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    Re: Background to Discovery: Pacific Exploration from Dampier to Cook
    From: George Huxtable
    Date: 2007 Sep 21, 10:28 +0100

    Andes Ruiz wrote-
    A very interesting book is available online:
    Background to Discovery: Pacific Exploration from Dampier to Cook.
    Berkeley University of California Press,  c1990 1990
    Howse, Derek, editor.
    (print view - 147 pages)
    The chapter "V. Navigation and Astronomy in the Voyages", (14 pages), speaks
    about the use of the Lunar Distances method at the beginning of the XVIII
    I agree with Andres about that chapter. Each chapter is by a different
    expert, and that one (pages 160 to 184) is by Derek Howse, who also edited
    the volume, and really knew what he was talking about where navigation was
    concerned. Other works by Howse, worth reading, are "Greenwich Time and the
    Longitude" (National Maritime Museum, 1997), and his section "The lunar
    distance method of measuring longitude", in "The Quest for Longitude", ed.
    W. Andrewes, (Harvard 1996).
    What is especially interesting, in the chapter which Andres recommended, is
    Howse's analysis of the going of the various timepieces taken on Cook's 2nd
    and 3rd voyages, which shows us the extent to which they required correction
    from astronomical observations. I haven't seen that study published
    elsewhere. It is indeed quite eye-opening, and corrects the widely-held view
    that possession of a timepiece was the complete answer to Cook's longitude
    problems. For a short ocean crossing of a month or two, many early
    chronometers did the job well, and they were particular useful in Cook's
    local coastal surveying. Used alone, they simply were not good enough for
    protracted voyages of several years, such as Cook's voyages would be; but
    used in conjunction with occasional lunars, they were just what he needed.
    Even if a timepiece had been carefully rated to keep good time before
    departure, an error in its rate, characteristally of the order of 10 seconds
    per day or more, could and did quickly develop, as Howse shows. If this had
    remained unchecked, and uncorrected, then over a three year period the
    accumulative error in time would build up to something like three hours, or
    45 degrees of resulting error in longitude! It was essential, then,  for
    corrections to be made at frequent intervals, and Cook would pay particular
    attention to that, particularly when he halted for a spell on-land.
    Howse is not infallible about lunars, though. Starting on page 174, he
    explains the navigator' procedure when taking a lunar, continuing-
    "But it is also necessary to measure the altitudes of the two bodies as
    nearly simultaneously as possible in order to correct for parallax and
    refraction. The lunar distances predicted in the almanac assume the observer
    is at the centre of the earth. In fact the observer is some feet above the
    earth's surface, and furthermore, both bodies appear too high because of
    atmospheric refraction., the amounts depending on their respective
    altitrudes ... Both these effects had to be allowed for; indeed, "clearing
    the distance", as it was called, was the most laborious part of the whole
    In that passage, Howse is confusing lunar parallax, which is important, with
    dip of the horizon, which isn't (not for a lunar, because the horizon isn't
    involved). And true to form, by the way, Sobel copied that same error into
    her book "Longitude" (page 98 of my 1996 edition).
    Now that "Background to discovery" is so accessible, Navlist members
    interested in the history of navigation shouldn't miss chapter 5. I found
    other chapters less valuable, though chapter 3 (pages 81 to 127) about
    French voyaging and technology, was of interest
    contact George Huxtable at george---.u-net.com
    or at +44 1865 820222 (from UK, 01865 820222)
    or at 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK.
    To post to this group, send email to NavList@fer3.com
    To unsubscribe, send email to NavList-unsubscribe@fer3.com

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