 What does the viewport allow Rikud to realize?
When nobody aboard ship remembers where it's going, how can they tell when it has arrived? 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. There was a strange thought, and Rikud's brain whirled once more down the tortuous course of half-formed questions and unsatisfactory answers. Rikud had been stopped in this action, although there was no real authority to stop him. Much of this Rikud could not understand, but he knew enough to realize that the reading machine had sided with the people against the elders, and it said the people had won. He could see the look of easy contentment on Chuls' face as the rays fanned down upon him, bathing his old body in a forgotten magic which, many generations before Rikud's time, had negated the necessity for a knowledge of medicine. Rikud often thought of his own death, still seventy-five years in the future, not without a sense of alarm. But if he wasn't reading himself, Rikud usually went to sleep. This was a new thought to Rikud, and he propped his head up on one elbow. 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." "Astronomy says some stars are variable," Crifer offered, but Rikud knew his lame-footed companion understood the word no better than he did. Over on Rikud's right, Chuls began to dress.
 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.