A time-exposure photograph of the Milky Way over the New Mexico desert. The word “galaxy” comes from the Greek for milk. CHRIS COOK/PHOTO RESEARCHERS

In 1610, Galileo Galilei published a small book describing astronomical observations that he had made of the skies above Padua. His homemade telescopes had less magnifying and resolving power than most beginners’ telescopes sold today, yet with them he made astonishing discoveries: that the moon has mountains and other topographical features; that Jupiter is orbited by satellites, which he called planets; and that the Milky Way is made up of individual stars. It may seem strange that this last observation could have surprised anyone, but 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 in the sky above the place where they live, and those who can see it are sometimes baffled by its name.

The stars have not become dimmer; rather, the Earth has become vastly brighter, so that celestial objects are harder to see. Air pollution has made the atmosphere less transparent and more reflective, and high levels of terrestrial illumination have washed out the stars overhead—a phenomenon called “sky glow.” Anyone who has flown across the country on a clear night has seen the landscape ablaze with artificial lights, especially in urban areas. Today, a person standing on the observation deck of the Empire State Building on a cloudless night would be unable to discern much more than the moon, the brighter planets, and a handful of very bright stars—less than one per cent of what Galileo would have been able to see without a telescope. Amateur astronomers sometimes classify nighttime darkness on the Bortle Dark-Sky Scale, which is based on a number of criteria, among them “limiting magnitude,” or the brightness of the faintest celestial objects that are visible without magnification. The scale, composed of nine points, was devised in 2001 by John E. Bortle, a retired Westchester County fire chief and a monthly columnist for Sky & Telescope. “One of the problems I was addressing was that younger amateur astronomers, especially east of the Mississippi, had never seen a dark sky at all,” he told me recently. “People will sometimes come up from the city and call me and say, ‘John, I’ve found this fabulous dark site, it’s totally black, you can’t imagine how good it is.’ So I’ll go and have a look, but it’s always poor. They have no comparison to work against.”

In Galileo’s time, nighttime skies all over the world would have merited the darkest Bortle ranking, Class 1. Today, the sky above New York City is Class 9, at the other extreme of the scale, and American suburban skies are typically Class 5, 6, or 7. The very darkest places in the continental United States today are almost never darker than Class 2, and are increasingly threatened. For someone standing on the North Rim of the Grand Canyon on a moonless night, the brightest feature of the sky is not the Milky Way but the glow of Las Vegas, a hundred and seventy-five miles away. To see skies truly comparable to those which Galileo knew, you would have to travel to such places as the Australian outback and the mountains of Peru. And civilization’s assault on the stars has consequences far beyond its impact on astronomers. Excessive, poorly designed outdoor lighting wastes electricity, imperils human health and safety, disturbs natural habitats, and, increasingly, deprives many of us of a direct relationship with the nighttime sky, which throughout human history has been a powerful source of reflection, inspiration, discovery, and plain old jaw-dropping wonder.

David L. Crawford earned his Ph.D. in astronomy in 1958 and spent nearly all his professional life at Kitt Peak National Observatory, on a mountaintop fifty-six miles southwest of Tucson, Arizona. By 1970, he had noticed, with alarm, a significant decrease in astronomical visibility. Tucson was growing rapidly, and so was its sky glow. With a colleague, he persuaded the city to adopt an ordinance governing exterior lighting, and later they persuaded other Arizona cities and counties to pass similar regulations. In 1988, Crawford and another friend formed a nonprofit organization called the International Dark-Sky Association. “We’re sort of a nighttime Sierra Club,” he told me, during a recent visit to Tucson. He retired from Kitt Peak in 1995 and has worked full time for the I.D.A. ever since, often putting in sixty-hour workweeks. He has the complexion of a man who doesn’t spend a lot of time outdoors during daylight, and speaks in the modulated tones of someone accustomed to talking while others are asleep. “We’re on a mission to change the world at night,” he said.

The I.D.A.’s headquarters is a warren of small offices, accommodating a dozen or so staff members and a shifting group of volunteers and researchers, around the corner from a (non-related) store that sells light fixtures. Crawford and his staff devote much of their time to proselytizing for dark-sky regulations and working with manufacturers to improve lighting products. Hanging on a wall in a conference room is a map that shows the geographical distribution of the organization’s eleven thousand members. The states with the highest representation are California (fifteen hundred and thirty), Arizona (six hundred and seventy), New York (five hundred and one), and Massachusetts (four hundred and eighty-two). The I.D.A. also has members in seventy-eight foreign countries, including Iraq and Iran, where astronomy is a popular hobby, especially among girls and young women. Authorities in Sa’adat-shahr, about four hundred miles south of Tehran, periodically cut off all electric power in the town in order to improve visibility at nighttime “star parties” conducted by a local teacher.

When the I.D.A. began, Crawford’s interest in outdoor lighting was limited to its impact on observatories; today, the organization’s brief covers everything from advising law-enforcement officers to assessing the effects of artificial lighting on wildlife. On the evening of my visit, while Crawford and I waited for the sky to grow darker, we went to dinner at a relatively new shopping mall on Tucson’s outskirts. As we drove up, Crawford explained that the mall had been of particular interest to the I.D.A.: “The original lighting system for this mall was put in by somebody from out of town, and it didn’t meet the Tucson code, so the developer had to call in a consultant and change it all. Now it’s one of the best in town, and we actually gave them an award a few years ago.”

The mall’s large parking lot was fully illuminated—as we walked from the car to the restaurant, I had no trouble reading notes that I had scribbled in my notebook—but it was free of what dark-sky advocates call “glare bombs”: fixtures that cast much of their light sideways, into the eyes of passersby, or upward, into the sky. Tucson’s code limits the brightness of exterior fixtures and requires most of them to be of a type usually known as “full cutoff” or “fully shielded,” meaning that they cast no light above the horizontal plane and employ a light source that cannot be seen by someone standing to the side. These are not necessarily more difficult or expensive to manufacture than traditional lights, and they typically cost less to operate. Calgary, Alberta, recently cut its electricity expenditures by more than two million dollars a year, by switching to full-cutoff, reduced-wattage street lights.