Sometimes, we can see clouds appearing as if they are behind the Sun or the Moon. Obviously, this is simply an illusion. But some flat-Earthers regard this as ‘evidence’ that the Sun and the Moon are close, closer than clouds ‘above’ them. They are wrong.
The phenomenon occurs because thinner clouds are transparent, and the light from the Sun or the Moon is relatively very bright. Our eyes and camera sensors are unable to distinguish the part of the Sun/Moon obscured by the thin clouds from the part that is completely clear from clouds. Parts of the clouds outside the Sun/Moon disc are still visible, making it appears as if they are behind the Sun/Moon.
We can simulate this using a light source and a few sheets of semi-transparent material, like old camera films. If the light source is bright enough, then the transparent material would appear as if it is not there, giving the illusion it is behind the light source.
- Picture courtesy of Mick West at Metabunk