From the introduction:
Are there really aliens in outer space? Most scientists think it likely. And many people have tried to guess what they would be like.
If they were very intelligent and had invented spaceships, they might want to see what's happening here. After all, Earth is a very interesting planet.
Imagine how an alien would feel, after travelling trillions of miles in a spacecraft, to see a beautiful blue-white sphere growing larger and larger in the star-studded black sky.
Imagine you are that alien - one living on Turoc, a planet far more advanced than ours - that you are an Earth Inspector.