 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, 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." The view had changed, and the strangeness of it made Rikud's pulses leap with excitement. Rikud couldn't remember, and he suddenly wished he could read Crifer's book on astronomy, which meant the same as stars.
 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.