The short answer
The Bortle scale rates how dark the night sky is at a given place, from class 1 (an excellent dark site, far from any town) to class 9 (an inner city sky). It was introduced by the amateur astronomer John Bortle in 2001 to give observers a shared way to describe their skies. Each class is defined by what you can actually see: how faint the faintest stars are, whether the Milky Way shows structure, and how much glow sits on the horizon.
The nine classes
The limiting magnitudes below are approximate values from Bortle’s original descriptions, for an experienced observer with fully dark-adapted eyes and no Moon. Most people, on most nights, will see a little less.
| Class | Description | Faintest naked-eye stars (approx.) |
|---|---|---|
| 1 | Excellent dark site | 7.6–8.0 |
| 2 | Typical truly dark site | 7.1–7.5 |
| 3 | Rural sky | 6.6–7.0 |
| 4 | Rural/suburban transition | 6.1–6.5 |
| 5 | Suburban sky | 5.6–6.0 |
| 6 | Bright suburban sky | 5.1–5.5 |
| 7 | Suburban/urban transition | 4.6–5.0 |
| 8 | City sky | 4.1–4.5 |
| 9 | Inner-city sky | 4.0 at best |
Magnitudes run backwards, so a higher number means fainter stars are visible. If the numbers are new to you, what does magnitude mean? explains the scale.
What changes from class to class
- Classes 1–2. The Milky Way is so detailed that it can cast faint shadows in class 1, and its dark lanes are obvious. Faint glows such as the zodiacal light are easy. Clouds look like black holes in the starfield, because there is no light below to illuminate them.
- Class 3. Some light domes appear on the horizon, but the Milky Way still shows plenty of structure and the Andromeda Galaxy is easy to the eye.
- Class 4. Light domes from towns are visible in several directions. The Milky Way is still impressive overhead but loses detail toward the horizon.
- Class 5. The Milky Way is weak or invisible near the horizon and looks washed out overhead. Clouds are noticeably brighter than the sky.
- Class 6. The Milky Way is only faintly visible near the zenith, if at all. The sky has a gray glow.
- Classes 7–9. The Milky Way is gone. The sky is gray or orange, and only the brighter stars and constellations remain. The Moon, planets and bright clusters such as the Pleiades are still good targets.
How to estimate your own Bortle class
There is no meter that reads out a Bortle number directly; it’s a judgment based on what you see. A few ways to get close:
- Check the Milky Way. On a clear, moonless night after full darkness, with your eyes adapted, is the Milky Way visible? Does it show structure, or only a faint band overhead? This alone separates the dark, middle and bright ends of the scale.
- Find your limiting magnitude. Pick a well-known constellation high in the sky, such as Ursa Minor or Cassiopeia, and compare the faintest stars you can see with a chart that shows magnitudes.
- Look at a light-pollution map. Satellite-based maps give a good first estimate for any location, though local conditions can differ.
- Use a sky quality meter. These measure sky brightness in magnitudes per square arcsecond. Higher readings are darker, and the darkest sites on Earth read around 22. Readings can be converted to an approximate Bortle class.
What the scale doesn’t tell you
A Bortle class describes the sky on a good night, without the Moon. On any particular night, other things can make it look worse:
- Moonlight can turn a class 3 sky into something closer to a suburb. See how moonlight affects stargazing.
- Twilight: the sky isn’t at its darkest until astronomical twilight ends. See what is astronomical twilight.
- Haze and humidity scatter city light, brightening the sky well beyond its usual level. See seeing and transparency.
- Direction: a site can be class 4 toward the countryside and class 6 toward the nearest town.
Using it to plan
The Bortle scale is most useful for matching targets to your sky. Under classes 7–9, focus on the Moon, planets, double stars and bright clusters; stargazing from a city has a full list. Classes 4–6 open up many star clusters and the brighter galaxies and nebulae, especially with binoculars. Classes 1–3 are where the Milky Way and faint deep-sky objects shine; browse the deep-sky catalog for ideas.
If you live under a bright sky, even a modest drive can help: light pollution falls off quickly with distance from a town, and a site with a dark horizon in the direction you want to observe can make a bigger difference than its overall class.