An alluring image of green streaks in a dark sky above Lake Superior glowed across digital billboards in Chicago, Kansas City and Denver this summer. "HAPPENING TONIGHT: NORTHERN LIGHTS," the message read, crowned with an Explore Minnesota logo.

Northern Minnesota has long been known for its beautiful forests and pristine lakes. Now, marketers and preservationists are promoting an easy-to-spot but less-celebrated gem: its dark skies.

In an age when an estimated 80 percent of people in North America cannot see the Milky Way, obscured by artificial light, dark night skies are taking on new cultural significance. Leaders and advocates in northern Minnesota are seeing the dark not only as a potential boost to tourism, but to human and animal health as well. The digital billboards are just part of a growing movement to promote the state's dark nights as well as preserve them by calling on governments, businesses and residents to curb light pollution.

In Cook County, tourism officials last year started marketing "Dark Sky Season," promoting the Northern Lights and stargazing during the month of December.

This week in Duluth, Starry Skies Lake Superior, the local chapter of the International Dark Sky Association, is hosting its first "Celebrate the Night Sky Week," with photography exhibits, readings and a seminar including the effects of light pollution. It follows an appeal to community leaders over the summer to designate areas as "night-sky friendly."

"As people realize … this is an amazing resource, this is going to continue to grow," said Randy Larson, coordinator of the event. "There's huge opportunity for every community around us. … It's just a matter of turning the lights down and turning them in a way that isn't disruptive."

Promoters partly credit social media for increasing the appreciation of dark skies, and science for providing a better ability to predict the chances of seeing the Northern Lights.

Timing is everything

"In a global sense, it's getting pretty good," said Bob Lysak, a physics professor at the University of Minnesota.

NASA satellites are now monitoring the sun, allowing scientists to see when it ejects a big blob of plasma. If the blob is headed toward Earth, it typically takes about two days to get here, Lysak said. Whether it produces Northern Lights depends on several factors, including the time of day, the location and the direction of the magnetic fields.

Northern Minnesota has been ranked by various groups as one of the best places to see Northern Lights in the Lower 48 states. Capitalizing on that, Explore Minnesota's billboards were timed to run on days when the forecast for Northern Lights was especially good.

"Some of our most popular photos on our Instagram channel have been from our Northern Lights," said Alyssa Hayes, of Explore Minnesota. "It's so stunning. When you capture a really great photo of the Northern Lights, it's a scroll stopper."

At Visit Cook County, marketing manager Kjersti Vick pointed out that maps of light pollution show the Arrowhead region among the darkest in the country, especially east of the Mississippi River.

She said leaders there chose the month of December to promote Northern Lights and stargazing because the sky darkens early, giving people plenty of time to stare into space.

"Being able to look at the stars is one of our big things," Vick said.

Human sensitivity

The Duluth event will also address the human and environmental cost of light pollution, taking into account everything from nocturnal animals to human circadian rhythms.

"I think the bottom line is that every place that people live is getting brighter and no place is getting darker," said Paul Bogard, who wrote a book called "The End of Night" and who will be speaking at the Duluth event this week.

While nobody is asking for complete darkness in populated areas, lighting can be more effective and less harmful, advocates say, by using fixtures that point it downward, for instance, or using lights that are not as bright.

"It's a huge waste of money. All the light that's being sent up into the sky is just wasted light, wasted money, wasted energy," Bogard pointed out. "People who have grown up in the last 30 or 40 years and have never left the city have no idea what a real night sky is supposed to look like. … They think a couple dozen stars is a starry night."

Studying the effects of light on humans and animals has been a hot topic among scientists recently, too.

A research arm of the World Health Organization classified shift work with circadian disruption as a "probable human carcinogen."

Harvard Medical School instructor Shadab Rahman, who's also speaking at the Duluth event, said humans are "very sensitive to light."

Working in an office and getting less light than expected during the day, then getting more than expected at night "works against our biology and contributes to adverse health effects," he said.

Larson said organizers of the Duluth event are already thinking about plans for next year's seminars, hoping an annual event will raise awareness in the state about keeping the nights dark.

"As the rest of the world continues to get brighter," Larson said, "we end up with this amazing resource of being able to see the stars."

Pam Louwagie • 612-673-7102