Have you ever tried to gaze up at the stars only to be greeted by a dull, hazy sky? That’s light pollution getting in the way of your stargazing. For those of us who dream of seeing the Milky Way stretch across the sky or watching distant galaxies sparkle, finding the perfect spot can be a challenge. But here’s where the Bortle Scale comes to the rescue!
Whether you're an amateur stargazer or an astrophotography enthusiast, the Bortle Scale helps you determine the best locations for viewing the night sky in all its glory. In this guide, we’ll break down how to use this scale to pick the right spot for your next stargazing adventure.
What Is the Bortle Scale?
The Bortle Scale is a simple, nine-level system that measures the darkness of the night sky in any given location, ranging from Class 1 (the darkest and clearest skies) to Class 9 (the most light-polluted, urban skies). It was developed by John E. Bortle in 2001 to help amateur astronomers and stargazers assess sky conditions based on light pollution levels.
Image credit: ESO/P. Horálek, M. Wallner
The Bortle Scale isn’t a scientific measurement of light pollution but a qualitative tool based on human perception. This makes it easy for stargazers to understand what celestial objects they can expect to see in different environments. For example, in Class 1 skies, deep-sky objects like distant galaxies and nebulae are easily visible to the naked eye, whereas in Class 9, even the brightest stars struggle to shine through the overwhelming city lights.
While the Bortle Scale is widely used and helpful, it’s important to remember that it’s subjective—what one person sees in a certain sky condition might vary slightly for another depending on factors like eyesight, altitude, and weather. For this reason, it’s often best to use the Bortle Scale alongside quantitative tools like Sky Quality Meters (which provide an objective reading of the night sky's brightness) or light pollution maps that give a more precise idea of where to find the darkest skies.
The 9 Classes of the Bortle Scale
Here’s a breakdown of each class on the Bortle Scale and what you can expect to see at each level of light pollution.
Class 1: Excellent Dark-Sky Site
What You See:
- The Milky Way is so bright that it casts shadows on the ground.
- Zodiacal light, faint meteors, and deep-sky objects like nebulae and galaxies are visible.
- The horizon shows no artificial light.
Where to Find It:
- Remote areas like deserts, high mountains, or isolated national parks far from human habitation.
Class 2: Typical Dark-Sky Site
What You See:
- The Milky Way is very prominent, with slight light pollution visible on the horizon.
- Fainter deep-sky objects are still visible, and Zodiacal light is noticeable.
Where to Find It:
- Wilderness areas and dark-sky reserves with little nearby artificial lighting.
Class 3: Rural Sky
What You See:
- Some light pollution is present, but the Milky Way remains visible, though slightly washed out.
- Fainter objects like nebulae are harder to distinguish.
Where to Find It:
- Rural areas far from cities, including national forests and some coastal regions.
Class 4: Rural/Suburban Transition
What You See:
- The Milky Way is faint and washed out.
- Skyglow from nearby cities is apparent.
- Fewer faint objects are visible, and the night sky appears dimmer.
Where to Find It:
- Rural areas within about an hour of a major city.
Class 5: Suburban Sky
What You See:
- The Milky Way is barely visible, typically only overhead.
- Skyglow from city lights dominates.
- Brighter celestial objects like planets and prominent stars are visible.
Where to Find It:
- Suburbs or small towns relatively close to urban centers.
Class 6: Bright Suburban Sky
What You See:
- The Milky Way is completely invisible.
- The sky is dominated by light pollution.
- Only the brightest planets and a handful of stars can be seen.
Where to Find It:
- Bright suburban areas close to cities or towns with heavy street lighting.
Class 7: Suburban/Urban Transition
What You See:
- Light pollution is heavy, and only the brightest stars and planets are visible.
- The sky appears washed out by artificial lights.
Where to Find It:
- Densely populated suburban areas near city centers.
Class 8: City Sky
What You See:
- The sky is heavily lit, and only the brightest stars, such as Vega or Sirius, are visible.
- Planets like Jupiter or Venus can still be seen, but most celestial objects are obscured.
Where to Find It:
- Large city environments with constant artificial lighting.
Class 9: Inner-City Sky
What You See:
- The sky is so bright from light pollution that it often appears gray or orange.
- Only the moon, planets, and a few of the brightest stars are visible.
Where to Find It:
- Inner-city areas with intense artificial lighting, such as downtown sections of major metropolitan cities.
Why the Bortle Scale Matters for Stargazing
If you’re serious about stargazing, the Bortle Scale is one of the best tools to help you determine how well you’ll be able to see celestial objects based on light pollution levels. Whether you’re planning to simply observe the stars or capture stunning astrophotography, knowing your location’s Bortle class can make all the difference.
Visibility of Celestial Objects
The Bortle Scale tells you what you can expect to see in the night sky, depending on the level of light pollution. For example:
- In Class 1 or 2 skies, the Milky Way is visible in remarkable detail, stretching across the sky. Fainter objects like the Andromeda Galaxy and distant nebulae are also visible to the naked eye. You can even see zodiacal light, the sunlight scattered by interplanetary dust.
- In Class 5 or 6 skies, you may still spot some planets and bright stars, but the Milky Way will be barely visible or not visible at all. Skyglow from nearby urban areas washes out much of the night sky.
- In Class 9 skies (inner-city), light pollution makes it nearly impossible to see anything other than the brightest stars and planets, like Jupiter and Venus. The night sky is often dominated by a dull glow, leaving the stars hidden.
Impact on Astrophotography
For astrophotographers, the Bortle Scale is an essential guide. Long-exposure photography, which captures the faint light of celestial objects, is particularly sensitive to light pollution. In brighter skies (Class 6 and above), skyglow from artificial lights interferes with image clarity, and faint objects like nebulae and galaxies may be completely washed out.
- In Class 1–3 skies, you can capture detailed long-exposure shots of objects like the Orion Nebula, Andromeda Galaxy, or star clusters without interference from nearby lights.
- In Class 4–5 skies, you may still get decent shots of brighter objects like planets or star fields, but the quality of images will decrease significantly for faint objects.
- Class 6–9 skies are difficult for astrophotography of deep-sky objects because the glow from streetlights, buildings, and other artificial sources dominates the night sky, making post-processing more challenging.
Benefits for Stargazers
Using the Bortle Scale helps stargazers of all levels pick the best locations for their specific interests. Whether you're planning a stargazing trip or just want to see what's visible from your backyard, knowing the Bortle class helps you set realistic expectations:
- Casual Stargazers: If you’re just hoping to spot some planets or a few bright stars, Class 5 or 6 skies are usually good enough.
- Deep-Sky Enthusiasts: For those looking to view or photograph deep-sky objects, a Class 1 or 2 sky is essential. These are often found in remote wilderness areas, national parks, or designated dark sky reserves.
How to Use the Bortle Scale to Choose Stargazing Locations
Ready to find the perfect stargazing spot? Here’s how you can use the Bortle Scale to make sure you choose the right location for your next night under the stars.
Step 1: Determine Your Stargazing Goals
- Planets and the Moon: These are bright enough to be visible even in light-polluted areas. If you're only interested in observing the moon or planets like Jupiter, Venus, or Saturn, you’ll be fine with a Class 5 or 6 sky.
- Deep-Sky Objects: If you're hoping to see distant galaxies, nebulae, or faint star clusters, you'll need to find a much darker location. For these objects, you’ll want to look for Class 1–3 skies, where light pollution is minimal, and the Milky Way is visible.
Step 2: Use Light Pollution Maps
Light pollution maps are incredibly useful for finding dark-sky areas near you. Websites like Dark Sky Finder, Light Pollution Map, and apps like Clear Outside provide color-coded overlays that show how light pollution levels change in different regions, based on the Bortle Scale.
- Locate Your Area: Enter your location or planned destination to find out what Bortle class the area falls into. These maps use colors to represent the different Bortle classes, with dark blue or black indicating Class 1–3 skies, and white or red showing heavy light pollution (Class 7–9).
- Explore New Spots: Look for areas in Class 1–4 zones, ideally far from urban centers. These maps make it easy to find the closest dark-sky areas, whether you're traveling or just planning a local outing.
Additional Tips for Stargazing in Low Bortle Scale Areas
Stargazing in low Bortle class areas (Class 1–3) offers a magical view of the night sky, but to make the most of these conditions, here are a few extra tips to enhance your experience.
Bring Proper Equipment
- Telescope or Binoculars: If you want to see details like the rings of Saturn or the craters of the Moon, a telescope is a must. But don’t underestimate the power of a good pair of binoculars (7x50 or 10x50 models).
- Red Flashlight: A red flashlight is essential for preserving your night vision.
- Star Charts or Apps: Bring along a star chart or download an app like SkySafari or Stellarium to help you find celestial objects.
- Warm Clothing: Dress warmly, as temperatures can drop significantly at night.
Final Thoughts on Stargazing with the Bortle Scale
The Bortle Scale is an incredibly useful tool for stargazers, helping you determine the level of light pollution in an area and guiding you toward the best spots for seeing the stars. Whether you're a casual stargazer hoping to see planets and bright stars or an experienced astrophotographer looking for Class 1 skies to capture deep-sky objects, understanding the Bortle Scale can make a huge difference in your night sky experience.
Now that you’ve got the knowledge, it's time to put it into action! Start planning your next stargazing trip using the Bortle Scale to guide you to the perfect dark-sky location. Whether you’re staying close to home or venturing out to a remote dark sky park, this scale will help you make the most of your night under the stars.