NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: Star-star distances for arc error
From: Frank Reed
Date: 2009 Jun 25, 21:54 -0700
From: Frank Reed
Date: 2009 Jun 25, 21:54 -0700
George H, you wrote: "I sympathise with his objection to the phrasing of the words he complains about." Heh. He did not object to the supposedly offensive phrase in his first reply where he quoted it back. Instead, as his errors multiplied, he rather suddenly announced that he was offended. It's a debate trick. :-) You don't recognize it?? And George, you wrote: "and quotes Dyson, in 1922 stating that mariners using lunars couldn't do better than about 20 miles. Which is in my view, is a fair assessment, but of course to measure a lunar to 20 miles or 20' at the equator requires measuring an angle to within 40 seconds, under at-sea conditions. Which already undermines Douglas' claim that one can't do better than 1', even from on land." And don't forget that an error of 20 miles included the error from the lunar distance tables in the Nautical Almanac. Since those were about 0.3' for the latter half of the period when lunars were commonly used, the implied error in the observations is even a little less. But of course, any such accuracy estimates are extremely difficult to validate since they depend so much on the particular decade and the circumstances of the observations. And Dyson simply wasn't knowledgeable on lunars. This is a "classic case" of that situation I have frequently warned about: early 20th century secondary sources on lunars cannot be trusted. Much as that 1906 "Lehrbuch" was flat-out wrong on limitations of Dunthorne's method (and indeed other aspects of lunars), anything after about 1850 is tainted by the obsolescence of the method in actual practice. -FER --~--~---------~--~----~------------~-------~--~----~ Navigation List archive: www.fer3.com/arc To post, email NavList@fer3.com To , email NavList-@fer3.com -~----------~----~----~----~------~----~------~--~---