And yet no true circle can be found in nature.
No true straight line or any regular geometric figure, either.
However, the Sun and moon both appear perfectly circular to the naked eye.
Because it just looks right? 
Eyes.
A perfect circle means he/she is looking directly at you.
Not sure about the moon. Enough irregularities are evident to make it fully plausible that they continue to the perimeter.
Planets/moons appear circular if large enough. Visual halos around objects. A spreading wave from an object dropped into a pond. Rings around a planet, like Saturn.
No, those aren’t perfect circles, but you don’t find perfect circles out of nature either; everything is imperfect.
That would be my point: Although the circle is strongly suggested by many aspects of the universe we live in, it is in fact entirely artificial and exists only as a mathematical concept.
Well, yes, but only in mathematics is a slightly imperfect circle not actually a circle, so it’s a wash.
Only if the person is really, really scared. Otherwise, the lids cut across the circle o f the iris.
Unless of course, you mean the pupil, but that only works for people with light colored eyes, a genetic mutation that’s only been around for 10,000 years or so.
No, no…
(It’s the little square in the corner instead of the little part-circle)
Brendon Small
It’s perfectly cromulent as the cross-section of a roller, though.