How to Read a Bortle Scale Map: Your Complete Guide

The Bortle Dark-Sky Scale was created by amateur astronomer John E. Bortle in 2001. It measures sky brightness on a scale of 1 (darkest) to 9 (brightest city center), based on what you can actually see with the naked eye.

Check your location's Bortle class on our Dark Sky Map.

Light pollution maps use color gradients to represent sky brightness. While exact colors vary by map provider, here's the typical scheme:

Standard Color Key:

• 🔵 Blue/Black (Bortle 1-2) — Pristine dark skies
• 🟢 Green (Bortle 3) — Rural excellence
• 🟡 Yellow (Bortle 4) — Good stargazing
• 🟠 Orange (Bortle 5-6) — Suburban skies
• 🔴 Red (Bortle 7-8) — Urban glow
• ⚪ White/Magenta (Bortle 9) — City center

Our Dark Sky Map uses NASA VIIRS satellite data to create precise, up-to-date light pollution overlays. The satellite measures actual light escaping into space, giving you real-world brightness levels.

Here's my step-by-step process for finding a stargazing spot:

Step 1: Find Your Starting Point Open our Dark Sky Map and zoom to your city. Note your home's Bortle class—this is your "baseline" for comparison.

Step 2: Look for Dark Pockets Zoom out and identify the nearest blue or green zones. In most of North America and Europe, you'll need to travel 60-90 minutes from a major city to reach Bortle 4.

Step 3: Trace the Route Follow roads from your location toward the dark zone. Look for:

• State/national parks (often have parking and facilities)
• Reservoirs and lakes (water creates natural buffer zones)
• Mountain overlooks (altitude helps, and you're above valley lights)

Step 4: Check the Terrain Dark maps show average brightness. A technically dark area surrounded by cities on three sides may have a limited view. Use satellite view to verify horizon obstructions.

Step 5: Verify Current Conditions Our dashboard shows real-time cloud cover, seeing, and transparency—not just static light pollution data.

A Bortle 3 site with bad conditions can be worse than a Bortle 5 with perfect transparency. Here's what else to check:

Transparency How clear is the air? Humidity, wildfire smoke, and dust all scatter light. High transparency nights make even moderate Bortle sites shine.

Seeing Atmospheric stability affects planetary and lunar detail. Turbulent air makes stars twinkle beautifully but ruins high-magnification views.

Cloud Cover Obviously, clouds block stars. But thin high cirrus can scatter light from distant cities, degrading otherwise dark skies.

Moon Phase A full moon at a Bortle 1 site is still a washed-out sky. Always plan around moonrise/moonset times. The week around new moon is prime time.

Light Domes Cities create "domes" of light visible from 50+ miles away. Even from a dark site, a city on the horizon will wash out low-altitude objects in that direction.

Know Your Light Tolerance For casual naked-eye stargazing, Bortle 5 is often fine. For Milky Way photography, you want Bortle 4 or darker. For serious deep sky imaging, aim for Bortle 3 or better.

Edge Zones Are Okay You don't always need to drive to the dark core. The edge of a green zone, where it meets yellow, is often accessible and still excellent.

Altitude Helps Higher elevation = less atmosphere = better transparency. Mountain sites often "punch above their weight" on the Bortle scale.

Direction Matters If you can't escape the city entirely, position yourself so the city is behind you. The sky in the opposite direction will be 1-2 Bortle classes darker.

Scout by Day Visit your stargazing spot during daylight first. Check for:

• Safe parking
• Line-of-sight obstructions
• Cell service (for emergency)
• Flat ground for setup

Our Dark Sky Map is specifically designed for astronomers and astrophotographers. Here's how to get the most from it:

Features:

• Toggle light pollution overlay on/off
• See your current location's Bortle class
• Check real-time weather and cloud cover
• View ISS and Starlink satellite passes
• Monitor aurora Kp index for northern lights

Quick Tips:

1. Allow location access for instant local data
2. Click anywhere on the map for point-specific info
3. Use the sidebar for detailed conditions
4. Save favorite locations (Premium feature)

Start planning your next dark sky adventure now—open the map.