Step outside a big city and the night sky transforms - thousands of stars appear, and the Milky Way itself becomes visible as a glowing band. Astronomer John Bortle mapped that transformation onto a 9-class scale in 2001. Drag the class slider below from 1 (darkest) to 9 (inner-city) and watch a seeded 260-star field thin out and the Milky Way fade, exactly as the published scale describes.
Published figures (John Bortle, Sky & Telescope, Feb 2001): naked-eye limiting magnitude ranges from about 7.6-8.0 at Class 1 (excellent dark-sky site) down to 4.0 or fainter at Class 9 (inner-city sky). The Milky Way is highly structured at Class 1-3, loses detail by Class 4-5, and is not visible from Class 8-9.
Drag to orbit and scroll or pinch to zoom. Drag the Bortle class slider, jump to a named sky, or toggle the Milky Way band and the horizon light dome on and off.
Bortle Scale Light Pollution 3D Explorer
Drag the Bortle class slider from 1 to 9 and watch a seeded 260-star sky field thin out star by star, the Milky Way fade, and a glowing horizon light dome rise - the same transformation you would see driving from a city center out into open countryside on a clear night.
Amateur astronomer John Bortle published this 9-class scale in Sky & Telescope in February 2001 to give stargazers a shared vocabulary for how dark a site really is, beyond a vague "pretty dark" or "kind of bright". Each class is anchored to a naked-eye limiting magnitude (NELM) range - the faintest star a well-adapted human eye can see - and a description of how much of the Milky Way survives.
- Drag the Bortle class slider from 1 (excellent dark-sky site) to 9 (inner-city sky), or jump straight to a named sky
- Watch the star count update live as fainter stars drop below the current class's limiting magnitude
- Toggle the Milky Way band and the horizon light dome independently
- Read the current class's NELM range and Milky Way description in the facts panel
- Drag to orbit, scroll or pinch to zoom
- Runs fully in the browser with the vendored three.js engine - no account, no upload
| Class | Description | NELM | Milky Way |
|---|---|---|---|
| 1 | Excellent dark-sky site | 7.6-8.0 | Casts visible shadows, high structure |
| 2 | Typical truly dark site | 7.1-7.5 | Highly structured, obvious dust lanes |
| 3 | Rural sky | 6.6-7.0 | Still complex structure |
| 4 | Rural/suburban transition | 6.3-6.5 | Impressive overhead but lacks detail |
| 5 | Suburban sky | 5.6-6.0 | Washed out overhead, invisible near horizon |
| 6 | Bright suburban sky | 5.1-5.5 | Visible only near the zenith, if at all |
| 7 | Suburban/urban transition | 4.6-5.0 | Almost or completely invisible |
| 8 | City sky | 4.1-4.5 | Not visible |
| 9 | Inner-city sky | 4.0 or less | Not visible; only brightest stars/planets |
The scale is not a precise brightness meter - a Sky Quality Meter (SQM) reading in magnitudes per square arcsecond gives a repeatable number, while a Bortle class is an observer's description of what is actually visible. Astronomers still use the two together: Bortle for a quick shared vocabulary, SQM for a measurement you can log and compare night to night.
For the magnitude number system itself - why a brighter star gets a LOWER number - see the Stellar Magnitude Explorer. This page instead applies that system to a real-world question: how much of the sky does a given light-pollution class erase.
Everything renders on your device with WebGL. The 3D engine loads once (about 0.7 MB) and is cached.
This is an educational approximation, not a live sky-brightness simulator - the star field is a fixed illustrative sample of 260 stars (not a real star catalogue) and the horizon light dome's height and color are a qualitative teaching device, not a measured mag/arcsec² skyglow reading. The naked-eye limiting magnitude ranges and Milky Way descriptions are John Bortle's published 2001 scale.
Frequently Asked Questions
What is the Bortle scale?
A 9-class scale (1 darkest to 9 brightest) that amateur astronomer John Bortle published in Sky & Telescope in February 2001 to describe how much light pollution affects what you can see at a given site, from an excellent dark-sky site (Class 1) to an inner-city sky (Class 9).
What does naked-eye limiting magnitude (NELM) mean?
It is the faintest star magnitude a well dark-adapted human eye can see at a given site. Lower magnitude numbers are brighter stars, so a HIGHER limiting magnitude means a darker, better sky - Class 1 sites reach about magnitude 7.6-8.0, while Class 9 inner-city skies drop to about magnitude 4.0 or fainter.
At what Bortle class does the Milky Way disappear?
It loses fine detail by Class 4-5, becomes visible only near the zenith by Class 6, and is essentially or completely gone by Class 7, disappearing entirely by Class 8-9 according to Bortle's published descriptions.
Is Bortle class the same as a Sky Quality Meter (SQM) reading?
No. An SQM measures sky brightness in magnitudes per square arcsecond as a repeatable number you can log. A Bortle class is a qualitative description of what an observer can actually see. The two are related but are not the same measurement.
Why do stars disappear as I drag the slider toward Class 9?
Each of the 260 seeded stars in this scene has a fixed "true" magnitude. As you move to a higher (brighter-sky) Bortle class, the naked-eye limiting magnitude drops, so any star fainter than that new limit is hidden - exactly mirroring what happens to a real observer's eye under real light pollution.
Is the horizon glow dome a real sky-brightness measurement?
No. It is a qualitative teaching device whose height and color scale with the Bortle class number to represent urban skyglow - it is not tied to a measured magnitude-per-square-arcsecond reading.