 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. Rikud, for his part, wanted to get back to the viewport and watch the one new bright star. "You know," Rikud said, sitting up now, "the stars in the viewport are changing." 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. 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.
 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.