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    Re: Star-star distances for arc error
    From: Frank Reed
    Date: 2009 Jul 3, 17:22 -0700

    Bill Morris, you wrote:
    "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?"
    For short distances, less than five or ten degrees, sure, of course, they're 
    similar, and identical at zero degrees, but there's a substantial difference 
    with larger angles. You 'swing the arc' (as George also has noted in a couple 
    of recent messages) when you bring the two star images together, and it's 
    much easier to see the alignment in those cases.
    You wrote:
    "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). "
    Sounds quite reasonable! Just about what I would expect with a SNO-T. Have you 
    tried this unclamped? Also, what do you find when you average sets of four? 
    Does your s.d. decrease by a factor of two? And may I ask, how are you 
    "clearing" these observations (just out of curiosity)?
    You wrote:
    "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'." "
    Yeah, that's an amazingly bad result (Gordon's 5' range, that is). It could 
    either be an unusually bad telescope or possibly that issue of dark 
    adaptation that Bill B. and I have experimented with. It turns out that many 
    people see bright stars as distorted, flared "blobs" when their eyes have 
    dark-adapted but the images are much sharper when we first step into the 
    dark. Traditionally, most navigators have considered full dark-adaptation a 
    pre-requisite for star sights in order to see the horizon clearly, but that 
    may work against star-star sights. Also, Gordon's daylight observations of 
    altitudes are much better than this 5' error range he finds with stars. 
    Something is clearly amiss.
    I don't think we should worry too much about Gordon's results way back in the 
    early 1960s. He was at that time a young, newly-graduated engineering student 
    with a passion for boating and an enthusiasm for celestial navigation. His 
    results are what they are: the observations and ruminations of one individual 
    using (mostly) one sextant. There are some nice points in his paper, but also 
    some very weak points.
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