Hey, smart people, Joe here.

(Describer) ...at sunset.

For pretty much all of human history, this has meant the end of our day. Sure, we harnessed fire and some artificial light, but we are not natural creatures of the night. But that all changed with one invention that has allowed us to be a part of the night like never before in human history, the light bulb. Try to imagine modern life without artificial light. It just isn't possible. But all of that light comes with a dark side.

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(Describer) Title: McDonald Observatory. In fast-motion, the domed roof opens as clouds pass and the sky darkens.

(Describer) Title: It's Okay to Be Smart.

On January 17, 1994, a powerful earthquake struck the Los Angeles area and caused a massive blackout. Nearby Griffith Observatory started receiving calls from residents asking about the strange sky they were seeing. What those people saw was the Milky Way. With no artificial light, the sight of the night sky was so unfamiliar, they didn't know what they were looking at. Today, more than 80% of the world and more than 99% of the U.S. and Europe live under a light-polluted skies. A third of humans on Earth can never see the Milky Way. And places like Singapore are so polluted by light that people's eyes never fully adjust to the dark. While researching light pollution the past couple months, I learned a new word, scotopic. It's the type of vision that we use in very low light levels. Whereas our normal bright light photopic vision is produced by three types of color-sensitive cone cells, dark scotopic vision is produced by the eyes' rod cells, which are great at sensing something's brightness, but can't discriminate different colors. Anyway, the reason I'd never heard of scotopic vision before is that most of us don't experience it much. Night has been taken over by light. It still gets dark at night. Unless you're near the North or South Pole in the summertime, the sun still goes down every day, but it's not real darkness. So then, what is real darkness? I've been struggling for a way to explain it. Because how do you describe the absence of something? Well, I figure you don't try to describe what's missing, you look at what was hiding there all along.

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(Describer) Dots of light fill a night sky, some grouped more densely together.

Now I'm not the world's best astrophotographer, but I've been lucky enough to take pictures in some of the darkest places left in North America, like Big Bend National Park, the Grand Canyon in Arizona, and here at McDonald Observatory in West Texas.

(Describer) The Milky Way passes.

And while I was out there, I met someone who's trying to save darkness. I'm Bill Wren, Special Assistant to the Superintendent at the University of Texas, Austin, McDonald Observatory. And my job responsibility is to keep the skies dark for ongoing astronomical research here at the observatory. The places where you can go to see a naturally dark starry sky are vanishing. They're shrinking. They're becoming fewer and farther between. You have to travel great distances from major cities in order to be able to see a naturally dark sky.

JOE: An amateur astronomer named John Bortle came up with a scale to measure the night sky brightness based on how many objects are visible. In perfectly dark skies, Bortle Scale of 1, there's between maybe 4,000 and 5,000 stars bright enough to be visible to the naked eye from any spot on Earth. I used some software called Stellarium to give you an idea of what that looks like. It's actually hard to even pick out constellations. But most Americans live in Bortle Scale 5 or higher, which means they aren't seeing 98% of the stars in the sky.

WREN: We see people all the time at our public star parties that have never seen the Milky Way and are just awe-inspired. Now the beauty of the night sky is one thing, but there's a bunch of other reasons that we should protect dark skies. For astronomy, I guess it's pretty straightforward. We need to be able to see the stars in the sky to be able to do astronomical research. And there's other questions about the exposure to too much artificial light at night not being so good for your health. And in fact, it affects the biorhythms of all living creatures on the planet. There's a cost-efficiency question in terms of how much light we're wasting into the night sky by poorly-designed, poorly- installed light fixtures. On the order of billions of dollars' worth of electricity costs alone are wasted into the night sky in the United States alone.

JOE: We've lit up streets, parking lots, buildings in every populated space we can, mostly to make the night more safe. So when you hear people saying that we should use less light at night, your first reaction might be, "Well, that'll make us less safe, right?" But as hard as it is to believe, no study has ever shown that more light leads to less crime. Most property crime occurs during the day. And even the worst crimes, like sexual assaults, that we normally associate with bad guys in dark alleys, are far more likely to occur indoors at the hands of someone the victim knows. And bad lighting can actually make bad guys harder to see. More light at night can actually make us less healthy too. For hundreds of thousands of years, humans evolved with the rhythms of night and day. Like other creatures, we have a natural biological clock, our circadian rhythm, that's controlled by the cycle of light and dark. In darkness, our bodies produce a hormone called melatonin that helps us sleep, it boosts our immune system, and helps a bunch of our organs function correctly. Light at night, especially blue light, can mess with that. And LED lighting, while it saves energy and money, often peaks in the blue part of the spectrum, making this problem worse. Night work has even been classified by some medical groups as a risk factor for many cancers. So consider putting this away at night. And if you do have to look at a screen, use night mode or an app to reduce the blue light given off by your screen. The more that scientists study this, the more it looks like we've underestimated the negative effects of light, that it truly is a pollutant and its effect on humans and on wildlife. There are countless wild species being negatively affected by our light pollution, whether they're nocturnal-- active at night, or crepuscular-- active during twilight. Everything from fireflies that can't find mates to dung beetles who can no longer navigate by the Milky Way, and from baby sea turtles when walking into roads instead of the sea, to the millions of birds killed every year in collisions with buildings.

(Describer) An animated bird crashes into a building.

Every September 11 in lower Manhattan, 88 7,000-watt searchlights shine into the night sky as a tribute in light. It's one of the brightest light installations ever constructed. And even though it's only on for one night a year, over a million birds have been lured in by these lights, disrupting their annual migrations, and many have died after colliding with buildings-- that is until scientists started working with the people running the tribute in light to keep that from happening. Now, if more than 1,000 birds are counted in the lights, they're turned off for 20 minutes. You can watch on this radar image as the lights alternate between on and off, and huge clouds of disoriented birds go safely on their way. I think this is an inspiring example of how humans and nature can coexist in a world of artificial light. Because stopping light pollution doesn't mean getting rid of all artificial light. That's crazy. Light pollution is not all light at night. It's the light out of place. The reason satellite views of the dark side of our planet look like this is because we're wasting that light by shining it up into space instead of using it to light our way down here.

(Describer) Earth light appears from space.

Light pollution is unique because it's the only kind of pollution we can clean up instantly. Polluted water and air take decades to cleanse them of human impact. But cleaning up the night skies is easy. Just turn out the light.

(Describer) The hazy Milky Way peeks out from behind a tree. Wren:

WREN: Definitely a sense of awe to stand, and look up at the Milky Way, and see the stars splashed across the sky, and realize the three-dimensional depth that you can see when you look into the plane of the galaxy. Just the scale that surrounds us, the dimensions of the universe on the grand scale is awe-inspiring. I do believe that seeing a naturally dark sky provides one with a sense of context, a sense of living in a very large space and being part of something on a very grand scale. And it does give one sometimes a chill up the spine to realize that we came out of this universe as opposed to being put into it. But we won't get too philosophical here. Well, Bill may not want to get philosophical, but I think that that beautifully sums up why dark is just as important to our lives as the light. Now, many of the beautiful shots of the night sky you've seen in this video are time-lapses. They're made with long exposure photos that show you a bit more than you can see with your own eyes. But I wanted to give you some idea of what you really can see with the naked eye under those dark skies that so few of us get to experience. And while it's not the crispiest shot in the world, I hope it captures a feeling for you.

(Describer) Outside...

I've been trying to think of a way to describe what it is like to see this, and it's really hard to come up with the words. It is something that you just have to see, and that's something that I hope that people still get the chance to see. It makes you feel small and big at the same time. It makes you feel far away and connected at the same time. And all I know for sure is it's good for you. Stay curious.

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(Describer) Sparkling dots of light and slight purple haze looms above and behind him. Title: PBS Digital Studios Accessibility provided by the US Department of Education.