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    Re: Sextant accuracy (was : Plumb-line horizon vs. geocentric horizon)
    From: Frank Reed CT
    Date: 2005 Feb 12, 23:41 EST
    Alex you wrote:
    "On your earlier message today on the human eye resolution.
    I think it is more complicated than that.
    "Resolution" is the ability to see two point sources apart.
    But frequently I see two stars collided in one spot, (so I already cannot
    tell them apart) but still understand
    that this collision is not perfect. Then I try to make this "spot"
    as small as I can. I believe that a human eye can be trained to increase
    this ability."
    Yes, there is definitely one step that you can go beyond the standard optics definition of resolution. And yet, I do find that the extra magnification is just what's required based on roughly 1.5 minute unaided resolution to get to 0.2 minute accuracy based on the standard definition. One example of a visual skill that's a step beyond resolution (that we've discussed before) is the ability to bring two large circles, like the images of the Sun and the Moon, into perfect tangency. There is still a resolution issue here, since you have to resolve the edge to define it, but the amazing thing is that we can do a pretty good job estimating when two circles would be in perfect tangency even if the actual point of contact is hidden from view. So you're right that there is definitely a larger perception issue at work here.
    And you wrote:
    "If you look at the marksmen results (with a rifle, with no optical sight)
    you can conclude that the human eye ability to aim a rifle is
    better than 1'."
    Interesting. I don't know anything about this. What can a typical trained marksman achieve in terms of angular accuracy? While I'm thinking of it, do we know if there are any people who have genetically superior foveas in their eyes with the cones more tighly packed to achieve superior resolution --and superior marksmanship?
    42.0N 87.7W, or 41.4N 72.1W.
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