Light pollution and the Bortle scale

7 min read

Key facts

What it is	Artificial light that shines where it is not needed, mostly upward or sideways
People under light-polluted skies	More than 80% of the world's population (2016 atlas)
People who cannot see the Milky Way	About one in three worldwide; about 80% of North Americans
Sky brightening trend	About 10% brighter per year (2011–2022 citizen-science counts)
Bortle scale	9 classes, from 1 (excellent dark site) to 9 (inner city)
Created by	Amateur astronomer John Bortle, published in Sky & Telescope in 2001
Dark sky certification	DarkSky International's International Dark Sky Places program, since 2001
Best fix	Fully shielded, warm-coloured, dimmable lights that are on only when needed

In short

Light pollution is wasted outdoor light that brightens the night sky and hides faint stars.
The Bortle scale rates a site's darkness from class 1 (pristine) to class 9 (inner city).
Artificial light at night disturbs birds, insects, sea turtles and our own sleep cycles.
Certified dark sky places protect natural night skies, and more than 200 exist worldwide as of 2026.
Shielding lights, using warmer colours and switching off unneeded lamps make a real difference quickly.

What light pollution is

Light pollution is outdoor artificial light that goes where it is not wanted. Streetlights, billboards, sports fields and security lamps send part of their light upward or sideways instead of onto the ground.

That stray light scatters off air molecules, dust and water droplets, forming a glowing dome over towns called skyglow. Glare (light shining straight into your eyes) and light trespass (light spilling onto a neighbour's property) are its close relatives.

Skyglow can be seen from more than 100 miles away from a large city. It is the main reason most people today never see the Milky Way from home.

The Bortle scale, class by class

In 2001 John Bortle proposed a nine-step scale so observers could describe how dark a site really is. It is judged by eye, using things like the Milky Way, faint glows and the faintest visible stars.

Class 1 is an excellent dark site: the Milky Way casts faint shadows, the zodiacal light is obvious, and the ground around you is almost invisible. Class 2 is a truly dark site, with the summer Milky Way showing detailed, marbled structure. Class 3 is a rural sky, where a little glow sits on the horizon but the Milky Way still looks rich.

Class 4 marks the rural–suburban transition: domes of light hang over distant towns and the Milky Way is visible but lacks detail. Class 5 is a suburban sky, where the Milky Way is weak or gone near the horizon. Class 6 is a bright suburban sky, with the Milky Way seen only overhead, if at all.

Class 7 is the suburban–urban transition: the whole sky has a greyish glow. Class 8 is a city sky, where only the brighter constellations stand out. Class 9 is an inner-city sky, where you may see just the Moon, the planets and a few dozen stars.

Effects on wildlife, health and energy

Many animals rely on natural darkness. Migrating birds can be drawn to lit buildings and collide with them, newly hatched sea turtles may crawl toward street lights instead of the sea, and huge numbers of insects die circling lamps.

In people, light at night can suppress melatonin, a hormone that helps regulate sleep. Researchers are still studying the long-term health links, but brighter, bluer light at night is the main concern.

Light that shines into the sky lights nothing useful, so it is also wasted energy and money. Poorly aimed lighting can even make streets less safe by creating glare and deep shadows.

Dark sky places

DarkSky International, formerly the International Dark-Sky Association, runs the International Dark Sky Places program. Flagstaff, Arizona, became the first International Dark Sky City in 2001.

Places can be certified as communities, parks, reserves, sanctuaries or urban night sky places. Each one commits to good lighting rules and public education, and as of 2026 there are more than 200 of them across several continents.

These sites are some of the best places to see a Bortle class 1–3 sky. Many national parks also run stargazing events and keep their lighting low and shielded.

How to find a darker sky near you

Online light pollution maps, built from satellite data, show the brightness of the sky around you. Look for areas well away from city domes, ideally on the side of town away from the largest city.

You can check your own sky by eye. After about 20 minutes in the dark, compare what you see with the Bortle descriptions above, or count the stars inside a known constellation.

Even a short drive can improve your sky by one or two Bortle classes. Pick a moonless night, since a bright Moon washes out the sky almost as much as a town.

What you can do

The fixes are simple and work immediately. Use fully shielded fixtures that point all light down, so the bulb itself is hidden from the side.

Choose warmer light (around 3000 K or lower), which scatters less in the sky and is gentler on wildlife. Use only as much brightness as you need, and fit timers or motion sensors so lights turn off when nobody is around.

You can also support local lighting rules, take part in citizen-science star counts such as Globe at Night, and talk with neighbours about aiming their lights down.