Distortion of the Solar Disk at Sunset
June 06, 2018
Atmospheric refraction is responsible for making celestial objects appear to ripple, rise, fall, stretch and compress, especially when near the horizon. The montage above shows a sunset sequence as viewed near Torre de Hércules, Spain on May 22, 2018. Variations of the index of refraction with altitude are most remarkable when the Sun is rising or setting -- when path length is at its maximum. Light from the bottom of the solar disk passes through more air than light coming from the top, so the bottom of the Sun is refracted upwards more noticeably than the top and thus looks flattened before sinking below the horizon.
However, atmospheric mirages and temperature inversions (typically a layer of warmer air lying just above a layer of cool air that's near the surface) can further exaggerate distortion, as observed above. Multiple inversions create the most bizarre shaped Suns.