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    Re: Sextant accuracy (was : Plumb-line horizon vs. geocentric horizon)
    From: Frank Reed CT
    Date: 2005 Feb 19, 19:15 EST
    Alex E wrote:
    "But my impression
    is that the resolution of 1" does not contradict any laws of physics."
    For imaging resolution, it does.

    "Besides, I am sure that all this was carefully studied by
    researchers in human vision, and finding it out only
    requires some search."
    Right. And apparently the standard *imaging* resolution of the human eye is indeed diffraction limited at about 30 arcseconds under ideal conditions. The array of cone cells in the fovea of the retina has approximately the same angular resolution. Ideal conditions are not normal conditions, of course, and under more typical conditions, the imaging resolution is closer to 60 arcseconds (that's 20/20 vision). In extreme bright light, when the pupil is contracted, the diffraction limit goes up and the resolution necessarily becomes worse, and in extreme low light conditions, the pupil dilates exposing a larger portion of the optically poor cornea and lense and the resolution worsens again. Indirect sunlight apparently yields the optimal pupil diameter and thus optimal resolution. (btw, it doesn't have anything to do with rods versus cones as long as we're talking about looking directly at the target object --there aren't any rods in the fovea).
    Beyond imaging resolution, this "vernier acuity" or "hyperacuity" means that the human vision system (which includes the eye and its components but also a big slice of the brain) has some ability to go beyond normal resolution by a factor of ten (or twenty!) in certain very specific tasks, some of which are definitely applicable to sextant use. There appears to be no consensus model that explains this extreme visual acuity except that it is probably happening in the brain, but the ability is not unusual. Almost everyone has hyperacuity except infants (they test them with teeny tiny sextants), and it is not dependent on other visual defects. For example, it turns out that people with moderately bad myopia can still pass 10 arcsecond vernier acuity tests. I'm going to try to put some sort of web-based vision test so that we can experiment with this ourselves --just for the fun of it.
    42.0N 87.7W, or 41.4N 72.1W.
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