Imagine that you are standing at the edge of a pond. A little farther out a fisherman is standing knee-deep in the water, and from your angle his lower legs appear ridiculously short. At the boundary separating air from water, light bends slightly toward the denser medium. Even though you may know better, the bottom of the pond appears higher than it really is and leaves little room for the fisherman’s legs.
Near the fisherman’s feet, a crab looks up at you, the line of its vision following the same bent path as yours. The crab notes with approval that your head appears somewhat flattened.
From the bottom of the air we look up at the setting sun. The sun’s light bends when it enters the density of the air for the same reason that the line of our vision bends when it enters the water. The sun, like your head as the crab sees it, is slightly flattened.
The next time you see the setting sun, tilt your head horizontally and you will see that it is not round.