Gazing up at the heavens from our backyards or a nearby field, most of us see an undifferentiated mess of stars--if, that is, we can see anything at all through the glow of light pollution. Today's casual observer knows far less about the sky than did our ancestors, who depended on the sun and the moon to tell them the time and on the stars to guide them through the seas. Nowadays, we don't need the sky, which is good, because we've made...