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    Re: Sextant accuracy (was : Plumb-line horizon vs. geocentric horizon)
    From: Fred Hebard
    Date: 2005 Feb 13, 19:42 -0500

    On another list, I recall 30 arc-seconds being quoted, with exceptional
    individuals able to go below 10 or 5.  My recollection is that the
    unaided eye is able to resolve things to about 0.1 mm.  One can assume
    a distance of about 18 inches, I hazard, when calculating the resultant
    angle, with exceptional individuals going to 0.02 mm or so.
    On Feb 12, 2005, at 11:41 PM, Frank Reed wrote:
    > 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?
    >  -FER
    > 42.0N 87.7W, or 41.4N 72.1W.
    > www.HistoricalAtlas.com/lunars

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