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    Re: Sextant accuracy (was : Plumb-line horizon vs. geocentric horizon)
    From: Fred Hebard
    Date: 2005 Feb 16, 15:02 -0500

    Frank,
    
    I read recently on another list that the density of photoreceptors in
    the eye is not a limiting factor in its resolution, apparently because
    the eye can move.  Unfortunately, I don't have time right not to dig
    this out and put it up for review.
    
    Fred
    
    On Feb 16, 2005, at 2:05 PM, Frank Reed wrote:
    
    >
    >
    >  Alex, you wrote earlier:
    > "However there are well documented cases of much higher resolution.
    > (One person tested in XIX century had resolution of 1",
    > but this seems to be the world record. Several people were described
    > who could see the phases of Venus with naked eye)."
    >
    > I mentioned before that I was skeptical, and it only dawned on me
    > while I was throwing out last week's back-of-the-envelope calculations
    > that one arc-second resolution is literally impossible. The
    > diffraction limit is imposed by the laws of physics and gives a lower
    > limit of 15 arcseconds under the very best of circumstances (under
    > typical circumstances, this optical limit is about 30"). If Nature
    > chose to pack more cone cells into our foveas than one for every 15"
    > of angle across the retina, they would not yield higher resolution.
    >
    > Can anyone think of a loophole I've missed here? The diffraction limit
    > has some fuzziness of definition (there's a pun in there somewhere),
    > but I don't think it's enough to help.
    >
    > -FER
    > 42.0N 87.7W, or 41.4N 72.1W.
    > www.HistoricalAtlas.com/lunars
    >
    
    
    

       
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