Hmmm...well if they were trying to cover the sky with a holograph light from behind would still be there. Think of a movie screen with light behind it. You could still see the movie. The light would just be there behind it. Things would just seem to be a little lighter in the room...or better yet imagine being in a drive in with a spot light just lightly hitting the screen. The movie would be be overlaid on the screen. The light would be surrounding the screen and going through it but would not be noticed as much as the movie. If a planetoid body was reflecting light from the sun at certain parts of the month and was covered by a holographic sky screen how would you know?