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    Re: Star-star distances for arc error
    From: Frank Reed
    Date: 2009 Jun 26, 19:31 -0700

    Gary, you asked:
    "I don't follow the discussions of lunar distance very carefully. What
    is the expected accuracy of the derived longitude in practice?"
    If you're doing lunars for their traditional purpose, namely to find GMT and 
    thence longitude (when compared against local time) then the accuracy in 
    longitude is approximately 3' of longitude (12 seconds of GMT) for ever 0.1' 
    error in the observed distance. Modern observers can get that level of 
    accuracy with well-adjusted good quality metal sextant using a 7x sextant 
    --IF they average four or more observations. Historical observations vary by 
    decade and other factors. Lunars were just about perfected right about the 
    time when they were no longer needed.
    You can also shoot lunars at KNOWN GMT to get a position fix (this was not how 
    they were used historically). This is a fix that does not require any type of 
    horizon, which is its principal advantage. In this case, you get both 
    latitude and longitude from a pair of lunars and the accuracy in each is 
    about 6 n.m. per 0.1' error in the observed distance.
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