 What do the characters refer to as "the world?"
True, this was the only case of its kind, the exception to the rule, but it proved the world was not perfect. You can find several of them under 'astronomy,' with a cross-reference under 'stars.' Something big and round, all grays and greens and browns, and something for which Rikud had no name. If he, Rikud, walked from one part of the world to another, it was with a purpose—to eat, or to sleep, or perhaps to bathe in the health-rays. Now if the world had walked from—somewhere, through the vast star-speckled darkness and to the great garden outside, this also was purposeful.
 Their ship, which is all they have ever known.
 Their ship, which is all they have ever known.  The men's quarters, which is all they have ever known.  The planet they are preparing to plummet onto.  They use it as a general term for the "universe."