The elliptical nature of sun during sunrise and sunset is due to refraction phenomenon as the ray of light bends before reaching the observer.
Sun has elliptical during sunrise and sunset Because of a density gradiënt in the lower layer of the atmosphere there is a gradual change in the air’s index of refraction. The closer a light ray is to the surface the stronger this effect is. This causes light rays to bend a little and things close to the horizon seem to be a bit higher than they really are. So the lower part of the suns disc is raised more than the upper part so it seems to be flattened.