Quick Answer
The Bortle Scale measures light pollution from 1 (darkest, pristine skies) to 9 (brightest, inner city). For astrophotography, aim for Class 3 or lower. For visual astronomy, Class 4 is acceptable.
What is the Bortle Scale?
Created by amateur astronomer John E. Bortle in 2001, the Bortle Scale is a 9-level system for measuring light pollution and sky darkness. It's the universal language astronomers use to describe sky quality.
The 9 Classes Explained
Class 1 - Excellent Dark Sky
The holy grail. Zodiacal light visible, Milky Way casts shadows. The Andromeda Galaxy is obvious to the naked eye.
Where to find: Remote deserts, mountains, oceans. Examples: Atacama Desert (Chile), Mauna Kea (Hawaii), NamibRand (Namibia)
Class 2 - Typical Truly Dark Site
Milky Way structure is highly detailed. M31, M33, M81 visible to naked eye. Zodiacal light still apparent.
Where to find: National parks far from cities. Examples: Cherry Springs State Park (PA), Death Valley (CA), Jasper Dark Sky Preserve (Canada)
Class 3 - Rural Sky
Light domes visible on horizon. Milky Way still impressive with good structure. Many Messier objects visible.
Where to find: Rural areas 50+ miles from major cities. This is the minimum for serious astrophotography.
Class 4 - Rural/Suburban Transition
Milky Way visible but washed out. Light domes in most directions. M31 visible but not spectacular.
Where to find: Outer suburbs, 20-30 miles from city centers. "Good enough" for casual stargazing.
Class 5 - Suburban Sky
Only hints of Milky Way at zenith. M31 barely visible. Clouds are noticeably brighter than sky.
Where to find: Typical American suburbs. Need light pollution filters for nebulae.
Class 6-9 - Bright Suburban to Inner City
Class 6: Milky Way invisible. Only brightest Messier objects visible.
Class 7: Entire sky has grayish color. M31 and M44 barely visible.
Class 8: Sky glows white/orange. Only Moon, planets, and brightest stars visible.
Class 9: Entire sky luminous. Zenith as bright as horizon.
Recommendation: Don't observe from these zones. Drive to Class 4 or better.
What Can You See At Each Level?
| Bortle Class | Milky Way | Naked Eye Galaxies |
|---|---|---|
| 1-2 | Casts shadows, highly detailed | M31, M33, M81, LMC, SMC |
| 3-4 | Clearly visible, some structure | M31, maybe M33 |
| 5-6 | Faint, only at zenith | M31 with difficulty |
| 7-9 | Invisible | None |
How to Check Your Local Bortle Class
Our interactive map shows real-time Bortle measurements based on NASA VIIRS satellite data.
Check Your Location →Why It Matters
- Visual Astronomy: Bortle 4 or better for meaningful observing
- Astrophotography: Bortle 3 or better for nebulae and galaxies
- Meteor Showers: Bortle 5 is acceptable (meteors are bright)
- Planetary Observing: Light pollution doesn't matter (planets are bright)
💡 Pro Tip
A Bortle 4 site with excellent atmospheric seeing beats a Bortle 2 site with poor seeing. Check weather forecasts for "transparency" and "seeing" before driving.