The Bortle Scale: How Dark Is Your Sky?

A dark, star-filled sky is rarer than most people think. A 2016 study of artificial sky brightness by Fabio Falchi and colleagues found that more than 99% of people in the US live under light-polluted skies, and that nearly 80% of North Americans can’t see the Milky Way from where they live. The Bortle scale gives you a way to describe where your own sky sits, and a goal to aim for when you look for a darker one.

Where the scale comes from

John E. Bortle published his Dark-Sky Scale in Sky & Telescope magazine in February 2001. He wanted to give observers a common language for sky quality. Instead of a single number, each of his nine classes describes what you can actually see: how much of the Milky Way shows, how faint a star you can make out, how bright the clouds look and how much glow there is on the horizon.

Bortle himself argued that the faintest star you can see, the naked-eye limiting magnitude, is a poor measure on its own, because it depends on your eyesight and how hard you try. (Magnitude is the astronomer’s brightness scale, in which larger numbers are fainter.) He judged a sky by the whole picture.

The nine classes

The descriptions below are summarized from Bortle’s original article. The limiting magnitudes are his rough figures for a typical observer.

Class	Name	Faintest star (approx.)	What you see
1	Excellent dark-sky site	7.6–8.0	The Milky Way is bright enough to cast shadows. Zodiacal light (a faint glow along the Sun’s path) is obvious, and even the faint glow opposite the Sun, the gegenschein, shows.
2	Typical truly dark site	7.1–7.5	The summer Milky Way is full of structure. Clouds appear only as dark holes against the stars.
3	Rural sky	6.6–7.0	Some light glow on the horizon, but the Milky Way still looks complex.
4	Rural/suburban transition	6.1–6.5	Light domes over towns are obvious. The Milky Way is still impressive but lacks fine detail.
5	Suburban sky	5.6–6.0	The Milky Way is weak near the horizon and washed out overhead. Clouds look brighter than the sky.
6	Bright suburban sky	about 5.5	The Milky Way is visible only near the zenith (straight up). The sky near the horizon is grayish white.
7	Suburban/urban transition	about 5.0	The Milky Way is invisible or nearly so.
8	City sky	4.5 at best	The sky glows so brightly you could read newspaper headlines by it.
9	Inner-city sky	4.0 or less	Only the Moon, planets and a few bright star clusters are worth looking at.

How to estimate your own class

You can get a rough idea on any clear, moonless night:

1. Let your eyes adjust for at least 20 minutes, away from direct lights. (See our dark adaptation guide.)
2. Look straight up. Can you see the Milky Way? If you can’t see it at all, you’re probably in class 7 or brighter.
3. If you can see it, is it a faint band only overhead (class 6), a band that fades toward the horizon (class 5), or a detailed river of light with dark lanes (class 3 or darker)?
4. Look at the horizon. Glowing domes over nearby towns point to classes 4 and 5. A horizon dark all the way around points to class 3 or better.
5. Look at any clouds. Clouds lit from below, brighter than the sky, mean a suburban sky. Clouds that appear as dark gaps in the stars mean a dark one.

Remember that your class can change from night to night and from one direction to another. Bortle’s own descriptions differ by direction, because a city on one horizon brightens that side of the sky far more than the other.

Bortle, SQM and other measures

The Bortle scale is visual and descriptive. For measured numbers, the National Park Service’s night-sky program uses several metrics:

• Sky Quality Meter (SQM) readings. A handheld meter measures the brightness of the sky overhead in magnitudes per square arcsecond. Higher numbers mean a darker sky, and the NPS gives about 22 as the darkest natural value.
• Naked-eye limiting magnitude. The NPS treats about 7.0 as good and 7.4 as excellent, while values below 6.3 mean the sky is degraded.
• All-sky light pollution ratio. This compares artificial light with natural sky brightness across the whole sky. Below 0.3 is excellent; above 2 means the natural sky is not readily visible.

The NPS does not publish an official conversion between Bortle classes and SQM readings. You’ll find tables online that match them up, but treat them as approximate.

How to find a darker sky

Light pollution falls off with distance from towns, so a darker sky is often closer than you think. Some practical steps:

• Head away from the glow. Drive away from the biggest city nearby, and avoid ending up next to a smaller town’s lights.
• Put hills or distance between you and town lights. A ridge or a valley that blocks the view of a city’s light dome helps.
• Look for certified dark-sky places. DarkSky International certifies parks, reserves, sanctuaries and communities that protect their night skies. Our dark sky page lists the certified places in each state, and every city page under tonight shows the nearest ones.
• Choose your direction. At a site with a town on one side, face the other way.
• Pick a moonless night. The Moon brightens the whole sky, and a long drive to a dark site is wasted under a full moon. Our best time to stargaze guide has more on timing.
• Check the weather and the air. Haze, humidity and smoke scatter light and brighten the sky, especially near the horizon.

Why it matters for what you see

The Bortle class sets the ceiling for almost everything. NASA’s 2021 estimates for the Perseid meteor shower, for example, range from about 40 meteors an hour under dark skies to one or two from inside a city. The same is true of the Milky Way, faint galaxies, the zodiacal light and the northern lights. The Moon, the planets and the brightest stars hold up in any sky, which is why they make good targets from the city while you plan a trip somewhere darker.