin was no authority on lunars. George, you have
    "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? "
    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. He also
    says that clearing a lunar requires knowledge of the observer's
    approximate position in order to correct for parallax, which is not
    true --only the Moon's altitude is required. Although a very
    approximate position is required for the oblateness correction, that's
    such a small thing that it's really irrelevant for practical
    And you wrote:
    "But he had experience of practical navigation at sea that far
    outweighs mine, or Frank's, or that of most of us on this list."
    Quite so. He knew *practical* navigation as a scientific yachtsman of
    the year 1875, and in 1875 practical navigation did NOT include
    Of the eminently quotable Lord Kelvin, you wrote:
    "He was a man with boundless intellectual curiosity, and a thoroughly
    logical mind"
    For the most part, yes. He was also a pompous man who made some of the
    most spectacularly wrong predictions of all time. "Heavier-than-air
    machines are an impossibility" (Kelvin in 1895), "there is nothing new
    to be discovered in physics. Only refinements of measurements at the
    sixth decimal place." (Kelvin in 1900) [both of these are paraphrases,
    but fairly close, I believe, to the original quotation]. These
    comments from Kelvin are relevant ONLY to remind us that people with
    enormous expertise in certain fields are not automatically experts in
    all fields. There's no question that Lord Kelvin was an expert in many
    aspects of real, practical navigation, but this does not make him an
    expert in methods, like lunars, that were fifty years obsolete by his
    And you wrote:
    "and must have been shipmate with many seamen who had been brought up
    on lunars."
    So you think he was an expert on lunars because he MAYBE knew some old
    men who had used lunars. Now that's funny!
    And you worried:
    "So I reckon that Frank does him wrong to belittle his authority in
    that way, unless he can offer hard evidence to back it."
    I do indeed belittle his authority on flying machines. 'Hey Lord
    Kelvin, guess what the Wright Brothers did!"  But I don't "belittle"
    his authority on lunars. 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. There's no reason he would have any significant
    expertise in lunars, but that does not invalidate his reasons for
    steering navigators away from them.
    I wrote that lunars were over and done with c.1825-1830 aboard British
    vessels and c.1850-1855 aboard American vessels. George, you replied:
    "That's another "sweeping statement" from Frank. I wonder what firm
    evidence he can provide."
    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.
    But I have much better evidence than that. I have personally studied
    over a hundred logbooks in the period from 1798 to 1912 in the
    collection of Mystic Seaport and also a fraction from other museums.
    These are mostly from American commercial vessels plus some US Navy
    vessels. Lunars are actively used from the beginning of this period as
    a backup or check on the longitude by dead reckoning. Around 1835, the
    preponderance of lunars are used as a backup or check on the longitude
    by chronometer. By the end of the next decade, lunars are rarely
    mentioned in the logbooks. After 1855, I have not seen a single
    instance of lunar distances in the logbooks. This is PRIMARY SOURCE
    historical evidence. It's not the reporting of some 20th century
    author who may know nothing more about lunars than he has read in yet
    another 20th century author's books. Want more? 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.
    Let's be clear, lunars did not vanish like some ghost in the middle of
    the 19th century. There were always a handful of enthusiasts in every
    decade after 1850 right up to the present day. But those faint echoes
    are not evidence of practice; they are evidence of historical interest
    and enthusiast/hobbyist interest, much like people practicing lunars
    According to Victor Slocum:
    "On one passage, which cannot have been earlier than 1872, his
    chronometer broke down but his vessel made port on schedule. But as it
    had always been rated by lunars, the mishap made no difference as far
    as the navigation of the vessel was concerned."
    Victor was maybe 4 or 5 years old when this occurred. He is at best
    reporting an old "tale" re-told by his father.
    And you wrote:
    "Victor Slocum, himself a ship master, adds: 'In the hands of skillful
    observers and patient computers the Lunar Method is reliable to within
    a quarter of a degree longitude, which would be the distance of a high
    landfall.' "
    It is a little absurd to quote Victor Slocum on lunars. He's yet
    another author who erroneously believed that his father had circum-
    navigated the globe *actively* using lunars --that lunars explain his
    lack of a chronometer. Furthermore, his work is a SECONDARY 20th
    century source. He was writing in the 1940s. You might as well quote
    Admiral Halsey as an authority on lunars.
    And you wrote:
    "So, those are two examples to contradict Frank's opinion about the
    early demise of lunars, as a backup to chronometers."
    History isn't like mathematics. A single counter-example does not
    disprove a model since history is all about shifting trends. After
    all, if a single instance of someone shooting a lunar distance implies
    the continued use of lunar distances, then why not just point to Alex
    Eremenko shooting lunars last month? Or John Letcher shooting lunars
    in the early 1960s? Or maybe list member Henry Halboth shooting lunars
    in the 1940s? These are ALL, like the occasional, very exceptional
    cases in the latter half of the 19th century, little more than echoes
    of an obsolete historical method. They are interesting echoes, of
    course, but they do not prove the continued usage of lunars at sea.
    Another good case from the 1850s would be one Capt. Toynbee whose
    lunar distance sights were the subject of considerable discussion in
    the pages of the Monthly Notices of the Royal Astronomical Society
    around 1855. Why? Because he was such an exception that articles by
    and about a sea captain who still used lunars at that date counted as
    And you wrote:
    "Kelvin may have thought that; I just don't know. But that isn't what
    he says in his lecture, as I read it. I ask Frank to recount what
    Kelvin said, to back up that view. True, Kelvin pointed to the big
    disadvantage of lunars, their imprecision; and so he should, and so
    did every other commentator."
    Kelvin significantly exaggerated the imprecision of lunars. Why? Most
    likely because he doesn't know any better, or perhaps because he's
    being proscriptive. He knows that lunars are obviously included in the
    widely available navigation manuals, and they're taught at many
    navigation schools, so a navigator might assume that they're useful.
    But Kelvin also knows that they have essentially zero practical value
    in the modern navigation of 1875.
    And you wrote:
    "Certainly, Lecky was dismissive of lunars, but I do not see Kelvin as
    being dismissive. He simply treated lunar distance as a way of
    obtaining longitude, with its defects."
    Really? Wow. Are we reading the same lecture?? [in case anyone else
    has been following along this far and would like to read this lecture
    (it's very good in most respects), you can find it online on... drum
    roll... googlebooks, of course]
    Lord Kelvin is every bit as dismissive of lunars as Lecky, and well he
    should be. They were long over and done with at sea by 1875 --except
    in the navigation schools, and in the fantasies of land-based,
    mathematically-inclined navigational theorists.
    You say that he simply treated it as one "way of obtaining longitude".
    Ok, then why aren't they discussed in the section entitled
    "Longitude"? Instead, lunars are mentioned in a brief section at the
    end of the astronomical portion of the lecture. He wrote of a certain
    level of accuracy, "to be able to do even so much as this is an
    accomplishment which not even a good modern navigator, NOW THAT THE
    HABIT OF TAKING LUNARS IS SO MUCH LOST by the use of chronometers, can
    be expected to possess." [emphasis added]
    Do you hear Kelvin? The "habit of taking lunars is so much lost."
    Next, Kelvin presents a sketchy and somewhat inaccurate account of the
    process of clearing a lunar distance. He mentions that there are
    methods and tables for this in Inman, Norie, Raper. Then he writes,
    "The conclusion of the process of finding Greenwich time by a lunar
    observation at sea I can best explain to you by reading from the
    Nautical Almanac for 1876." [Really?? He's going to read from the
    Nautical Almanac rather than one of those excellent navigation manuals
    he's just mentioned??] And sure enough, he then reads VERBATIM the
    instructions in the almanac on interpolation including the details on
    the "proportional logarithm of difference" and even the paragraph
    introducing non-linear interpolation which was a matter of exquisite
    triviality, almost never needed in any practical situation. This part
    of working a lunar observation, the interpolation between the
    distances listed in the almanac, is actually very quick, and it's not
    difficult. So why then is he reading this technical section from the
    explanation in the back of the Nautical Almanac? And remember, this is
    in a public lecture to a mixed audience where he previously had to
    explain that a circle is divided into 360 degrees. The answer is
    simple: it's for a laugh. He's trying hard to make lunars sound like a
    lot of trouble, so he reaches for the technical manual. I've done a
    lot of public lecturing on astronomy and celestial navigation, and
    this is a well-known trick. Reading from a tech manual is a sure way
    to make any topic sound painfully difficult, and it will get a laugh.
    As is to be expected with a quote intended to drive home his point,
    this is the very last thing he has to say on lunars. This is indeed
    dismissive and INTENTIONALLY so. He's guiding prospective navigators
    away from a method that has long since ceased to be useful in
    practical navigation.
    You tossed in this comment:
    "What Frank has provided above is the Frank Reed view of lunars,
    rather than the Kelvin view of lunars."
    Ok, George. I don't even know what that means.
    "By the way, the "best backup for a chronometer" was two extra
    chronometers, not just one. If you only had two, and they disagreed,
    you were hardly any better off, as you didn't know which to trust."
    Nonetheless, in the logbooks, there are references to navigators
    comparing two chronometers. So although your logic may not see any use
    for two, practicing navigators did. Two chronometers are better than
    one. Naturally, three is better than two, four better than three, and
    so on. As soon as chronometer became cheap enough that several did not
    seem overly expensive, lunars had no remaining purpose.
    You wrote:
    "It's clear, then, that on long slow sailing-vessel voyages, the
    chronometer was not necessarily the answer to the maiden's prayer,
    when adjusting for temperature by a different technique can have such
    a large effect on the result. At least, with lunar distances, the
    error didn't grow with time; with a chronometer, it certainly did. "
    This sort of logic is, I think, at the heart of your mistaken notions
    about the longevity of the lunar method. You've come up with --well-
    thought out-- theoretical reasons why navigators SHOULD have continued
    to use lunars, so you then conclude that they DID continue to use
    lunars. That's not history.
    You added:
    "So I suggest that a large influence on the replacement of lunars by
    chronometers was the introduction of steam. Voyages were then got over
    in a shorter time"
    Lecky says the same thing (maybe that's where you heard it?). It's not
    unreasonable to suggest this as part of the solution, but I would say
    that the decline in the cost of chronometers is sufficient explanation
    all by itself, and it fits with the two decade delay on American
    I reiterate: with very rare exceptions, lunars were long gone on
    British vessels by the time Lord Kelvin was lecturing. It's worth
    noting that there were British colonial adventurers in Africa and
    elsewhere using lunar distances on land decades after they had ceased
    being used commonly at sea.
    42.0N 87.7W, or 41.4N 72.1W.

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