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    Re: Index error by a star-star distance
    From: Frank Reed
    Date: 2008 Sep 22, 23:58 -0400

    Andres, you wrote:
    "I am writing an article about �Index error by a star-star distance�." 
    
    I think a title that would make more sense (at least in English) would be 
    'sextant calibration by star-star distance' or 'arc error by star-star 
    distance'. 
    
    And you wrote:
    "Unfortunately there is not much information about this subject, or I can�t 
    found it. Anybody has any source of information; old books, papers, � ?" 
    
    Historically, have a look at Simms' book on the sextant published in 1858. A 
    link to it on google books is in the list I assembled here: 
    http://www.fer3.com/Mystic2008/navbooks1.html. 
    
    Several people have already mentioned John Karl's book. There is also John 
    Letcher's book which included a rather neat procedure for clearing star-star 
    sights. Like John Karl's tables, the method ignores aberration which limits 
    the method to an accuracy of about 40 arcseconds at best. This is easy 
    enough to fix. If you tabulate any distances, you need to include monthly 
    values. There have also been plenty of discussions of this on the list in 
    years past. Go to the archive and search on "star-star" and maybe also 
    "star-to-star" and "interstellar". 
    
    BUT NOTE THIS: most of the people that I have discussed this with over the 
    years get disappointing results from star-star distances, usually two or 
    three times worse than they get from lunar distances. So why not just use 
    lunar distances?? Several people, including me, believe that star-star 
    distances suffer from the distorted diffraction patterns around star images. 
    Stars never look like little dots with well-defined centers. Instead they 
    are spikey things that in many ways resemble a child's drawing of a star. 
    And each person, depending on the imperfections of their eyes, sees a 
    different pattern of spikes in the asymmetrical image of the star. 
    
    Incidentally, I will make a little pre-announcement here. I have spent the 
    past two months perfecting a table-top method of calibrating a sextant arc 
    to determine arc error. It's an optical technique, which I do not intend to 
    describe in further detail, at least not yet, that allows me to measure arc 
    error to a precision of +/-2 arcseconds at every ten degrees, or any other 
    convenient step, from 0 to 120 degrees (at 0, arc error is identical to 
    index error). It takes about an hour to do one sextant, and it's repeatable. 
    So now all I need to do is build that time machine and set up "Frank's 
    Sextant Calibration Shoppe". 
    
     -FER 
    
    
    
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