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Re: Real accuracy of the method of lunar distances
From: Fred Hebard
Date: 2004 Jan 12, 16:06 -0500
From: Fred Hebard
Date: 2004 Jan 12, 16:06 -0500
Frank, You are correct in pointing out that Chauvenet did not say that "at least one of the objects would need to be a star or planet." That was my extrapolation. Yes, he was talking about eliminating errors. He also explicitly mentions averaging separately leading and tailing (east and west) sun lunars. Certainly this would not be useful at sea as they would occur at separate times of the month. He may have been alluding to Bolte's and Behrens' data sets, or similar sets of data. I believe Chauvenet mentions star-star distance measurements in Volume II in the section on sextants, and derives the recipe for calculating them, including refraction. I mentioned star-star distance measurements to John Luykx of Navtrak Nauticals (he passed away last year, was a Pres of the navigation foundation, American) as a method of determining eccentricity and other "uncorrectable" sextant errors. John said he thought it might take upwards of 500 observations or sets of observations to assemble enough data. It would be tough. I think one of those fancy 20x scopes would be useful here, to superimpose the stars accurately. But holding them in the field of view would be physically demanding. A motor-driven equatorial mount for the sextant would be very helpful! It would be interesting to see how the fiction that star-star distances do not need to be corrected for refraction worked its way into even Bowditch, I believe, as well as other modern sextant texts. The old guys knew better. Perhaps the modern guys are just thinking of using it for rough checks of sextant accuracy. Fred ------------------------------------------------------------------------ Frederick V. Hebard, PhD Email: mailto:Fred@acf.org Staff Pathologist, Meadowview Research Farms Web: http://www.acf.org American Chestnut Foundation Phone: (276) 944-4631 14005 Glenbrook Ave. Fax: (276) 944-0934 Meadowview, VA 24361 On Jan 12, 2004, at 3:32 PM, Frank Reed wrote: > Fred, you earlier wrote: > "One clear point that came out of Bolte's paper from the 1870s > referenced by Jan Kalivoda was that there's little difference in > precision between lunars taken with stars and lunars taken with the > sun. Additionally, taking two lunars, one in either direction from the > moon (and equally spaced), is more accurate than taking one. Then at > least one of the objects would need to be a star or planet. I vaguely > recall Chauvenet also making this second point. " > > In this message, if I understood you correctly, you were saying that > sights on both sides of the Moon would yield better results because > "at least one of the objects would need to be a star or planet." (this > was at the point when you were contesting my statement that Moon-Sun > lunars are easier to shoot accurately than Moon-star lunars). You > agree that that's not what Chauvenet was getting at, right? He was > talking about eliminating errors from inadequately determined index > correction and, if you're lucky enough to find objects the same > distance on either side of the Sun, eccentricity error. It's > interesting that Chauvenet comments that errors of eccentricity are > "not readily determined". Apparently, star-to-star sights did not > occur to him. > > Frank E. Reed > [X] Mystic, Connecticut > [ ] Chicago, Illinois >