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    Re: Star-star distances for arc error
    From: Gary LaPook
    Date: 2009 Jun 26, 01:43 -0700

    I don't follow the discussions of lunar distance very carefully. What
    is the expected accuracy of the derived longitude in practice?
    
    gl
    
    On Jun 25, 9:54�pm,  wrote:
    > 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
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