At this brief moment, the sun's light penetrates the greatest possible amount of atmospheric fluid possible for direct viewing. This is a good thing, of course, in that all that air acts as a great filter, protecting our eyeballs from damage during one of the two times of day (sunrise being the other) when we can peer directly at our nearest star. When the sunset is visible against a flat horizon, this also is when we can see plenty of visual effects, mostly involving assorted distortions of the glowing orb through a long fetch of atmosphere. Of course, the balloon-shaped sun is an illusion, but a rather common one at this time of day. Air layers of different density direct and redirect light at different rates, loosely akin to the bending of light passing from air into a glass of water and back.
Sanibel FL (30 Dec 8) Looking WSW