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    Re: Using star-star distances
    From: Bill B
    Date: 2008 Sep 26, 00:02 -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. "
    
    Frank replied:
    > 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.
    
    "Any error which is repeatable in multiple trials is no error at all."
    
    It is an election year.  What are you running for?   I understand your
    calibration argument, but the verbiage tickles my sense of the ridiculous.
    How can a "repeatable error" be "no error?" "No" and "repeatable" are
    modifiers of "error." Semantics. If a calibrated 1 gram weight consistently
    measures 1.5 grams on scale, there is an error, one way or the other. The
    difference can be compensated for if you trust the standard.
    
    The "bad news" and frustration is if you have checked mirror alignment,
    eccentricity of the micrometer drum etc. and the advertising or
    certification sheet claims accuracy within plus/minus 18" along the arc, and
    the observations are 1!0 off at some point.  It can be compensated for, but
    none-the-less a bummer.  Especially--as from what I can conclude from the
    list archives and what I have seen--there is no pattern to the "blips."  88d
    OK, 90d off by 1', 92d OK.  Testing at 10d intervals is interesting, but if
    the next tooth in the gear is flawed the testing is all but useless for
    establishing a pattern of "no" error.
    
    Good to see you posting again. After spending time in the halls of Purdue's
    Math/Science Bldg. with Alex playing with your laser-calibration method I
    look forward to the posting of your latest invention for calibration.
    
    Bill B.
    
    
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