Light pollution and the Bortle scale: how dark is your sky?

A sky most people have never seen

Artificial light at night scatters in the atmosphere and makes the whole sky glow. The 2016 World Atlas of artificial sky brightness found that more than 80% of the world's population, 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 a third of humanity.

And it is getting brighter. A study of citizen-science star counts published in 2023 estimated that the visible night sky brightened by roughly 10% per year between 2011 and 2022, much faster than satellite measurements had suggested, partly because satellites miss the blue light of modern LEDs.

The Bortle scale in plain words

In 2001 amateur astronomer John Bortle described nine classes of sky by what you can see in them:

• Class 1–2, excellent dark site: thousands of stars, the Milky Way is so bright it looks structured and can cast faint shadows, and the zodiacal light is obvious. Rare; usually far from any town.
• Class 3, rural: the Milky Way is still detailed, with some glow from distant towns on the horizon.
• Class 4, rural/suburban transition: the Milky Way is clearly visible overhead but fades toward the horizon; towns show as domes of light.
• Class 5, suburban: the Milky Way is weak and washed out near the horizon.
• Class 6–7, bright suburban to suburban/urban transition: the Milky Way is barely visible or gone; the sky is greyish.
• Class 8–9, city and inner city: only the Moon, planets, the brightest stars and a few constellations remain.

Measure your own sky with your eyes

The simplest method is to count stars inside a known constellation. Projects such as Globe at Night ask you to compare what you see in Orion or another seasonal constellation with a set of charts showing the sky at different limiting magnitudes. In a city you might see six stars in Orion; at a good dark site, dozens.

For a number, a Sky Quality Meter (SQM) reads the sky's brightness in magnitudes per square arcsecond: around 22 at the darkest sites on Earth, about 19 in a suburb, and 17 or less in a city. Phone apps can estimate it, though less reliably.

How far to drive

Light domes from a big city reach surprisingly far: a city of a million people can brighten the horizon 50–100 km away. A rough rule is to put a range of hills or 50 km of countryside between you and the nearest large town, and to choose the side of town away from where you want to look.

Light-pollution maps, built from satellite measurements of light at night, show where the darker patches are. Combine them with a clear, moonless night and you will see a different sky.

What you can still enjoy from a city

Light pollution hides faint things, not bright ones. From a city you can still follow the Moon, all five bright planets, the brightest stars, ISS passes and bright satellite flares, and the brightest meteors. Binoculars help a lot: they pull hundreds of stars back out of the glow.

Small things that help

• At home: shield outdoor lights so they shine down, use warm-coloured (amber) bulbs, and put them on timers or motion sensors.
• Support dark-sky places: dark-sky parks and reserves protect some of the best skies left, and many are open to visitors at night.
• Report what you see: star-count projects build the long-term record of how fast our skies are changing.