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    Sobel and Longitude. was:: Re: David Thomson and his lunar tables
    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  george@hux.me.uk
    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
    
    
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