The Bortle scale is a nine-level rating of how dark the night sky is at a given spot. It runs from Class 1, the darkest skies on Earth, to Class 9, an inner-city sky. Amateur astronomer John E. Bortle published it in the February 2001 issue of Sky & Telescope to help skywatchers judge and compare observing sites.
The Bortle Scale in Brief
The Bortle scale sorts night skies into classes by what a person can see with the naked eye from one spot. Each step from Class 1 to Class 9 adds light pollution and takes faint objects out of view.
Bortle described each class through landmarks in the sky: the Milky Way, the zodiacal light, star clusters, and how clouds look against the background. The classes also line up with two measurements: naked-eye limiting magnitude (NELM) and sky quality meter (SQM) readings of skyglow.
At the bright end of the scale, light domes hang above the horizon and the Milky Way vanishes. Fewer stars and planets show. At the dark end, light domes appear only toward cities or not at all. Stars crowd the sky and the zodiacal light glows with color.
A Class 4.5, the semi-suburban or transition sky, sits between Class 4 and Class 5.
Bortle Class by Class
Each Bortle class has a title, a NELM range, an approximate SQM range and a set of sky markers you can check by eye.
| Class | Title | NELM | Approx. SQM | What you see |
|---|---|---|---|---|
| 1 | Excellent dark-sky site | 7.6 to 8.0 | 21.76 to 22.0 | Colorful zodiacal light; gegenschein and airglow readily visible; Milky Way in Scorpius and Sagittarius casts obvious shadows; M33 visible to the naked eye |
| 2 | Typical truly dark site | 7.1 to 7.5 | 21.6 to 21.75 | Zodiacal light bright enough to cast shadows at dusk and dawn; clouds show as dark holes; summer Milky Way highly structured |
| 3 | Rural sky | 6.6 to 7.0 | 21.3 to 21.6 | Some light pollution at the horizon; clouds lit near the horizon and dark overhead; summer Milky Way still complex |
| 4 | Brighter rural | 6.3 to 6.5 | 20.8 to 21.3 | Light domes in several directions; Milky Way impressive well above the horizon but lacking detail |
| 4.5 | Semi-suburban/transition | 6.1 to 6.3 | 20.3 to 20.8 | Clouds glow gray overhead; Milky Way vague 10 to 15 degrees above the horizon; Great Rift still obvious overhead in good transparency |
| 5 | Suburban sky | 5.6 to 6.0 | 19.25 to 20.3 | Light pollution in most directions; clouds brighter than the sky; Milky Way invisible near the horizon and washed out overhead |
| 6 | Bright suburban sky | 5.1 to 5.5 | 18.5 to 19.25 | No zodiacal light; sky within 35 degrees of the horizon glows grayish white; Milky Way only near the zenith |
| 7 | Suburban/urban transition | 4.6 to 5.0 | 18.00 to 18.5 | Whole sky light gray; clouds brightly lit; Milky Way nearly or totally invisible |
| 8 | City sky | 4.1 to 4.5 | Below 18.00 | Sky light gray or orange and bright enough to read by; familiar constellations weak or missing |
| 9 | Inner-city sky | 4.0 or less | Below 18.00 | Many constellations invisible; the Moon, planets, bright satellites and a few bright star clusters remain |
A wide database of sky quality meter readings lets anyone estimate a Bortle rating for any place in the world.
How to Estimate Your Own Sky
You can estimate your Bortle class by eye on any clear night, using the markers Bortle built the scale around. Work through these checks:
- The Milky Way. Highly structured means Class 2 or darker. Complex but softer points to Class 3. Impressive without detail suggests Class 4. Washed out overhead suggests Class 5. A faint cloud or nothing at all means Class 7 or brighter.
- The clouds. Dark holes against the stars mean Class 2. Clouds lit only near the horizon point to Class 3, while clouds brighter than the sky mean Class 5. Brightly lit clouds belong to Class 7.
- The ground. At Class 2 your surroundings are barely visible as silhouettes. At Class 4 they are clearly visible even at a distance, and at Class 8 you can read easily.
- The horizon. Light domes in several directions mean Class 4. Domes in most or all directions mean Class 5, and an evenly light gray sky means Class 7.
- The constellations. If familiar patterns look weak or broken, you are in Class 8 or Class 9.
A sky quality meter gives your sky a number you can match to the table. A reading above 21.6 points to Class 2 or darker.
Your area's Bortle class can also be estimated before you leave home. The Wikipedia entry for the Bortle scale links an interactive light pollution map you can use for this.
Limits of the Class Descriptions
The darkest class descriptions can promise more than a typical observer will see. A 2014 paper by Andrew Crumey argued that Bortle may have overestimated how visible dim objects are, even under the darkest skies. Diffuse objects such as M33 are harder to see than single stars of the same brightness, so naming M33 as a naked-eye object sets a high bar.
The same paper treats a NELM above 7.1, which Bortle gives for Class 2, as a big step up in expectation. A NELM of 6.0 to 6.5 is the more commonly cited figure. Bortle knew that NELM changes with each observer's eyesight, and he wrote the scale as an alternative to raw NELM.
If you cannot pick out M33 or count very faint stars, lean on the Milky Way, cloud and horizon checks above.
Why the Bortle Scale Matters for Seeing the Milky Way
Your Bortle class decides what kind of Milky Way you will see, from a shadow-casting band at Class 1 to a faint cloud, or nothing, at Class 7.
- Class 1: the Scorpius and Sagittarius regions of the Milky Way cast obvious shadows, and fainter constellations get lost among the stars.
- Class 2: the summer Milky Way is highly structured.
- Class 3: the summer Milky Way still looks complex.
- Class 4: the Milky Way well above the horizon is impressive but lacks detail.
- Class 4.5: the band is only vaguely visible low in the sky, though the Great Rift stays obvious overhead when the air is clear.
- Class 5: the Milky Way disappears near the horizon and looks washed out overhead, and the winter Milky Way is subtle even straight up.
- Class 6: the Milky Way shows only near the zenith.
- Class 7: the Milky Way is nearly or totally invisible and resembles a cloud.
For a Milky Way with visible structure, aim for Class 3 or darker.
The stargazing and dark sky guide covers the rest of the night-sky basics.
Finding Bortle Class 1 and 2 Skies
Bortle published the scale because growing light pollution was putting true dark sites out of reach for many amateur astronomers. Ongoing urbanization and the switch to LED lighting have continued that trend. The higher color temperature of LED lights has been a particular contributor.
For one park in detail, read the guide to stargazing at Bryce Canyon.
At Dark Light Collection, we are building glass pyramids on private land at two sites, opening in 2027. One is about 15 minutes from Bryce Canyon National Park in Utah, and the other is within 15 minutes of Glacier National Park's West Entrance in Montana. Each pyramid has a king bed positioned for sky-gazing and a private deck with a personal telescope. Our page on why dark skies matter has more.
Each pyramid comes with nightly downloadable astrophotography taken from its own coordinates. See what else is planned under Experiences & Adventures.
Frequently asked questions
What Bortle class is the darkest?
Class 1, called an excellent dark-sky site, is the darkest. Its markers include a colorful zodiacal light, a readily visible gegenschein, and Milky Way regions in Scorpius and Sagittarius bright enough to cast shadows.
What Bortle do you need to see the Milky Way?
The Milky Way is still visible near the zenith in a Class 6 sky, and it turns nearly or totally invisible at Class 7. For a summer Milky Way with clear structure, look for Class 3 or darker.
Who created the Bortle scale?
Amateur astronomer John E. Bortle created the scale. He published it in Sky & Telescope in February 2001 to help skywatchers evaluate and compare observing sites.
Sleep Under a Darker Sky in 2027
Founding deposits are open for glass pyramids near Bryce Canyon and Glacier national parks.
Reserve Your Pyramid Why dark sky →
