Sun appears flattened at the sunrise and sunset?

  The light rays entering to earth from sun undergoes refraction.As the light enters from vacuum                    (rarer medium) to earth atmosphere                               (denser medium) bending of light happens towards normal.The rays coming from top and bottom portion of sun are refracted by different degrees causing apparent flattening of sun but the rays from side of sun bend in equal amount.So,the sun appears flattened or oval in sunrise and sunset.