Why does the sun appear bigger and seems to possess an oval shape during sunrise and sunset?

During sunrise and sunset, the sun is at the horizon and refractive index of the atmosphere of the earth decrease with height. Due to this, light reaching the earth’s atmosphere from different parts of the vertical diameter of the sun enters at different heights in earth’s atmosphere and so travels in media of different refractive indices at the same instant and hence, bend unequally. Due to this unequal bending of light from the vertical diameter, the image of the sun gets distorted and it appears oval and larger. However, at noon when the sun is overhead, then due to normal incidence of light‘there is no bending of light and hence, the sun appears circular.