Light traveling through space
goes at a much faster speed than light waves passing through
very moist air near the surface of the earth. If you watch
the sun setting on a very humid, warm day, you'll observe
that just as the sun nears the horizon, the previously round
disk seems to have a flattened bottom; it is not a perfect
circle. The light waves coming from the bottom portion of
the sun are refracted by the moisture in the air to create
this optical illusion.
Rays from top of sun are also refracted, but not as much
because they enter the atmosphere at a less oblique angle.
Thus, the top of the sun is also flattened, but not as much
as the bottom.
If you are still unclear about the concepts, please refer
the animation below.