What the Bortle scale is
The scale was proposed in 2001 by the American amateur astronomer John E. Bortle in Sky & Telescope magazine. It describes the darkness of the night sky in nine classes, using things that any observer can check by eye: how many stars are visible, whether the Milky Way shows structure, whether faint glows such as the zodiacal light can be seen.
The 9 classes
The "limiting magnitude" is the faintest star you can see with the naked eye: the higher the number, the fainter the stars you can see.
| Class | Sky | Naked-eye limiting magnitude | What you see |
|---|---|---|---|
| 1 | Excellent dark site | 7.6–8.0 | the Milky Way casts shadows; the zodiacal light is striking |
| 2 | Typical truly dark site | 7.1–7.5 | the Milky Way is highly structured, like veined marble |
| 3 | Rural sky | 6.6–7.0 | some light domes on the horizon; Milky Way still complex |
| 4 | Rural/suburban transition | 6.1–6.5 | light domes in several directions; Milky Way visible but less detailed |
| 5 | Suburban sky | 5.6–6.0 | Milky Way weak, washed out near the horizon |
| 6 | Bright suburban sky | 5.1–5.5 | Milky Way only near the zenith, if at all |
| 7 | Suburban/urban transition | 4.6–5.0 | grayish sky; Milky Way invisible |
| 8 | City sky | 4.1–4.5 | only the main constellations |
| 9 | Inner-city sky | 4.0 at best | the Moon, the planets and a few bright stars |
Why it matters
According to the World Atlas of Artificial Night Sky Brightness published in 2016 by Fabio Falchi and colleagues, the Milky Way is hidden for about one third of humanity, and Italy is among the countries with the most widespread light pollution. Moving from a class 7 to a class 3 sky means going from a few hundred visible stars to thousands, and seeing the Milky Way as a river of light instead of nothing at all.
How to find a dark sky
- Move away from cities: light domes are visible for tens of kilometers. Look for places with mountains or hills between you and the towns.
- Go up: high plateaus and mountain passes often have cleaner, drier air.
- Choose the right night: the days around the new Moon are the darkest. A full Moon brightens the sky as much as a few classes of light pollution.
- Check the weather: clear sky, low humidity and no haze.
- Let your eyes adapt: your eyes need 20–30 minutes in the dark to reach full sensitivity. Use only red light and avoid looking at your phone.
How to estimate the Bortle class yourself
Look at a well-known constellation and count the faintest stars you can see, or simply check the Milky Way: if you see dark lanes and a lot of structure, you are in class 3 or better; if it is faint or invisible, you are in class 5 or worse. More precise measurements use a sky quality meter, which gives the brightness of the sky in magnitudes per square arcsecond.
Dark sky spots on Starlyze
Starlyze includes a map of the best places to observe the stars away from light pollution, each with its Bortle class and altitude. Observers can add new spots for the community, check the Moon phase and the year's sky events, and share what they saw. Not sure what to point at? Read what you can see with the naked eye, binoculars and a telescope.
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