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    Re: accuracy of Cook's lunars
    From: John Huth
    Date: 2013 Jan 9, 10:19 -0500
    Alex (et. al.) - 

    Here are some answers from my student regarding her project on Cook's lunars.  

    I tend to agree with her that a lot of work on this kind of thing is one of successive approximation.   You try to fit the data, the fits raise questions, you look at the data in another way, etc etc.   (I omit my questions, but they're along the lines of what we've discussed).

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    As much as I would like to duck behind bravado, I have to say I'm pretty stumped. You've raised many good questions, and I'm not sure I have satisfactory answers. But here's a try.
     

    Most of Cook's coordinates were taken at sea, but he is careful to make distinctions. The journals are useful in that they do not merely log data, but flesh-out measurements within the context of a narrative. For example,

    Thursday 12/14 (1769): In the evening, having split the shore and mizen (?) topsails, we brought the ship under her courses; and at midnight, we wore, and stood to the southward till five in the morning ... we discovered that we had fallen much to the leeward since yesterday morning. At noon, our latitude by observation was 34 degrees 6 minutes south ... and at noon the next day we were in latitude 34 degress 10', longitude 185 degrees 45' W and by estimation about seventeen leagues from the land. 

    Being a sponsored scientific expedition, Cook and his crew made conscious efforts to be precise about their measurements. Most (though not all) longitude & latitude sightings were described in relation (or lack thereof) to land. It's for instance clear that the coordinate readings in the above excerpt were not made on land, but in the middle of sea. Others, however, were recorded and refined on land:

    Monday 12/11 (1769): Early in the morning, we stood in with the land, seven leagues to the westward of Doubless Bay, the bottom of which is not far distant from the bottom of another large bay, which the shore ... being separated only by a low neck of land, which juts out into a peninsula that I have called Knuckle Point. About the middle of this bay, which we called Sandy Bay, is a high mountain, standing upon a distant shore, to which I gave the name of Mount Camel. The latitude here is 34 degrees 51' S and longitude 186 degrees 5'.

    I also read in a secondary source that the majority of lunar readings were made by Charles Green, assistant to Astronomer Royal Nevil Maskelyne (the lunar guy himself). To be precise, he recorded hundreds of sets of lunar observations for locations to be incorporated into new maps. This would not have been possible in mid-ocean ship conditions (I think?).

    In other words, most of the time, Cook was clear about location: observations at sea meant observations at sea, those referring to capes or beaches or islands are usually accompanied by descriptions of local flora & fauna, meaning that the ship made a stop on land. Cook is also very good at being specific about the layout of the land itself: islands often have western and eastern extremities, as does beaches and even inland sea routes (e.g. entrance to Queen Charlotte's Sound). And of course, much of this coordinates were meant to be incorporated into new maps, and so maps made immediately after the expedition are good sources for location. 

    Nonetheless, the question about precision still remains: exactly how precisely can we assign modern/accurate coordinates to Cook's coordinates, so that the error calculation actually mean something. Admittedly, this problem made me break out in cold sweat throughout the project--especially when dealing with measurements based on dastardly vague "cape-this" and "cape-that". Here I assume that "cape", "bay", "point" refer to shores where Cook's ships harbored and where observations were made. I threw out locations when such "cape" "bay" or "point" referred to a wide stretch of land. For the locations I worked with,  "north" vs "south" of a "bay" or "beach" is significant only in seconds of longitude. However, because Cook's measures were almost always precise only to minutes of longitude, I decided to allow this imprecision in seconds of longitude. This means I had to round all error calculations to minutes of longitude rather than seconds (unfortunately, not sure if this is acceptable methodology).

    The problematic nature of historical data demands careful handling. My experience with Cook's journals is definitely a learning process, and frustratingly layered by multiple attempts to redo things. To be honest, the "final turned-in" product is still not good, and the data could benefit from additional cleaning and scrutiny. But I do believe that there is enough resources and primary documents out there to render this sort of error analysis meaningful. It just requires a lot of research and care. 

    On Tue, Jan 8, 2013 at 1:11 AM, Geoffrey Kolbe <geoffreykolbe---.com> wrote:
    I seem to recall that Cook's survey of the coast of the Northern Island of New Zealand was amazingly accurate in latitude, but that was offset by some 25 miles (from memory) in longitude. I recall that he landed at least once to make astronomical observations. No doubt somebody will be better informed than I on this subject..

    Geoffrey


    At 15:24 07/01/2013, you wrote:

    I have a more general question:
    What observations of Cook's expedition from known places on land
    are available?
    Except Point Venus, Thaiti, that I know.

    Alex.





       
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