Imagine a space traveler arriving on our planet after a voyage of some years. She lands during the daytime, near the United Nations in New York City, and when she steps from her ship she has to shield her eyes. She has been away from her sun, from any sun, for so long that she’s unaccustomed to daylight; space is dark, though dotted with a billion points of light. Soon she’s ushered indoors to meet a long succession of diplomats and functionaries. Sometime that evening, she steps outside agai...