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    Re: Longitude by lunar altitudes
    From: Frank Reed
    Date: 2010 Jan 5, 22:55 -0800


    Thanks that's interesting. I didn't know it was in Thoms. Of course, history is all about exceptions. Thoms' navigation textbook was a minor one. Most American navigators in that era used Bowditch while a few used Norie or one of the other well-known English textbooks. Thoms was second-rate.

    I don't know why you're worried that you led me astray when I guessed Chauvenet. It was just a guess, and it's in there, too. :-)

    You shouldn't trust Thoms on navigational practice though you can certainly still cherish that "time capsule". His contribution was minor and I have found, often eccentric. Note right away that he is proposing all these different methods for finding GMT by the Moon from 1856 forward. But chronometers had completely taken over by that date. Very nearly no one used lunars at sea after about 1850, even on American ships where they were still fairly popular in the 1840s.

    You wrote:
    "Thoms statement of the precision of the method damns it as useless. After working that lengthy computation (see page 208), one can only expect to know one's longitude within a degree. The uncertainty of the result, when compared to the accuracy of a true lunar or longitude by chronometer would have relegated this method to the 'interesting but useless' pile nearly right away."

    Which method? His weird declination method? Yes, that would certainly be true. It's cumbersome, needlessly complicated, AND inaccurate. I would call it a "stupid human trick" (yes, it can be done, and it might be amusing, but it makes you scratch your head). So... you should then ask... why the heck did he put that in the book??

    But the statement of inaccuracy above definitely does not apply to longitude by lunar altitudes --when done correctly. And that's always the critical clause in navigation, isn't it... "when done correctly." It's easy to prove that this or that navigation method is a piece of junk by describing its behavior under circumstances where it is KNOWN to fail. But you have to assess it within the rules of the game.

    In another message, you mentioned that Thoms worked an example of longitude by lunar altitudes using the contemporary Nautical Almanac. Does that example prove to you, by its mere existence, that he did it right?? :->

    Finally, I would like to add something that I've said before on a number of occasions: the history of navigation is not equivalent to the history of navigation textbooks. Navigation textbooks were often wildly at odds with actual practice. If you want to learn about the REAL history of navigation, then look to the logbooks and the notebooks and the scrap paper, like you did early last year.


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