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    Re: Using star-star distances
    From: Frank Reed
    Date: 2008 Sep 23, 00:15 -0400

    Bill, you wrote:
    "He was on, I kept coming up 1' off, despite other pairings being on. We
    swapped sextants, I hit the mark with his and he was 1' off with mine.  That
    pretty much tells the tale.  Not an IC problem, not personal error, so
    definitely a scale error. "
    
    Just to emphasize, this is not bad news. It's good news. Arc error is a 100%
    correctable error, every bit as much as index error, if it has been measured
    in some way. Because arc error has been a bit of a mystery and difficult to
    assess, many navigators get the feeling that a sextant with arc error is
    "bad". But this is not necessarily the case. Any error which is repeatable
    in multiple trials is no error at all. It's just like a clock that gains
    exactly one second every day. That's a good clock. Predictable, repeatable
    errors are not a problem. Arc error is only a true source of error when the
    calibration table is wrong or when the arc error varies randomly between
    sights. In some of my recent sextant calibration tests, I've had the
    pleasure of watching, in real time, the calibration of a plastic sextant
    glide up two minutes of arc postive for a couple of minutes (of time) then
    drift in the opposite direction until it was zero and then negative by a
    couple of minutes of arc. Something like that simply cannot be corrected in
    live navigation.
    
     -FER
    
    
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