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    Re: Sextant accuracy (was : Plumb-line horizon vs. geocentric horizon)
    From: Fred Hebard
    Date: 2005 Feb 17, 20:39 -0500

    It was on a very high-traffic list, arstechinca.com.  I've been
    wracking my brain, but nothing has come to mind yet.  I'll continue to
    ponder it, although I've been very busy the last month.  A google
    search would probably be more productive.
    The basic conclusion both with Frank's searching and on that other list
    is the same: the angular resolution of the eye is finer than the
    spacing of the rods & cones.  Frank's resolution limits also were in
    accord with those on the other list.
    In neither of them, was the question of how the eye achieves this
    resolution addressed in much depth.
    On Feb 17, 2005, at 6:35 PM, Alexandre Eremenko wrote:
    > Fred,
    > Maybe you can give a reference for this other web list?
    > The explanation you suggest coinside with my own conclusions
    > when I was trying to think about this incredible case 30 years ago.
    > Alex.
    > On Wed, 16 Feb 2005, Fred Hebard wrote:
    >> 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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