Astrophysics reigns as the most humbling of scientific disciplines. The astounding breadth and depth of the universe deflates our egos daily, and we are continually at the mercy of uncontrolled forces. A simple cloudy evening—one that would stop no other human activity—prevents us from making observations with a telescope that can cost $20,000 a night to run, regardless of the weather. We are passive observers of the cosmos, acquiring data when, where, and how nature reveals itself to us. To know the cosmos requires that we have windows onto the universe that remain unfogged, untinted, and unpolluted. But the spread of what we call civilization, and the associated ubiquity of modern technology, is generally at odds with this mission. Unless something is done about it, people will soon bathe the Earth in a background glow of light that will block all access to the frontiers of cosmic discovery.

The most obvious and prevalent form of astropollution comes from streetlamps. All too often, they can be seen from your airplane window during night flights, which means that these streetlamps illuminate not only the streets below but the rest of the universe. Unshielded streetlights, such as those without downward-facing shades, are most to blame. Municipalities with these poorly designed lamp housings find themselves buying higher-wattage bulbs because half the lamplight points upward. This wasted light, shot forth into the night sky, has rendered much of the world’s real estate unsuitable for astronomical research. At a 1999 conference entitled “Preserving the Astronomical Sky,” participants rightly moaned about the loss of dark skies around the globe. One paper reported that inefficient lighting costs the city of Vienna $720,000 annually, London $2.9 million, Washington, D.C., $4.2 million, and New York City $13.6 million. Note that London, with a population similar to that of New York City, is more efficient in its inefficiency by nearly a factor of five.

The astrophysicist’s dilemma is not that light escapes into space but that the lower atmosphere supports a mixture of water vapor, dust, and pollutants that bounce some of the upward-flowing photons back down to Earth, leaving the sky aglow with the sig­nature of a city’s nightlife. As cities become brighter and brighter, dim objects in the cosmos become less and less visible, severing urban dwellers’ access to the universe.

It’s hard to exaggerate the magnitude of this effect. A penlight’s beam, aimed at a wall across a darkened dining room, is easy to spot. But gradually brighten the overhead light, and watch how the beam gets harder and harder to see. Under light-polluted skies, fuzzy objects such as comets, nebulae, and galaxies become difficult or impossible to detect. I have never in my life seen the Milky Way galaxy from within the limits of New York City, and I was born and raised here. If you observe the night sky from light-drenched Times Square, you might see a dozen or so stars, compared with the thousands that were visible from the same spot when Peter Stuyvesant was hobbling around town. No wonder ancient peoples shared a culture of sky lore, whereas modern peoples, who know nothing of the night sky, instead share a culture of evening TV.

The expansion of electrically lit cities during the twentieth century created a technology fog that forced astronomers to move their hilltop observatories from the outskirts of towns to remote places such as the Canary Islands, the Chilean Andes, and Hawaii’s Mauna Kea. One notable exception is Kitt Peak National Observatory in Arizona. Instead of running away from the spreading and brightening city of Tucson, fifty miles away, the astronomers stayed and fought. The battle is easier-won than you might think; all you have to do is convince people that their choice of outdoor lighting is a waste of money. In the end, the city gets efficient streetlamps and the astronomers get a dark sky. Ordinance No. 8210 of the Tucson/Pima County Outdoor Lighting Code reads as though the mayor, the chief of police, and the prison warden were all astronomers at the time the code was passed. Section 1 identifies the intent of the ordinance:

The purpose of this Code is to provide standards for outdoor lighting so that its use does not unreasonably interfere with astronomical observations. It is the intent of this Code to encourage, through the regulation of the types, kinds, construction, installation, and uses of outdoor electrically powered illuminating devices, lighting practices and systems to conserve energy without decreasing safety, utility, security, and productivity while enhancing nighttime enjoyment of property within the jurisdiction.

And after thirteen other sections that give strict rules and regulations governing citizens’ choice of outdoor lighting, we get to the best part, section 15:

It shall be a civil infraction for any person to violate any of the provisions of this Code. Each and every day during which the violation continues shall constitute a separate offense.

As you can see, by shining light on an astronomer’s telescope you can turn a peace-loving citizen into a Rambo. Think I’m joking? The International Dark-Sky Association (IDA) is an organization that fights upward-pointing light anywhere in the world. With an opening phrase reminiscent of the one painted on Los Angeles Police Department squad cars, the IDA’s motto says it all: “To preserve and protect the nighttime environment and our heri­tage of dark skies through quality outdoor lighting.” And, like the police, the IDA will come after you if you transgress.

I know. They came after me. Not a week after the Rose Center for Earth and Space first opened its doors to the public, I received a letter from the IDA’s executive director, scolding me for the upward-pointing lights embedded in the pavement of our entrance plaza. We were justly accused—the plaza does have forty (very low wattage) lamps that help delineate and illuminate the Rose Center’s granite-clad arched entryway. These lights are partly functional and partly decorative. The point of the letter was not to blame the bad viewing conditions across all of New York City on these itty-bitty lamps but to hold the Hayden Planetarium accountable for setting a good example for the rest of the world. I am embarrassed to say that the lights remain.

But all that’s bad is not artificial. A full Moon is bright enough to reduce the number of stars visible to the unaided eye from thousands to hundreds. Indeed, the full Moon is more than 100,000 times brighter than the brightest nighttime stars. And the physics of reflection angles endows the full Moon with more than ten times the brightness of a half Moon. This moonglow also greatly reduces the number of meteors visible during a meteor shower (though clouds would be worse), no matter where you are on Earth. So never wish a full Moon upon an astronomer who is headed off to a big telescope. True, the Moon’s tidal force created tide pools and other dynamic habitats that contributed to the transition from marine to terrestrial life and ultimately made it possible for humans to thrive. Apart from this detail, most observational astronomers, especially cosmologists, would be happy if the Moon had never existed.

A few years ago I got a phone call from a marketing executive who wanted to light up the Moon with the logo of her company. She wanted to know how she might proceed. After slamming down the phone, I called her back and politely explained why it was a bad idea. Other corporate executives have asked me how to put into orbit mile-wide luminous banners with catchy slogans written across them, much like the skywriting or flag-dragging airplanes you see at sports events or over the ocean from a crowded beach. I always threaten to send the light police after them.

Modern life’s insidious link with light pollution extends to other parts of the electromagnetic spectrum. Next at risk is the astronomer’s radiowave window to the cosmos, including micro­waves. In modern times we are awash in the signals of such radiowave-emitting devices as cellular telephones, garage-door openers, keys that trigger “boip” sounds as they remotely lock and unlock car doors, microwave relay stations, radio and television transmitters, walkie-talkies, police radar guns, global positioning systems, and satellite communications networks. Earth’s radiowave window to the universe lies cloaked in this technologically induced fog. And the few clear bands that remain within the radio spectrum are getting progressively narrower as the trappings of high-tech living grab more and more radiowave real estate. The detection and study of extremely faint celestial objects is being compromised as never before.

In the past half century radio astronomers discovered remarkable things, including pulsars, quasars, molecules in space, and the cosmic microwave background, the first evidence in support of the big bang itself. But even a wireless conversation can drown such faint radio signals: modern radio telescopes are so sensitive that a cell-phone encounter between two astronauts on the Moon would be one of the brightest sources in the radio sky. And if Martians used cell phones, our most powerful radio telescopes would easily nab them, too.

The Federal Communications Commission is not unmindful of the heavy, often conflicting demands that various segments of society place on the radio spectrum. The FCC’s Spectrum Policy Task Force intends to review the policies that govern use of the electromagnetic spectrum, with the goal of improving efficiency and flexibility. FCC chairman Michael K. Powell told the Washington Post (June 19, 2002) that he wanted the FCC’s philosophy to shift from a “command and control” approach to a “market-oriented” one. The commission will also review how it allocates and assigns bands of the radio spectrum, as well as how one allocation may interfere with another.

For its part, the American Astro­nomical Society, the professional organization of the nation’s astro­physicists, has called on its members to be as vigilant as the IDA folks—a posture Iendorse—in trying to convince policy makers that specially identified radio frequencies should be left clear for astronomers’ use. To borrow vocabulary and concepts from the irrepressible Green movement, these bands should be considered a kind of “electromagnetic wilderness” or “electromagnetic national park.” To eliminate interference, the geographic areas surrounding the protected observatories should also be kept clear of human-generated radio signals of any kind.

The most challenging problem may be that the farther an object is from the Milky Way, the longer the wavelength and the lower the frequency of its radio signals. This phenomenon, which is a cosmological Doppler effect, is the principal signature of our expanding universe. So it’s not really possible to isolate a single range of “astro” frequencies and assert that the entire cosmos, from nearby galaxies to the edge of the observable universe, can be served through this window. The struggle continues.

Today, the best place to build telescopes for exploring all parts of the electromagnetic spectrum is the Moon. But not on the side that faces the Earth. Putting them there might be worse than looking out from the Earth’s surface. When viewed from the Moon’s near side, the Earth looks thirteen times bigger, and shines some fifty times brighter, than the Moon does when viewed from the Earth. And the Earth never sets. As you might suspect, civilization’s chattering communication signals also make Earth the brightest object in the radiowave sky. The astronomer’s heaven is, instead, the Moon’s far side, where the Earth never rises, remaining forever buried below the horizon.

Without a view of Earth, telescopes built on the Moon could point in any skyward direction, without the risk of contamination from the Earth’s electromagnetic emanations. Not only that, night on the Moon lasts nearly fifteen Earth days, which would enable astronomers to monitor objects in the sky for days on end, much longer than they could from the Earth. And because there is no lunar atmosphere, observations conducted from the Moon’s surface would be as good as observations of the cosmos from Earth orbit. The Hubble Space Telescope would lose the bragging rights it now enjoys.

Furthermore, without an atmosphere to scatter sunlight, the Moon’s daytime sky is almost as dark as its night, so everybody’s favorite stars hover visibly in the sky, right alongside the disk of the Sun. A more pollution-free place has yet to be found.

On second thought, I retract my earlier callous remarks about the Moon. Maybe our neighbor in space will one day become the astronomer’s best friend after all.

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Neil deGrasse Tyson, an astrophysicist, is the Frederick P. Rose Director of New York City’s Hayden Planetarium and a visiting research scientist at Princeton University.

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