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A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Chauvenet
From: George Huxtable
Date: 2004 Sep 16, 21:20 +0100
From: George Huxtable
Date: 2004 Sep 16, 21:20 +0100
We were recently discussing William Chauvenet's "A Manual of Spherical and Practical Astronomy". I've just discovered an interesting 20-page pamphlet by Chauvenet, "Astronomy, comprising suggestions to U.S. naval officers, bearing on points connected with nautical astronomy, astronomical geography, and general astronomy", dated 1868. This was No. 1 of a series of "Navy scientific papers", issued by the Bureau of Navigation, Navy Department. That's a series I have not come across before, and don't know how many papers it extended to. Nor do I recall ever seeing any reference to Chauvenet's pamphlet elsewhere. It seems to have vanished from the record. It offers lots of useful and practical advice to navigators. I don't know where, or how, or even if, Chauvenet acquired his maritime experience. At that time he was Chancellor of Washington University, Saint Louis, Missouri; about as far from the sea as it's possible to get. He seems to have been an academic astronomer and mathematician: does anyone know better, about any maritime experience? There's useful stuff about meridian and ex-meridian latitude observations, about Sumner lines, about the behaviour of chronometers. He is a strong advocate of using lunars for checking chronometers, even as late as 1868, and of his own "approximate" method for clearing lunars, which he published in the American Ephemeris for 1855 (also published separately as Chauvenet: "New method of Lunar Distances"), and which he claims to be good to 2 arc-seconds, using tables to 4 figures. Presumably, this is the same (rather long-winded) method given in his Spherical and Practical Astronomy, 5th ed. 1863. He has some interesting words to offer about the state of predictions of the Moon's position. "The ephemeris of the moon has until within a few years been inaccurate. It is well known that for a number of years previous to the first publication of the American Ephemeris and Nautical Almanac in 1855, the ephemeris of the Moon in the British nautical almanac (of which the American almanacs at that time were only reprints), and in other European almacs of the same kind, was greatly in error. Foe example, the so;ar eclipse of July 28, 1851, proved that the moon's right ascension as given in the British alanac was at that date in excess of 28" (in arc,) and consequently the distance of the moon's limb from any star in her path on that date, could it have been used at that time, would have been given in the almanac with an error of about the same amount. The error of the ephemeris, however, was not constant, but varied during each lunation, so that the error of the lunar distances was usually less than 20", and sometimes less than 5", but it never disappeared entirely." To me, this was something of an eye-opener. I was aware that in Cook's time and Vancouver's time, the moon's predicted position could be nearly-an-arcminute out in celestial longitude, which could put Earth-longitudes derived from lunars out by 30 arc-minutes or sometimes more. But I had thought that by the mid 1850s lunar predictions would have improved to the point at which errors in the predictions were negligible compared with the errors in the measurement. Not so, however, according to Chauvenet. Clearly, he takes some justified pride in the fact that the still-newish nation has started to be able to tell the old world a thing or two, going on to tell us that "The improvement in the lunar ephemeris, first introduced in the American Almanac, by the use of Peirce's Tables of the Moon, and subsequently in the British and other European Ephemerides, by the use of Hansen's tables, reduces the mean error of the lunar distance to about 5"." It's all interesting stuff, and, coming from Chauvenet's pen, highly authoritative. I wonder if anyone knows of other titles in that series of "Navy scientific papers"? 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. ================================================================