 What does the viewport allow Rikud to realize?
Every day for a week now, Rikud had come to the viewport to watch the great changeless sweep of space. The stars—it was a meaningless concept to Rikud, but that was what everyone called the bright pinpoints of light on the black backdrop in the viewport—were not apparent in the speckled profusion Rikud had always known. Rikud knew that something which he could not explain had entered the elder man's head for a moment, but it had departed almost before Chuls knew of its existence. Rikud, for his part, wanted to get back to the viewport and watch the one new bright star. But if he wasn't reading himself, Rikud usually went to sleep. "You know," Rikud said, sitting up now, "the stars in the viewport are changing." The view had changed, and the strangeness of it made Rikud's pulses leap with excitement. All the stars were paler now than before, and where Rikud had seen the one bright central star, he now saw a globe of light, white with a tinge of blue in it, and so bright that it hurt his eyes to look. But how could a star change into a blinking blue-white globe—if, indeed, that was the star Rikud had seen earlier? Rikud couldn't remember, and he suddenly wished he could read Crifer's book on astronomy, which meant the same as stars. Presently Rikud became aware that his eyes were not tearing any longer, and he turned to look at the viewport. It seemed neatly sheered down the middle, so that on one side Rikud saw an expanse of brown and green, and on the other, blue. Rikud turned again to the port, where he could see nothing but an obscuring cloud of white vapor, murky, swirling, more confusing than ever. And besides, Rikud had the distinct feeling that here was something far vaster than the gardens, although all of it existed in the viewport which was no wider than the length of his body. For a whole week that view did not change, and Rikud had come to accept it as fact. Rikud regretted that he never had had the chance to read that book on astronomy. 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.
 There is more to the world outside of the ship they are on.
 There is more to the world outside of the ship they are on.  The viewport is not a flat space, and objects can pass through it.  The stars are indeed changing.  The garden outside is moving.