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    Re: Almost a sextant
    From: Frank Reed
    Date: 2009 Dec 4, 14:57 -0800

    Bruce, you wrote:
    "Now there is an Iphone app that uses the camera to measure angles. Match that 
    with celestial navigation apps and we would have our digital pocket sextant."
    
    But it's still an iKamal. And what I mean by that is that it is limited to the 
    angular accuracy that can be obtained with any simple "angle-filling" device 
    that is held in the hand. Expect no better than half a degree of acccuracy 
    EVEN IF the internal vertical is absolutely accurate (and it's not).
    
    However, I do know a way of using -two- iPhones to make a good sextant. You 
    see, the face of the iPhone is quite reflective :-). So we take two iPhones, 
    power them down, affix them perpendicular to a backplate, one on a rotating 
    arm, and then when held so that sunlight or starlight bounces from one iPhone 
    to the next, the angle between a direct ray from the horizon and the 
    reflected ray from the two iPhones will not change when it is rotated about 
    an axis perpendicular to the backplate. Ha! Yes, it's just a sextant with the 
    two iPhones being used as horizon and index mirrors. Until the phone can 
    capture the video and analyze the image (presently it cannot except by 
    hacking), the iKamal cannot beat the accuracy that comes from 
    double-reflection. 
    
    And also, needless to say but I will say it anyway, the accelerometers for 
    determining the vertical in these new phones (many models now have them --not 
    just the iPhone) are sensitive to observer accelerations unless some rather 
    sophisticated processing is performed to average those out.
    
    -FER
    
    
    
    -- 
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