How dark is your night sky? An observer's guide

When I was a young boy growing up in New York City, I would always look forward to those occasions when our family visited my Aunt Irma and Uncle Ron.  

Irma and Ron lived in Mahopac, New York, 42 miles (69 kilometers) north of the bright lights of midtown Manhattan. From my home in the Bronx, I always had difficulty seeing very many stars. On many nights, for instance, I had trouble seeing Megrez, the star in the Big Dipper that joins the bowl with the handle.  

But it was a different story in the-then rural community of Mahopac (population 1,300 in 1968). Up there, the nights were incredibly dark; the sky teemed with myriads of stars. The Milky Way was so bright that, on the very best summer nights, its light could cast faint shadows!  

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Believe it or not, that was the way things were more than a half century ago within a 50-mile (80 km) radius of New York City. Looking south toward the big metropolis, there was hardly any hint of a huge city with 8 million people; only the slightest bit of glare or light haze was apparent from beyond the horizon. 

It was from my aunt and uncle's house that I made many of my earliest astronomical observations and dutifully reported them to Sky & Telescope magazine, where they were published. Back then, I told myself that when I got older, I would move to the lower Hudson Valley, where I could enjoy a star-studded sky on almost every clear night of the year. Eventually, I got my wish: In 2002 I bought a house in Putnam Valley, just 7 miles (11 km) west of Mahopac. 

But it's a far cry from the way things were back in the 1960s.

Times change 

Thanks to the tremendous increase in light pollution over the past 50 years, the pristine night sky of my youth is no more. Instead of dark, starry skies, a typical night as seen from my front door verges more toward a charcoal gray. Looking south toward New York City, a blush-gray dome of hazy light reaches nearly halfway up into the sky. The light glare to my south is not due just to the bright lights of New York City but also to those of nearby cities in adjacent Westchester County, such as Peekskill and White Plains. I estimate that the night sky from my home base in Putnam County is at least six times brighter now than it was half a century ago. 

With today's pervasive light pollution, it might be difficult to believe that there was once a time when clouds were the greatest hurdle to a star-filled sky. Factor bright moonlight and atmospheric turbulence into the mix, and good viewing opportunities might seem few and far between these days. However, with a little bit of planning you can still enjoy some good views of the heavens. 

First, be advised that the amount of light pollution from your backyard can vary. For example, in the after-midnight hours, most local strip malls shut off their lights. Also, during winter months, the amount of snow cover, which reflects light up into the sky, can have a dramatic impact on sky brightness.  

In addition, the effects of light pollution are accentuated by the transparency of the sky. The hazier the sky is on any given night, the more pronounced the glow from light pollution will be.

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'Bowled over'

To test how dark your sky is, pick a night that follows a clear day with low humidity and deep blue skies. Then, during the evening after it gets fully dark, look to the north and find the four stars that make up the bowl of the Little Dipper. If you live under badly light-polluted urban skies, you're going to see just two of the four stars that comprise the dipper's bowl. Those two stars are Kochab and Pherkad, the so-called "Guardians" of the North Star (Polaris). They are always in the sky and appear to wheel a course around Polaris, acting like celestial sentries. 

Now for some numbers.

Kochab is a second-magnitude star and Pherkad is third-magnitude. Magnitude is the astronomer's yardstick for determining how bright a particular sky object appears. The bigger the number, the fainter the object. A one-magnitude difference means that the object with the lower number is 2.5 times brighter than the object with the higher number. If you can see only Kochab and Pherkad, then the limiting magnitude (the brightest of the faintest stars visible to your naked eye) will likely be around 3.1. 

So, using the bowl of the Little Dipper to monitor the limiting magnitude of your location from night to night and hour to hour is a good way to get a feel for how variable the effects of light pollution can be. 

For most suburbs, on many nights, you might glimpse a third star in the Little Dipper's bowl, the one that joins the bowl with the handle. That's Zeta, at just over magnitude four. If you can make out Zeta, the limiting magnitude is probably close to 4.5 — "typical" suburban light pollution. If you can discern all four stars in the bowl of the Little Dipper (the faintest is Eta, a fifth-magnitude star) and also make out the brighter parts of the Milky Way, consider yourself lucky to be under a dark suburban sky.  

Sadly, however, only about one in five Americans regularly enjoys a sky like this.

Shedding light on dark skies

Of course, in those places where the population is low — remote parts of Earth or out over the ocean — the night sky will resemble what I can remember from my younger days in Mahopac. And one person has developed a way to judge how we might rank just such an exceptional sky. 

More than two decades ago, one of the most assiduous observers of the night sky developed a new dark-sky rating scale. His name is John E. Bortle, and he is very well known in astronomical circles as an expert in the field of comets, having observed more than 300 in his lifetime. He has also made over 200,000 variable star estimates in more than 60 years of observing. To aid observers in judging the true darkness of a particular site, Bortle created a nine-level scale based on his many years of observing experience. The lower the number, the better your sky.

Which of the following describes your night sky?

Class 9: Inner-city sky. The entire sky is brightly lit, even at the zenith. Many stars making up familiar constellations are invisible. The only celestial sights that provide pleasing telescopic views are the moon, planets and (if you can find them) a few of the brightest star clusters. 

Class 8: City sky. The sky glows white, gray or even orangish, and you can read newspaper headlines without difficulty. Some of the stars making up the familiar constellation patterns are difficult to see or are entirely absent. 

Class 7: Suburban/urban transition: The entire sky background has a vague, grayish-white hue. The Beehive star cluster or the Andromeda galaxy might be glimpsed with the unaided eye but are very indistinct. Clouds are brilliantly lit. 

Class 6: Bright suburban sky. Any indication of the Milky Way is apparent only toward the zenith. The Andromeda galaxy is only modestly apparent to the unaided eye. Clouds anywhere in the sky appear fairly bright. You have no trouble seeing eyepieces and telescope accessories on an observing table.  

Class 5: Suburban sky. The Milky Way is very weak or invisible near the horizon and looks rather washed out overhead. Light sources are evident in most if not all directions. Over most or all of the sky, clouds are noticeably brighter than the sky itself. 

Class 4: Rural/suburban transition. Over population centers. fairly obvious domes of light pollution are apparent. The zodiacal light is clearly evident but doesn't even extend halfway to the zenith at the beginning or end of twilight. (The zodiacal light is a faint, elongated cone of light extending from the horizon along the plane of the planets' orbits, which is apparently caused by the reflection of sunlight off dust and ice particles.) The Milky Way well above the horizon is impressive but lacks all but the most obvious structure. 

Cloud 3: Rural sky: Some indication of light pollution is evident along the horizon. Clouds appear faintly illuminated in the brightest parts of the sky near the horizon but are dark overhead. The Milky Way appears bright and complex. The zodiacal light is striking in spring (early evening) and autumn (early morning). Your telescope is vaguely apparent at a distance of 20 or 30 feet (6 or 9 meters). 

Class 2: Typical truly dark site. The summer Milky Way is highly structured to the unaided eye, and its brightest parts look like veined marble when viewed with ordinary binoculars. Any clouds in the sky are visible only as dark holes or voids in the starry background. Many Messier objects are distinctly visible to the naked eye.

Class 1: Excellent dark sky site.  The zodiacal light, gegenschein, and zodiacal band are all visible. The Scorpius and Sagittarius region of the Milky Way casts obvious diffuse shadows on the ground. If you are observing on a grass-covered field bordered by trees, your telescope, companions and vehicle are almost totally invisible.  

"This," notes Mr. Bortle, "is an observer's Nirvana!"

Joe Rao serves as an instructor and guest lecturer at New York's Hayden Planetarium. He writes about astronomy for Natural History magazine, the Farmers' Almanac and other publications. Follow us on Twitter @Spacedotcom and on Facebook.