A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Sobel and Longitude. was:: Re: David Thomson and his lunar tables
From: George Huxtable
Date: 2010 Mar 18, 23:38 -0000
From: George Huxtable
Date: 2010 Mar 18, 23:38 -0000
Oh, dear... We're on to Sobel again. One of the many problems with Sobel is that, as a journalist rather than a navigator, she didn't understand what was going on, but thought she did. Which was how she came to write such stuff as (on page 5) "Precise knowledge of the hour in two different places at once - a longitude prerequisite so easily accessible today from any pair of cheap wristwatches - was utterly unattainable up to and including the era of pendulum clocks". It is of course true that the most difficult part of that problem, the knowledge of Greenwich Time, was resolved by the introduction of the chronometer, and later that job could be done by a cheap wristwatch. But she has the notion that to get the longitude you only have to compare it with local time, to be read off the other cheap wristwatch. There's no mention of any need for the important step of going out and taking an observation for local time, if the sky is clear enough to make that possible. The only words about that topic are to the effect that "the navigator resets his ship's clock to local noon when the sun reaches the highest point in the sky...", a statement that's defective on so many levels, that it shows nothing but her lack of understanding. And a similar problem besets Frank's recent posting, when he writes- "Lunars were very common in navigational practice before chronometers became widely available, but not in the way one might at first imagine. The system was not lunars in place of chronometers but rather dead reckoning, or what they called "longitude by account," supplemented by lunars. And after chronometers became common enough in the USA, the rule was longitude by chronometer supplemented by lunars, at least for a while." Even with a chronometer on board, longitude by chronometer was available only after taking an observation for local time, for an object well away from the meridian. That might be made from the Sun, at morning or afternoon, or from a star at twilight, if the the sky was clear at the auspicious moment. In between such moments, and whenever the sky was obscured by cloud (and it was unpredictable how long that might last) the ship's position was determined by dead reckoning, from the last fix. The introduction of a chronometer did not dispose of the need for dead reckoning. It was the only prime means of short-term navigation, available in all weathers, until satellites appeared. The introduction of the chronometer made big changes to the way navigation could be done, in providing a reasonably dependable source of Greenwich Time which remained good for a month or two, the duration of a voyage between sighted headlands. It wasn't a difference in principle, though, if a navigator carried some sort of pocket-watch that he could trust to a minute a day, or thereabouts. For a short time, after a lunar observation had provided a check on the time showed by the watch, that watch could be used, just like a real chronometer, to bridge over the interval to the next lunar. Lunars were unavailable over a few days around new moon, and such a technique could be used to bridge over any such gap. Frank wrote: "Navigators didn't shoot lunars every day or even close to every day." No they didn't shoot lunars every day, for one thing because lunars weren't available every day. If a navigator was competent to measure star-lunars, they were available over many more days than Sun-lunars. Nor was it necessary, even when lunars were being relied on for longitude, to take them every day, if a ship was in mid-ocean, when her longitude mattered little. But in the lunar era, a conscientious navigator would take a lunar when the opportunity arose, when the Moon was visible in a clear sky. It was part of the "professional pride" that Henty has referred to. But more rarely possible, if the navigator didn't have star-lunars in his toolbag. Frank's account seems to allow for the use of Sun lunars only. Was there really no mention at all of any star lunars? If so, it may be that the navigational standards on the vessels to whose logs Frank has had access may have been somewhat more casual. George. contact George Huxtable, at firstname.lastname@example.org or at +44 1865 820222 (from UK, 01865 820222) or at 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK. ----- Original Message ----- From: "Frank Reed"
To: Sent: Wednesday, March 17, 2010 7:13 PM Subject: [NavList] Re: David Thomson and his lunar tables John, Aboard American vessels, lunars were used for longitude right through the 1840s though almost never after that. One of my favorite examples is the gold rush ship "Sabina" in 1849. I wrote up its navigation and use of lunars for NavList back in 2005. You can read about it here: http://fer3.com/arc/m2.aspx?i=024812. Lunars were very common in navigational practice before chronometers became widely available, but not in the way one might at first imagine. The system was not lunars in place of chronometers but rather dead reckoning, or what they called "longitude by account," supplemented by lunars. And after chronometers became common enough in the USA, the rule was longitude by chronometer supplemented by lunars, at least for a while. Lunars were never used alone, which was probably the vision that many of the nautical astronomers had for them back in the 18th century (and sometimes the impression that people get today when they hear about them). Navigators didn't shoot lunars every day or even close to every day. The rough pattern was to shoot lunars for a few days around First Quarter and a few days around Last Quarter. That check on the longitude every fortnight was a nice way of locking out the slowly accumulating uncertainty in a longitude by dead reckoning or a longitude by a potentially cranky chronometer. As for Sobel's book, it definitely oversells the immediate victory of the chronometer and converts Maskelyne into more of a villain than he probably was (though there were plenty at the time who thought him a bit of a villain, too). It took decades, in fact nearly three-quarters of a century, to build up the little industry of artisans that could manufacture chronometers reliably and in sufficient quantities to bring the price down and satisfy the market. The version of history told in Sobel's "Longitude" was the standard version of the story among most people who told the history of navigation in the latter half of the twentieth century. She didn't invent it. Certainly the image of a "lone genius" beating the system resonates... even if it didn't quite happen that way. Apart from this fairly significant flaw, I think there's an enormous amount of value and some very fine prose in Sobel's "Longitude". For an alternate opinion, on the NavList main page, search for the phrase "despicable little bookling". :-) And the idea that the astronomers and mathematicians were irrationally opposed and in some cases dogmatically opposed to "mechanics" and their devices still has plenty going for it. Maskelyne and a few others just couldn't see the simplicity of a solution based on complicated machines. They didn't get it. I'm attaching a page from a logbook from 1809 with an interesting example of a lunar worked out. The text towards the lower left reads "Running for Madagascar from Cape Agulhas. this obs. agrees very nearly with acct." (in other words, the lunar longitude agrees very nearly with the dead reckoning longitude --so he's happy). At the very bottom of the page, with respect to another lunar observation, he writes "Cape Orfui bearing NW 6 Leagues which would make its long. about 51-10 --by Bowditch its long. is 51-38." The actual longitude of Cape Guardafui, the eastern tip of the Horn of Africa, is about 51-15 so he's doing pretty well with lunars! Also it's interesting to note that he is carrying a copy of Bowditch and using it as a reference for longitudes of places, but he has NOT worked his lunar observations using the procedures recommended by Bowditch back then. And I'm also attaching a photo of a set of Australian postage stamps commemorating Cook. It shows the geometry of shooting a lunar in the middle pane. I could nitpick: the details aren't quite right, but it's impressive to see something navigational on a stamp anyway. -FER ---------------------------------------------------------------- NavList message boards and member settings: www.fer3.com/NavList Members may optionally receive posts by email. To cancel email delivery, send a message to NoMail[at]fer3.com ----------------------------------------------------------------