 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. But ever since the engines somewhere in the rear of the world had changed their tone, from the steady whining Rikud had heard all twenty-five years of his life, to the sullen roar that came to his ears now, the feelings had grown. 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. Instead, there was more of the blackness, and one very bright star set apart by itself in the middle of the viewport. 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 can find several of them under 'astronomy,' with a cross-reference under 'stars.' "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. Something big and round, all grays and greens and browns, and something for which Rikud had no name. A section of it had expanded outward and assumed the rectangular shape of the viewport, and its size as well. 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.
 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.