The Bortle scale: how dark is your sky really?

Anyone standing under a truly dark sky for the first time gets a small shock. The Milky Way is not a pale band but a three-dimensional structure with detail, dark clouds and glowing zones. Andromeda is clearly visible to the naked eye as a distinct patch. The sky feels heavier, fuller, more real.

Anyone who grew up in a large German city, on the other hand, may not know this sight at all. For them, night is an orange lit-up sky with twenty visible stars. The difference has a name and a number: the Bortle scale.

What is the Bortle scale?

The Bortle scale was developed in 2001 by the American amateur astronomer John E. Bortle and published in the magazine Sky & Telescope. It divides sky darkness into nine classes, from class 1 (absolutely dark, practically deserted) to class 9 (inner city, sky lit up orange).

Before the Bortle scale there was no common language for sky darkness. Astronomers said things like "good sky" or "so-so" and meant completely different conditions. Bortle changed that by defining concrete visual criteria for each class.

The nine classes at a glance

Class 1: absolutely dark sky

The holy grail of observers. Zodiacal light is so bright that it casts shadows. The gegenschein is visible to the naked eye. The Milky Way casts measurable shadows. Such sites barely exist in Europe any more; you have to go to the Sahara, to Scandinavia or to the Canary Islands.

Class 2: true dark sky

Zodiacal light clearly visible, Milky Way complexly structured and rich in detail. M33, the Triangulum Galaxy, is visible to the naked eye. In Germany you find such conditions at best in the low mountain ranges far from towns, and even then only on the best nights.

Class 3: rural sky

Faint glow at the horizon towards towns. Milky Way still impressive, many faint structures visible. A realistic target for astrophotographers willing to drive an hour.

Class 4: rural to suburban

Light domes over towns already clearly visible, but the zenith still fairly dark. Milky Way well visible, fainter objects require more effort. For many hobby astronomers in Germany the best reachable class.

Class 5: suburban sky (default)

The SkyGrade default and for many users the most realistic estimate. Milky Way faint but visible, light domes dominate the horizon. Deep-sky objects are imageable, but the brightened background requires longer exposures and good post-processing.

Class 6: bright suburban sky

Milky Way visible only on the best clear nights, light domes everywhere. Many faint objects barely recognisable in the eyepiece. Astrophotography is still possible, but the sky background increasingly dominates the image.

Class 7: suburban to urban

Milky Way practically invisible. Bright Messier objects still doable, fainter objects vanish in the noise. Moon and planets work well, deep-sky imaging becomes a challenge.

Class 8: urban sky

The sky is grey or orange. Only the brightest objects are imageable. Planetary imaging still makes sense, deep-sky only with considerable effort and special narrowband filters that filter out city light.

Class 9: inner city

Only the Moon, planets and the brightest stars are visible. The sky glows bright orange or white. Astrophotography in the classic sense is barely possible. But the pizza around the corner is probably good.

Bortle class and SkyGrade

In SkyGrade you can enter the Bortle class of your location manually. It feeds directly into the deep-sky score: a dark site with class 2 gets clearly more points than the same site with class 7, even with identical weather and seeing.

Don't know your Bortle class? No problem. On lightpollutionmap.info you can click any point on the map and read off the Bortle class. The map is based on satellite data and is surprisingly accurate.

Practical tips

• Drive out. Just 20 to 30 kilometres out of the city can mean a difference of two to three Bortle classes
• Use lightpollutionmap.info to find dark spots in your region
• Narrowband filters (H-alpha, OIII) can make the difference in the city: they filter out artificial light and let only certain wavelengths through
• Moon and planets are barely affected by the Bortle class: they are worth it even from the city
• Enter your real Bortle class in SkyGrade, not the optimistic guess, then the rating is more honest

Further reading

• What is seeing? – besides light pollution, seeing is the most important factor for sharp astrophotos.
• Transparency: why a cloudless sky can still be poor – light pollution and haze reinforce each other.
• Astronomical twilight: when does real night begin? – even the darkest sky is useless if the Sun is still too high.