The edge of the sun doesn’t have to be absolutely sharp in order to look that way to us. it just has to go from being transparent to being completely opaque in a distance that is smaller than your eye can resolve. remember from astronomical instruments that the ability to resolve detail depends on the size of the telescope’s aperture. the pupil of your eye is very small relative to the size of a telescope and therefore is very limited in the amount of detail you can see. in fact, your eye cannot see details that are smaller than 1/30 of the diameter of the sun (about 1 arcminute). nearly all the light from the sun emerges from a layer that is only about 400 km thick. what fraction is this of the diameter of the sun? how does this compare with the ability of the human eye to resolve detail? suppose we could see light emerging directly from a layer that was 300,000 km thick. would the sun appear to have a sharp edge?