What causes flattening of sun during …

Apparent flattening of the sun’s disc at sunrise and sunset is due to atmospheric refraction. The density and the refractive index of the atmosphere decreases with altitude, so the rays above and below the sun on the horizon are refracted by different degrees. This causes the apparent flattening of the sun. But the rays from the sides of the sun on a horizontal plane are generally refracted by the same amount, hence the sun still appears circular along its sides.

The rays of light from the bottom of the sun are refracting more than those from the top, flattening the sun's circular shape. “Flattened sun” can also refer to the Sun appearing in an oval shape close to the horizon during sunrise or sunset. The lower half of the sun appears darker in colour than the top half. 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. That is why the sun appears oval or flattened at the time of sunrise and sunset.