The policeman tells Christopher he's being arrested for assault. When he hears this, he feels much calmer, because it's the kind of thing cops say on TV.
The policeman puts him into the back of his police car, and Christopher looks out the window up at the stars. He takes this opportunity to tell us a little about the universe. It turns out he likes scientific facts, because they are things "you can work out in your own mind [...] without having to ask anyone" (17.13).