NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: Precision of lunars
From: Frank Reed CT
Date: 2007 Apr 23, 23:27 -0700
From: Frank Reed CT
Date: 2007 Apr 23, 23:27 -0700
I wrote that Kelvin was no authority on lunars. George, you have asked: "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 navigation. 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 lunars. 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 time. 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 today. 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 *newsworthy*. 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. And: "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 vessels. 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. -FER 42.0N 87.7W, or 41.4N 72.1W. www.HistoricalAtlas.com/lunars