“In Galileo’s time people assumed that the Milky Way must be some kind of continuous substance. It truly resembled a streak of spilled liquid—our word “galaxy” comes from the Greek for milk—and it was so bright that it cast shadows on the ground (as did Jupiter and Venus).

Today, by contrast, most Americans are unable to see the Milky Way. Those who can are baffled by its name. The stars have not become dimmer; rather, the Earth has become vastly brighter. Air pollution has made the atmosphere less transparent and more reflective, and washed out the stars overhead.