FAQs About the Bortle Scale
When planning a camping trip, there is always one thing we check before ever clicking the reserve button. How much light pollution is in the area? By knowing this little piece of information, we are better able to enjoy our favorite part of RVing which is stargazing.
In this short article, I’ll answer a few questions related to the Bortle scale so you can also enjoy the night skies and better plan your camping trips. If you would like to know how we pick our campsite when RV, check out this article on how to find a dark sky campground.
So, let’s dive in and find out what is a light pollution scale.
What is the Bortle Scale?
The Bortle scale is a general way to measure light pollution with categories from 1 to 9. It is based on the night sky brightness in an area with 1 being a location with the least amount of light pollution and 9 having the most light pollution. It uses easily identifiable objects in the night sky as a reference at each level of the scale. We like to think of it as a “cheat sheet” type of scale that gives you a rough idea where there is and isn’t light pollution. It simply makes finding a dark sky easier for amateur astronomers and astrophotographers like us.
Who Created the Bortle Scale?
The Bortle Scale was first published in Sky & Telescope magazine in 2001 by amateur astronomer John E. Bortle. He is a multiple award winning astronomer also known for his “Comet Digest” section of the same publication in addition to his easy to understand light pollution scale. For decades, he also edited the monthly American Association of Variable Star Observers (AAVSO) circular, and has recorded thousands upon thousands of celestial observations over the years.
What is the Purpose of the Bortle Scale?
The purpose of the Bortle scale is to help astronomers better evaluate the quality of their skies. It simplifies stargazing. Using a technique such as NELM (naked-eye limiting magnitude) assumes everyone has the same vision when trying to find faint objects. With NELM, one person’s magnitude 6.5 sky might be another’s 6.2 or 6. 7 sky because of how well their eyes detect celestial objects at night.
With my vision, I might have more trouble seeing the same faint object as you. The Bortle scale makes it easier to judge darkness and gives you a rough idea that is easy to understand and share when explaining light pollution and sky quality to others. If you need more accurate data, buy a sky quality meter and take some SQM readings (measured in magnitude per square arcsecond). The Bortle scale maps most amateur stargazers use are for simplicity, so getting the most precise data on sky darkness probably isn’t that big of a deal.
At What Bortle Scale Can You See the Milky Way?
At Bortle scale level 4, we have found it easiest to see a clearly defined Milky Way core. The white starry line of the Milky Way is easier to see extending across a larger portion of the sky. At level 5, you sometimes can only see it up above at zenith extending a little below depending on light pollution on the horizon. Level 4 classification areas are places we tend to visit regularly during Milky Way season since we are pretty much guaranteed to see the core of the galaxy on a clear night. These level 4 areas are usually classified as “green” on a dark sky map.
Is the Bortle Scale Linear?
The Bortle scale is not linear. For example, a Bortle 2 sky can have SQM measurements from 21.89 to 21.99, which is a range of 0.1 versus a Bortle 4 sky which can have measurements span from 20.49 to 21.69, which is a range of 1.2. You might ask: Is it a logarithmic scale? No. Some of the brighter Bortle scale numbers such as class 6 and 7 have differences of 0.56. It is a scale with plenty of subjectivity because it is based on what you can actually see in the night sky versus using an instrument to measure it.
What Can You See in Each Bortle Level?
At each Bortle level, you are able to see a number of celestial objects. Here’s the basic breakdown to help you understand what you can see starting with the most light polluted sky to least light polluted as explained by John E. Bortle.
Level 9 – Inner-city Sky
- Bright star clusters
- The Moon
- Bright planets
Level 8 – City Sky
- M31 and M44 with a modest size telescope
- Stars down to magnitude 4.5
- Possibly a couple familiar constellation patterns
Level 7 – Suburban/Urban Transition
- M31 and M44 possibly with naked eye
- Brightest Messier objects are “pale ghosts”
- Milky Way is totally invisible still
Level 6 – Bright Suburban Sky
- M33 is possible with binoculars
- Zodiacal light not visible
- M31 seen with naked eye
- Possible trace of the Milky Way straight above at zenith
Level 5 – Suburban Sky
- Zodiacal light seen on clear fall and spring nights
- Milky Way is very weak or still invisible near the horizon
Level 4 – Rural/Suburban Transition
- Winter Milky Way mostly visible
- Zodiacal light is clearly evident
- M33 is tough to see with averted vision
- Milky Way is impressive but lacks the most obvious structure
Level 3 – Rural Sky
- Milky Way appears complex
- M4, M5, M15, M22 visible with the naked eye
- M33 easy to see with averted vision
- Zodiacal light is strong in spring and autumn with a hint of its color
Level 2 – Typical Truly Dark Sky
- M33 seen easily with direct vision
- Airglow can be faint along the horizon
- Summer Milky Way is highly structured with naked eye and mable appearance with binoculars
- Zodiacal light appears yellow against blue-white Milky Way
- Before dawn and after dusk, zodiacal light bright enough to cast weak shadows on the ground
- Many Messier globular objects can be seen with the naked eye
Level 1 – Excellent Dark Sky Site
- Zodiacal light and gegenschein are all visible with the naked eye
- Zodiacal band spans across the entire sky
- Airglow is more noticeable just above the horizon
- M33 is obvious naked eye object
- Sagittarius and Scorpius areas of the Milky Way bright enough to cast clear shadows on the ground
What is the Most Accurate Light Pollution Map?
With numerous light pollution maps online to choose from, some are more up-to-date than others. If you Google “dark sky map”, the result that typically pops up first is darksitefinder.com. If you search for “light pollution map”, lightpollutionmap.info comes up.
Dark Sky Finder is using light pollution data from David Lorenz based on 2006 data while Light Pollution Map is using data from 2020. This more recent data is based on David Lorenz’s work using Pierantonio Cinzano’s original atlas. Our own RWTadventures.com stargazing campground map also uses the same data from this updated map. On our dark sky map, you can can even enter your zip code to see your Bortle number.
So, for the most accurate light pollution map, I would use either our map because you love us our the World Atlas of Artificial Night Sky Brightness since that is the main source others are getting info from.