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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
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 companies. 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- www.huxtable.u-net.com/lewis01.htm 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. George. ================================================================ 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. ================================================================