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    Lewis & Clark lunars and latitudes was:[NAV-L] No Lunars Era
    From: George Huxtable
    Date: 2004 Dec 7, 17:52 +0000

    Leave to one side all this discussion about L&C's knowledge of lunars. They
    couldn't even manage to measure LATITUDES!
    I've now completing quite a detailed study of their astronomical
    observations, during their early voyage to the Missouri and their winter
    stay near St Louis, up to their official departure in May 1804.
    It's already been pointed out, by Rudner and Heynau,  in "Revisiting Fort
    Mandan's Latitude" (We Proceeded On, Nov 2001) that L&C made a common error
    in all their altitudes, throughout the journey, which were taken by
    reflection in an artificial horizon.
    Instead of correcting the observation for index error, and then halving for
    the reflection, as they should, they first halved the sextant reading, then
    corrected for index error. This meant that an error of half the index
    correction would always appear in their result. It was bad enough for
    sextant altitudes, for which the index correction was more than 8
    arc-minutes (negative). For Sun meridian altitudes in summer, the reflected
    altitude was beyond the 120-deg range of a sextant, and so they had to use
    an octant in back-observation mode. For that instrument the correction was
    more than +2 deg 11', so resulting in a latitude error of half that, or
    about 66 miles!
    Another error occurred (in the early voyage, at least) when using the
    sextant, which had an inverting scope. When used with the artificial
    horizon, the observers must have lost touch with the number of inversions.
    We can deduce from the results that all such sextant observations must have
    actually been made of an upper-limb contact, but were noted in the journal
    indiscriminately, sometimes as UL, sometimes as LL. Using the octant, which
    didn't invert, L&C usually noted the limb correctly.
    As a result of these and other smaller errors, L&Cs latitudes were NEVER
    anywhere-near correct.
    Patterson's instructions, in the form of the "Astronomical Notebook",
    should take a large part of the blame. Although L&C were embarking on an
    inland journey, in which no natural horizon would be visible, Patterson
    provided no written advice whatsoever about using an artificial horizon.
    Perhaps he was himself familiar only with a marine context.
    I agree that the involvement of Jefferson, who showed a tenuous grasp of
    the principles of lunars, didn't help.
    Early attempts, particularly those by Clark, to calculate latitudes from
    those observations, were absurdly in error. I suspect that Clark was trying
    every fiddle he could devise to get good latitudes from his bad data.
    Altitudes measured from now-known locations in that early journey can now
    be checked, using the recorded sextant readings, and allowing for those
    now-known errors. In nearly all cases the observations themselves, when
    properly processed, give VERY precise latitudes. There was nothing the
    matter with the artificial-horizon altitudes themselves, particularly those
    taken by Lewis.
    My conclusion: both Lewis and Clark, whatever their qualities as explorers
    and leaders of men, were quite inept as navigators and surveyors. They were
    innocents abroad.  They badly needed a mariner in the party, or one of the
    skilled surveyors that had already explored much of Canada for the trading
    The "Astronomical Notebook", instructions by Robert Patterson, was carried
    on the voyage, and my transcription, with commentary for modern navigators
    (recently updated) is available at-
    Anyone interested in the recorded details of that part of the journey
    should consult Vol 2 of Gary Moulton's magnificent 13-vol set of "The
    Journals of the Lewis and Clark expedition", published by the University of
    Nebraska Press in 1986. I understand that now there's also a single-volume
    abridged edition, but it's unlikely to contain the necessary detail.
    contact George Huxtable by email at george@huxtable.u-net.com, by phone at
    01865 820222 (from outside UK, +44 1865 820222), or by mail at 1 Sandy
    Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK.

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