Light pollution and stargazing

For most of human history, the night sky was a routine experience. Every person who ever lived, until roughly six generations ago, walked outside in the evening and saw the Milky Way arch overhead. They didn’t have to drive anywhere. They didn’t have to plan for the new moon. The galaxy was just there, every clear night, the way trees and clouds are there.

That changed faster than people realize. By 2016, a global study by Falchi and colleagues found that 80% of the world’s population lives under skies polluted by artificial light. In the United States and Western Europe, the figure is over 99%. The Milky Way, for the vast majority of people alive today, is not part of their lived experience. It’s something you have to travel to see.

The three kinds of light pollution

Light pollution is a bag with several different things in it, and the distinctions matter.

The biggest one, the one that hides the Milky Way, is sky glow. Sky glow is the diffuse white-blue haze you see above any city at night. It happens because outdoor lighting — streetlights, billboards, car parks, security lights — sends some of its light upward, where it scatters off particles in the atmosphere and turns the whole sky into a giant glowing dome. It doesn’t matter where the light is pointed originally; once it’s bouncing around in the atmosphere, it ends up over everyone.

The second is glare, which is direct light hitting your eye from a poorly-shielded source. Glare is annoying when you’re driving past a badly-designed streetlamp, but it isn’t what hides the stars. Sky glow does that.

The third is light trespass, which is the neighbor’s security light shining into your bedroom. Annoying but local.

For stargazing, sky glow is the killer.

How much it’s stealing from you

Under a truly dark sky — what astronomers call Bortle 1, no artificial light contribution — a human eye can see roughly 6,500 stars in the sky over the course of a night. The Milky Way casts visible shadows. Andromeda is a small spiral oval to the naked eye, not just a smudge.

Step that down to Bortle 4, which is what you get at the edge of typical European suburbs, and you can see maybe 2,500 stars. The Milky Way is faint but visible. Andromeda is a smudge if you know exactly where to look.

Step it to Bortle 7, typical of large suburbs and small cities, and you’re down to 300 to 500 stars. The Milky Way is gone. Only the brightest constellations are recognizable. Most of the sky is a featureless gray-white wash.

In Bortle 9, the centers of London, Tokyo, or New York, you have maybe 30 stars visible. The Moon, three or four planets, a handful of first-magnitude stars. That’s it.

The progression isn’t subtle. You don’t notice it because you’ve lived under your local Bortle class your whole life and it’s normal. The first time most people see a Bortle 1 or 2 sky, they describe the experience as overwhelming. There are simply too many stars to process at first.

Why blue LEDs made it worse

Around 2010, cities started replacing their old yellow-orange sodium streetlights with white-blue LEDs. The new lamps were more efficient, cheaper to run, and gave nicer-looking light. Astronomers and ecologists argued against the switch on the grounds that blue light scatters in the atmosphere much more than yellow light does, and the result would be a measurable worsening of sky glow even though the lamps were using less power overall.

Astronomers lost that argument almost everywhere, and the prediction has come true. A satellite study by Sánchez de Miguel and colleagues, published in 2023, found that global sky brightness rose by 9.6% per year between 2011 and 2022. At that pace, a child born in a Bortle 5 neighborhood will live in Bortle 7 conditions by the time they’re a teenager. The Milky Way is receding from human view at a measurable rate.

The good news

Light pollution is, unlike almost every other environmental problem, completely reversible. You don’t need to wait fifty years for a generation of trees to grow back or a fishery to recover. You just turn the lights off, or shield them better, and the very next night the stars return.

A few cities have done this. The town of Flagstaff, Arizona mandated shielded, warm-color lighting in 1958 to protect the nearby Lowell Observatory; the result is a fast-growing city of 70,000 that maintains Bortle 4 skies in its center. Tucson did the same in the 1970s. Several Italian regions, Slovenia nationally, and various French départements have ordinances on the books. They work.

What you can do, personally and politically:

Personally, the biggest single thing is to drive somewhere darker when you want to see stars. A 30 km trip in the right direction usually drops you from Bortle 7 to Bortle 4, which is a tenfold improvement in visible detail. The next biggest is to use red light only when you’re observing, because red light doesn’t disrupt your dark adaptation the way white light does.

Politically: support shielded-lighting ordinances in your municipality, support warm-color LED mandates (3000K or warmer, not 5000K daylight-white), and support dark sky preserves. The International Dark-Sky Association tracks ordinances and has templates city councils can use.

What SkyMinute does about it

We don’t currently factor your local Bortle class into the stargazing score, which is a limitation we’re aware of. A “Magic night” rating from a Bortle 7 suburb will not look like a “Magic night” from a Bortle 3 site, even though both got a 5/5 rating from us. The clear-sky forecast is the same; the dark sky is not.

The fix, for now, is to combine our forecast with a drive. If we tell you tonight is a 5/5 night for clouds and there’s no bright moon, that’s the night to take the car somewhere dark. It really does make a much bigger difference than people expect.