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    Re: Photo sextant sights
    From: Frank Reed
    Date: 2008 Aug 02, 23:43 -0400

    George H, you wrote:
    "This may perhaps suggest a possible enhancement to Frank Reed's lunar
    distance website. Instead of just multiplying the discrepancy in lunar
    distance by a constant factor of 30, to obtain longitude error, I wonder if
    it would be possible (and relatively easy) to actually compute that error,
    allowing for such geometric misalignments (and perhaps also, differences in
    the angular speed of the Moon). That would then at least draw the attention
    of a user to the fact that he was measuring a lunar distance in a silly
    direction; such a warning would have been useful here."
    
    I've thought about this issue a number of times over the past four years
    since the calculator first came online. It depends on what people are
    "really doing" when they're measuring lunar distances today. MOST people,
    that I have spoken with, are just trying it out to see what it's like and to
    experience a famous historical method of navigation. For them, the
    meaningful error in terms of longitude is the equivalent error in a typical
    lunar observation --and that's why I designed it that way. They're not
    necessarily worried about the exact longitude error in a specific case,
    though they do want the exact error in terms of the observation itself; they
    want to know, "how close was I? did I do well?". Then SOME people are using
    lunars to calibrate their sextants. For them, they want to know the exact
    error in the observation, and the 30x value is useful simply because it
    reveals an extra digit of precision in the observational error. And SOME
    people (like me) are using the calculator to analyze historical lunar
    observations, and in those cases, the longitude is adjusted via the DR
    longitude, to whatever accuracy is desired, and the "longitude error" in the
    analysis page is simply an indication of how close the DR is to the correct
    position given the observed lunar distance (under the assumption of no error
    in the observation). Finally, some people may, in the future, want to know
    the error in the corresponding "lunar LOP" and in that case a different
    measure of error is required.
    
    But I am still thinking about it, and perhaps the best option would be a
    click-through to a rather long explanation of the various possibilities
    INCLUDING the true error in longitude, calculated along the lines you've
    described, resulting from the error in observation.
    
     -FER
    
    
    
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