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    Re: Land Surveyors vs. Navy Lunars 1884 Nile Valley
    From: Frank Reed
    Date: 2010 Apr 5, 04:24 -0700

    Antoine, in reply to a post by Robin, you wrote:
    "Here are (again) the data published by you Robin :
    Absarat, Nubia, Nile Valley, 31st December 1884
    Moon-Markab (Star alpha Pegasi) LUNAR" [etc.]

    Robin joined us at Mystic Seaport in June, 2008 which is where his interest in celestial navigation was re-invigorated (correct me if I'm wrong on that, Robin!). He will be delivering a presentation on his complex analysis techniques during the weekend of June 4-6 at Mystic Seaport this year.

    Among the books which I pointed out during one of my lunars presentations in 2008 was "Longitude by Lunar Distances: illustrated with examples worked out step by step" by H. Wilberforce Clarke, published in 1885. There's a copy in the library at Mystic Seaport. There's also a digitized copy on Google Books, link below. I have described this as a book "designed to make lunars appear as difficult as humanly possible". I wouldn't wish it on my worst enemy! Clarke seems to have adored the complexity and the absurdly minute calculational details. It should be noted that this was written long after lunars had ceased to be practical at sea, but they were still being observed by some explorers and adventurers in Africa, the Middle East, and in other parts of the world still incompletely mapped in that period. Some of the more important longitudes along the Nile during the early part of the search for its headwaters were found by lunars. You can dig up lots of this material with such simple search terms on Google Books as 'Africa "lunar observations"' with dates from 1850 to 1899. For example, there's this tidbit regarding that famous British missionary-hero, David Livingstone:
    "Livingstone was perhaps the most exact computer of astronomical observations who has yet appeared among African explorers. With reference to the instruction he had received from Captain Donaldson, he remarks in the above-quoted letter to Mr. Watt: "Tho captain of our vessel was very obliging to me, and gave me all the information respecting the use of the quadrant in his power, frequently sitting up till twelve at night for the purpose of taking lunar observations with me. The captain is of a most agreeable nature, a well-informed, shrewd Scotchman, but no Christian." "

    If you want to see the lunar distance data you were puzzling over as originally published in Clarke's book, go to the book here:
    http://books.google.com/books?id=mtoMAAAAYAAJ
    and search in it for "Absarat". This is NOT a book that will teach you anything about actual "practice" at this period of time. The data as listed does not appear to match the data you posted unless I missed something, so I suspect that Robin applied the usual "pre-clearing" steps to the observation, taking out the semi-diameters before continuing.

    Beware: Wilberforce Clarke's "Lunar Distances" book is quicksand.

    -FER


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