NavList:

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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