A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
From: Henry Halboth
Date: 2009 May 8, 20:52 -0700
Thanks for bringing a most interesting viewpoint on Lunars to our attention. Although again probably past the prime era of the method, it reinforces my often stated opinion that, given more time, innovation, and practice, it would have been significantly perfected in its accuracy and consistency.
I too have some difficulty in deciphering the E/W and W/E notation, but assume that it may refer to sextant position in observing, i.e., with the instrument right side up and then upside down so as to measure the distance from both directions, which was contrary to generally accepted practice at the time.
We are also not advised as to whether or not the Captain had
access to the newly calculated Lunar Tables which came out about the time of
the published observations – I know only that the American Almanac first
contained the new tables in 1855, but that does not necessarily mean that they
might not have been available elsewhere before that date. Unfortunately, both
officers allow to having discarded sights; the question here obtains to why
they were considered poor, as opposed to there just not agreeing with the
premises proposed in the article, which latter circumstance would go directly
to the matter of consistency. The chronometer error had been established at
Madras and, but for apparent heavy weather experienced in rounding the Cape of
Good Hope, could have again been established via the Lloyd’s Signal Station
then there maintained. I certainly
suspect that they had a pretty good idea of both the CE and its reliability –
sufficiently so to reject any error completely out of the ballpark, especially
as they had other chronometers for comparison. Chauvenet, at about this time, as I recall had some rather earthy remarks regarding Lunars and CE determined therefrom which, as I recall, basically advised that if the determined error agrees with that carried forward, fin, if not, forget it.
I do not question your often stated accuracy of LD observations to only within 1’ of arc when using the instruments available at inception of the method, i.e. the late 1700’s, however, by the 1850’s much improved instruments were available and accuracies to within 10” of arc have been opined by writers of that era. In my opinion, for what it may be worth, an LD error of 1” of arc, by an experienced observer using a well-calibrated modern instrument would be enormous – no so, however, to quickly state, in an altitude measured on a questionable sea horizon. My last full, single observer, LD, worked up on July 27, 2007 (Greenwich date), in a known Longitude, cleared by Borda’s Method, using 6-place Logarithms, with careful attention to the 2nd Correction, produced results to within 3-seconds of the true GMT; this result was posted on this List at the time.
--- On Fri, 5/8/09, George Huxtable <firstname.lastname@example.org> wrote:
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