Why does the sun appear flat during sunrise and sunset?
Answer: During sunrise and sunset, the sun appears slightly tilted up and down due to the refraction of light in the atmosphere. At this time the lower edge of the sun is closer to the horizon than the upper edge. As a result, rays coming from the lower end of the Sun are bent more by refraction in the atmosphere, and rays coming from the upper end are bent less by refraction.
Because of this, the vertical diameter of the Sun appears to be less than the horizontal diameter. Therefore, the sun appears to tilt slightly up and down during sunrise and sunset.