Question

the sun appears oval in shape at sunrise and sunset but appears circular at noon

Answer 1

Hello Friend...
At sunrise and sunset, the sun is near the horizon. The rays of light from the upper part and lower part of the periphery of the sun bend unequally on travelling through earth's atmosphere. ... Hence the sun appears circular at noon.
Hope it helps you...☺

Answer 2

it is all because of rarefaction of the image of actual sun if you read ncert you will find a para saying sun appears earlier than actually its not.

in rarefaction fraction image rays passes through many layers of different densities atmosphere which causes rarefaction hence sun appears oval in shape

secondally angle of emergence and incidence also matter