I had a random idea for what the seven skies could be in terms of space travel:
The first sky is Earth orbit.
The second sky encompasses lunar orbit and the space between.
The third sky extends to the Solar system.
The fourth sky includes interstellar space, and is the path to the fifth sky.
The fifth sky includes other star systems.
The sixth sky includes intergalactic space, and is the path to the seventh sky.
The seventh sky includes other galaxies.
I suppose that this could be useful in SF, particularly regarding a space opera.