"It begins here, with a commonly observed and reported phenomenon: the light coming through the cracks between the leaves in the trees hits the ground or a piece of white cardboard and is round. Now you know that the spaces between the leaves aren't round. They're all irregular. The round spots of light are images of the sun.
"One thing that occurred to me when I looked at this was that there must also be an image of the sky surrounding the sun. The light from the sky must get through the holes, too. But we aren't normally aware of it, because the blue light of the sky is so dim compared to the sun. The sun is really bright - about a million times brighter than the moon. And the moon is even brighter than the clear blue sky.