The winter cold can be challenging for stargazers, but it offers rewards to the warmly dressed. That’s because cold air can’t hold as much water vapor as warm air, so winter night skies are relatively free of the subtle haze that in summer can obscure faint stars or dim the sparkle of the brightest. Observatories are often built in locations that besides lacking light pollution also have predominantly cold, dry air.
Another factor enhancing the perception of winter stars is that night falls earlier. On summer evenings twilight lingers as people return home from work, school, and errands, stopping to look at the sky perhaps before going indoors. But in winter at those same times it can be fully dark outside. The stars seem brighter and the constellations more prominent now than they do at the same points in our summer routines.
A few weeks ago, I wrote in these pages about my first experience of a truly dark sky — it was at Mauna Kea in Hawaii, where there is relatively little human-caused light pollution. That’s where it became startlingly obvious to me that the darker the sky, the more stars we can see.
Whether a sky is “dark” or not is a relative thing. The Outer Cape’s night sky is darker than Boston’s, while the night sky of Cherry Springs State Park in Potter County, Pa. — one of the best skies in the northeastern U.S. — is darker than that of the Outer Cape.
Anyone interested in preserving the Outer Cape’s relatively dark skies should start with a good understanding of light pollution — artificial nighttime illumination that escapes from or exceeds its intended use and causes negative visual effects. You’ll see it in at least three distinct forms: glare, light trespass, and sky glow.
The negative effects of light pollution are not just aesthetic. If you’ve had to squint and avert your eyes from the bright headlights of oncoming cars, maybe with your car swerving as you look aside, you’ve experienced glare. You’ve probably also experienced glare as a pedestrian. Have you ever walked around an area with bright streetlights at night and found you can see well only by looking down at your feet? In that situation, a person standing 20 feet away may be invisible to you because of the overhead glare; the same goes for a car approaching the crosswalk as you step into the street.
Though light trespass may sound like a mild criminal offense, it’s not. Still, if your neighbor has motion-activated backyard lights that shine into your bedroom whenever a raccoon skulks by. you know how annoying light trespass can be. It’s light going where it isn’t needed or wanted. Another example is a parking lot that’s fully illuminated all night, long after every shop is closed.
Then there’s sky glow. It’s the net effect of the other forms of light pollution, as our collective wasted or unwanted light shines up into the sky. There it scatters off air molecules, suspended dust, and water vapor. Some reflects toward the ground, forming a diffuse background sky glow. Its color ranges from faint yellow to orange red. The glow is brighter near the horizon because you’re looking through more of the atmosphere, and it fades toward the zenith (straight overhead), where you’re looking through less of the atmosphere. This is also why sunsets and sunrises look red. Sky glow is what takes the stars away.
Faint sky glow is as bright as the faintest stars, rendering them invisible to us. As the intensity of the sky glow increases, brighter stars are swallowed up. In the most heavily light-polluted skies, such as in major cities, most of the night sky appears dull orange and only a handful of the brightest stars can shine through. Most constellations are lost. It’s a sad sight for anyone fortunate enough to have experienced a dark sky.
That sadness is not just figurative. Research studies have shown that exposure to excessive artificial light at night suppresses melatonin, a hormone produced by the brain that regulates and supports many vital bodily functions. It’s been linked to increased risk for depression, obesity, heart disease, and other ailments. And of course it interferes with sleep, the disruption of which is also linked with those same diseases and more.
Light pollution also affects wildlife. Building lights can disrupt bird migration. Headlights and streetlights near beaches can lure sea turtle hatchlings toward traffic rather than the sea. Light spilling over wetlands and salt marches affects the breeding behavior of amphibians, which can reduce their populations and have spillover effects onto other animals above and below them in the food chain. And our backyard and parking lot lights can interfere with nocturnal predators and their prey, making both stealth and concealment more difficult. These are just a few examples.
Don’t worry. I am not advocating for shutting off all the lights. We still need lights at night for safety and the essential functions of a 24-hour human civilization. And the occasional light for convenience isn’t going to collapse our ecosystems.
But what is needed is better, smarter illumination. The good news is that better lights are safer, more effective, and cheaper than bad lighting. I plan to return to this subject again with more on efforts underway to protect the night sky, some of it by DarkSky International (a conservation and advocacy organization) and, closer to home, some by rangers and staff of the Cape Cod National Seashore. Maybe even by some of us ordinary appreciators of the stars. Clear skies!
