Why Can't You See Many Stars From a Bright City?

1. City Lights Brighten the Air, and the Faint Stars Get Lost in It

The U.S. National Park Service defines light pollution as excess or inappropriate artificial light outdoors, and says it shows up in three ways: glare, light trespass, and skyglow[1]. Glare is light shining straight into your eyes. Light trespass is light spilling onto someone else's property, such as a streetlight shining into a bedroom. The third one, skyglow, is the one that steals the stars. It is the brightening of the night sky by human-made light that has scattered in the atmosphere[1].

Some of the light from lamps, signs, and windows travels upward or sideways, and some of it scatters off particles in the air and spreads across the sky. The result is a faint glow over the whole sky. The park service says the scattered light lowers the contrast, which makes stars and planets harder to see. It adds that the glare and glow also stop your eyes from fully adapting to the dark[1].

A simple way to picture it is a flashlight. Its beam is easy to see on a dark night and nearly invisible at noon. The beam did not get weaker. The background got brighter. A faint star is like a very dim flashlight far away, so it gets lost first.

2. More Than Four in Five People Live Under a Sky Brightened by Artificial Light

In 2016 an international team led by Fabio Falchi published a world atlas of artificial sky brightness in Science Advances. It combined satellite images of city lights, a computer model of how light spreads in the air, and sky-brightness readings taken on the ground[2]. About a fifth of the ground readings used to check the model came from citizen scientists with inexpensive handheld meters[3].

The authors report that more than 80% of the world's people, and more than 99% of people in the United States and Europe, live under light-polluted skies. The Milky Way is hidden from more than one-third of humanity, including 60% of Europeans and nearly 80% of North Americans[2]. For comparison, the paper puts the natural brightness of a clear, moonless sky far from the Milky Way at about magnitude 22 per square arcsecond, a standard way astronomers measure how bright a patch of sky is[2].

The atlas also shows that the damage is uneven. In Western Europe, only a few small areas still have relatively unpolluted skies, including parts of Scotland, Sweden, Norway, Spain, and Austria[3]. Light pollution is not only a big-city problem, but the darkest places are now rare.

3. The Glow of a City Can Be Seen Hundreds of Miles Away

You do not have to live in a city to lose some of the sky. The National Park Service says the glow from cities has been documented at distances of more than 200 miles (about 320 kilometers) from national parks[1]. In a very dark place, the sky straight overhead may look fine while the horizon still shows a bright dome of light from a faraway town[1].

The air itself matters too. Dust, soot, and other particles, along with clouds, scatter artificial light further and make the sky even brighter[1]. Cleaner air does not fix the problem by itself, but hazy nights make it worse.

The trend is upward. The park service reports that from 2012 to 2016, satellites showed Earth's artificially lit outdoor area growing by 2.2% a year, and that citizen-science measurements suggest that the measured night skies became brighter by an average of 9.6% a year from 2011 to 2022, which would double the brightness in about eight years[1]. The two kinds of measurement count different things, so the numbers should not be treated as the same rate. The park service also notes exceptions: places that created and enforced lighting rules, such as Tucson and Flagstaff in Arizona[1].

4. Artificial Light Can Mislead Baby Sea Turtles Heading for the Ocean

Skyglow is not only a problem for stargazers. Researchers at the University of Western Australia and partners tracked green turtle hatchlings swimming near the shore at Ningaloo Reef in Western Australia. They found that 90% of the tracked hatchlings swam toward artificial light, apparently mistaking it for the moon, and lost their way on the journey offshore[4]. The lead scientist noted that it was already widely known that lights near nesting beaches attract hatchlings emerging from nests and can make it hard for them to find the sea. This study looked at what happens once they are in the water[4].

The National Park Service adds that light pollution also disrupts ecosystems, wastes energy, and can have adverse health effects in humans, including through the body clock that dark nights help keep in sync[1]. Turtles are one clear example of why scientists treat the night as a habitat, not just a view.

5. Aiming Light Where It Is Needed Can Bring Some of the Dark Back

Outdoor lighting is useful. The park service says it is often necessary for a productive modern society[1]. The aim is to waste less of it, and to keep it from lighting the sky and the neighbors' windows.

Falchi's team shows that this is not automatic. As the world switches to LED lights, they warned that switching to LEDs without careful thought about their color and brightness could lead to a two- to threefold increase in skyglow on clear nights[3]. Better, cheaper bulbs do not help the sky if more of them are installed and they are aimed carelessly.

There is evidence that rules can work. According to the National Park Service, the cities of Tucson and Flagstaff in Arizona, and parts of northern Italy, did not follow the fast rise in light pollution, and the common thread is that lighting regulations were created and enforced[1]. The sources we read do not say exactly which rules made the difference, so that is a good thing to look up yourself.

6. Count Stars on Different Nights to Feel the Difference Yourself

With an adult, pick one patch of sky, such as the area around a bright constellation you can recognize, and count the stars you can see in it. Let your eyes adjust for a few minutes first, and avoid looking at your phone, since bright light undoes the adjustment. Write down the place, date, and whether it was clear. With an adult, try again somewhere with fewer lights, such as a nearby park or a relative's house outside town. Your counts will be rough, but the difference is often large.

Stay with an adult, and do not go onto dark roads or unfamiliar places to find a better view. Check which lights near your home point upward or sideways and which are shielded. The sources below, especially the National Park Service pages and the 2016 atlas, are the best next step. One open question from our reading: how much do the color and direction of LED lights change the glow you see? It would be a good one to chase.

Sources

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