The Bortle dark sky scale measures night sky quality from Class 1 (the darkest skies on Earth) to Class 9 (inner-city skyglow). Developed by John Bortle in 2001, it's the universal standard for comparing light pollution and predicting what objects are visible to observers. Click a class below to see what you can expect.
Select Bortle Class
Tap a number to see what's visible at that light pollution level
Sky Conditions
Bortle Scale Quick Reference
| Class | Name | Limit Mag | Milky Way |
|---|
How to Use the Bortle Scale Night Sky Guide
The Bortle scale gives you a concrete framework for evaluating night sky quality before planning a stargazing trip or astrophotography session. Use this guide to understand what each class means in practice and find locations that meet your observing goals.
Step 1: Find Your Current Bortle Class
Use lightpollutionmap.info to look up the light pollution at your location. The map uses Bortle scale color coding — dark blue and black are Class 1–2, gray-green is Class 3–4, orange through red is Class 5–7, and white is Class 8–9. The Globe at Night citizen science program also lets you contribute and compare observations.
Step 2: Understand Limiting Magnitude
Limiting magnitude is the faintest star visible to the naked eye under those conditions. The higher the number, the darker the sky. Under Class 1 skies you can see stars at magnitude 7.6+. Under Class 6 (typical suburban sky), you're limited to about magnitude 5.1. Each full magnitude step represents about 2.5× difference in brightness.
Step 3: Plan Your Observing Session
For visual observing of star clusters, galaxies, and nebulae: Bortle 4 or better is the practical minimum. For astrophotography: Bortle 3 or better is preferred, though modern CMOS sensors and software like DeepSkyStacker can manage in Bortle 5–6 for bright targets. Solar system objects (planets, moon) look the same regardless of Bortle class — atmospheric seeing matters more.
Step 4: Find Dark Sky Sites Near You
The International Dark-Sky Association (IDA) certifies Dark Sky Places — parks, reserves, and communities with protected night sky ordinances. Search the IDA website or the DarkSkyFinder app for certified dark sky parks near you. Driving 30–60 minutes from most cities can take you from Bortle 8 to Bortle 4–5, dramatically improving your experience.
Frequently Asked Questions
Is this Bortle scale guide free to use?
Yes, completely free with no signup required. Browse all 9 Bortle levels with full descriptions instantly.
Is my data safe and private?
Yes. This is a reference tool. Everything runs in your browser with no data collection.
What is the Bortle scale?
The Bortle scale is a nine-level numeric scale developed by John E. Bortle in 2001 to measure the darkness of the night sky. Level 1 is the darkest sky on Earth (remote wilderness), while level 9 is the brightest (downtown city center). It uses naked-eye limiting magnitude and visible features like the zodiacal light to define each level.
What Bortle level is good for stargazing?
Bortle 4 (rural/suburban transition) is the minimum for serious visual observing — the Milky Way is clearly visible and most Messier objects are easy targets. Bortle 3 (rural sky) reveals the zodiacal light. Bortle 1–2 is world-class for astrophotography. Most backyard observers are in Bortle 6–8.
How do I find my Bortle rating?
Use the Light Pollution Map at lightpollutionmap.info, the Globe at Night app, or the Dark Sky Finder. These overlays color-code the sky brightness from satellite data. You can also estimate by checking if the Milky Way is visible — if you see it faintly, you're around Bortle 4–5; if it's bright and detailed, you're at Bortle 2–3.
Can I see the Milky Way from a Bortle 5 sky?
At Bortle 5 (suburban sky), the Milky Way is faintly visible above the horizon but lacks detail. The core is recognizable but washed out. For a truly impressive Milky Way view with structure and dark lanes, you need Bortle 3 or better. Bortle 4 shows the Milky Way clearly but with some sky glow on horizons.
What deep sky objects can I see at Bortle 6?
At Bortle 6 (bright suburban), the Andromeda Galaxy (M31) is visible to the naked eye but without structure. Most Messier objects are visible in a 6-inch telescope. The Virgo Cluster, Hercules Cluster (M13), and Orion Nebula are accessible. The zodiacal light is invisible and the gegenschein cannot be seen.