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    Re: Star-star distances for arc error
    From: Bill Morris
    Date: 2009 Jul 3, 13:36 -0700

    You wrote, in commenting on R B Gordon's paper in the Journal of the Royal 
    Institute of Navigation (1964) 17. pp 125 - 147:
    "The author describes 
    his attempts to measure INDEX ERROR using stars. He describes his miserable 
    results which he clearly cannot explain, and then from that experience he 
    asserts (without evidence; without data; much as you have done) that 
    star-star sights for arc error wouldn't work. He has ZERO data on the topic 
    of using star-star distances for arc error."
    What you say about his lack of data is fair enough, but isn't estimating index 
    error by using a star the simplest case of a star-star sight? If one accepts 
    this, may we then accept that sights using two stars are likely to have 
    similar errors? In my case, using a SNO-T sextant with x6 telescope clamped 
    atop a theodolite tripod the standard deviation of index error observations 
    using a star was 0.17 minutes (n = 30). 
    I am not a fan of Galilean 'scopes and Gordon seems to have been particularly 
    unlucky with his if he found "...a star image will superimpose  over a range 
    of 5'."
    Bill Morris
    New Zealand
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