Knowing how dark your sky is is key to joyous stargazing and the Bortle Scale is one tool we can use to measure that.
As we’ve noted in another article, we can see about 9,000 stars of up to magnitude 6 shining overhead, which is about the dimmest that we can see with the naked eye.
When you’re stargazing in a town with a little light pollution, the number of stars you can see might drop to 900. The bright lights of a big city would reduce it even further to less than 100, which is hardly sufficient for effective stargazing!
Learn the best ways to stargaze in a city
In this article, we’re going to help you learn how to grade the darkness of your sky so that you’ll understand what objects you can and can’t see, and if it is dark enough for you to force your eyes and equipment to their limits.
When you know what Bortle your sky is, you’ll be able to discuss with other astronomers, such as on Cloudy Nights, what objects you should expect to see from your location.
Measuring the Darkness of the Sky
For a long time, astronomers used the naked-eye-limiting magnitude (NELM) to measure the faintest star that they could see with the naked eye at a particular location.
As the name suggests, NELM measures the faintest star that can be perceived near the zenith (overhead) when the observer’s eyes are dark-adjusted. We’ll give you a concrete example of how you can do this for yourself later on in this article.
That is okay for a simple comparison between sites, but each observer has different eyesight and, the more you get used to looking at the sky, the more detail you can see. It is also affected by the seeing quality, which is different from measuring darkness.
So using individual perceptions is not a scientific way of grading and comparing dark sky sites.
What astronomers needed was something more rigorous. Introducing…
The Bortle Scale
The Bortle Scale was introduced to the astronomy community by John E. Bortle in 2001, inside the pages of Astronomy Magazine. In effect, Bortle gave us all a light-pollution scale.
This 9-grade scale sought to help stargazers classify their sky not only with reference to a naked-eye limiting magnitude but also what can (or can’t) be seen within each grade.
It provides information about celestial objects in general, not just the stars, including Messier objects, galaxies, zodiacal light, and more. This way, astronomers can evaluate the quality or brightness of their night sky.
Level 1 defines the darkest possible night sky on Earth, which is pitch black and where the faintest naked-eye objects are visible, including these phenomena that need a really dark sky.
Level 9 is a sky with extremely bright light pollution, which is mostly found over major cities; the night sky is literally bright enough to read under. Under level 9 conditions, only the moon, the nearest planets, and a handful of brightest stars are visible. We’ll take a detailed look at each level in the section after the next.
In defining what can and can’t be seen at each level of the scale, Bortle uses the following parameters:
- The amount of visible light pollution
- Key Messier objects that can be seen
- Zodiacal light (the faint arc of light we can see in spring after dusk and fall before dawn)
- The brightness of clouds compared to the background sky
- How visible the Milky Way is
- How much of your surroundings you can see
What Each Grade Means on the Bortle Scale
In the table below, airglow refers to the phenomenon caused by radiation in Earth’s atmosphere. It is the reason why the sky is never completely dark.
The limiting magnitude is given as two measures, firstly the faintest object that is seen with a 12.5″ reflecting telescope, and secondly with the naked eye.
| Level (Color) | Sky Type | Celestial objects that are visible | Lim. Mag* |
| 1 (Black) | Darkest possible | – The huge number of stars makes it harder for faint constellations to be recognized – The Triangulum galaxy is a naked-eye object – Many Messier objects and globular clusters are seen – Airglow is seen all over | 17.5 / 7.6-8.0 |
| 2 (Gray) | Truly dark | – Several Messier objects can be seen with the naked eye – Zodiacal light is bright enough to cast shadows – Airglow is only seen near the horizon | 16.5 / 7.1-7.5 |
| 3 (Blue) | Dark (Rural) | – Light pollution begins to appear on the horizon – The Triangulum galaxy (M33) is visible but you need to use averted vision – Clouds illuminated at the horizon but dark overhead | 16 / 6.6-7.0 |
| 4 (Green/ Yellow) | Rural/ Suburban Transition | – Light pollution is more apparent – The Milky Way becomes less detailed but still vibrant – M33 is only seen high up in the sky | 15.5 / 6.1-6.5 |
| 5 (Orange) | Suburban | – Light pollution is seen in all directions – Zodiacal light is only visible near the horizon – Clouds are brighter even at night – The Milky Way is only slightly visible | 15 / 5.6-6.0 |
| 6 (Red) | Bright suburban | – Zodiacal light is no longer visible – The lower third of the sky glows grayish – M33 is no longer visible, and M31 only modestly so | 14.5 / 5.1-5.5 |
| 7 (Red) | Suburban urban | – Light pollution affects the entire sky – The Milky Way is not visible – Clouds are brightly lit | 14 / 4.6-5.0 |
| 8 (White) | City | – Pollution is so bright you can read by it – Constellations are only slightly visible – Even with a telescope, only the brightest Messier objects are seen | 13 / 4.1-4.5 |
| 9 (White) | Inner-city | – The sky is bright – Key stars of major constellations are not visible – Only the Moon, close planets, and brightest stars can be seen | <4 |
Problems With the Bortle Scale
Although the Bortle scale is a great improvement on simple NELM, it still has some downsides.
Firstly, it is still partially subjective. What I might judge to be visible you might not. That’s why some users, such as btr209 on Cloudy Nights have a problem with it.
However, this is combated by having an approximate surface brightness measure (magnitude per arcsecond squared) for each of the divisions in the scale. (Whether any of us want to go so far as buying a sky-quality meter to measure that is another matter.)
Using this measurement, we discover several DSOs that can easily be found in a light-polluted sky.
Secondly, the transition from one grade to another can be quite marked, plus, each grade covers a slightly different level of light pollution. The counter to that is each grade also sets half magnitude steps for the naked eye limiting magnitude.
For the average astronomer, neither of these presents a big issue. Instead, it makes more sense for us backyard stargazers to understand how dark our own sky is using Bortle’s measures.
What Bortle is My Sky?
Using the methods below, I know that I’m fortunate enough to live under grade 5 skies. In reality, if I travel to a nearby area protected from the worst of the local light pollution, that improves to a grade 4.
Let’s use all that we’ve learned so far to understand how to measure the Bortle grade of your observing site.
There have been efforts such as the World Atlas of Artificial Sky Brightness to measure how dark (or bright) the sky is at a given location. Mapping efforts such as these have one rule of thumb: the brighter the area, the fewer objects you can see.
The Light Pollution Map is a great resource for discovering how dark (or bright) your local sky is. It colors a map of the world using the Bortle Scale colors giving an easy ‘ready reckoner’ for the levels of light pollution in your area.
Use the embedded version of their map below to see what Bortle grade is given to the place you stargaze from. A special thank you to the Light Pollution Map team for permitting me to include it in this article.
The Dark Site Finder is a similarly useful tool for you to understand how dark your sky is. We can use this resource the other way around though: to find the nearest dark sky to us if we have the opportunity to travel for our astronomy.
The map has markers for US locations of national parks, conservation areas, and even craters that have been designated as “dark sky sites” where you will have the best chance of stargazing in that area.
Clicking on those markers you can see in the image above will give you information about the place if camping is allowed, and even how much the stay would cost.
Every state in the U.S. has at least one location that measures four or darker on the Bortle Scale. This list of dark sky sites in the US shows the best dark sky in each state, and it opens in a new tab so you won’t lose this page.
Measuring Your Bortle Grade With Local Knowledge
You don’t have to rely on online charts to assess the Bortle grade of your local sky.
As I mentioned earlier, the maps say I’m stargazing under a grade five sky but, in reality, I can get this to be a grade four with not too much difficulty.
To clarify the darkness of your sky with your local knowledge, use the flowchart below to discover which category your sky falls under.
Begin with the question in the top right corner, i.e. ‘Can you see all seven of the main stars of Ursa Minor?’ and move from there depending on whether you answer yes or no. Eventually, you’ll end at an assessed score based on your personal experience.
How to Measure the Naked Eye Limiting Magnitude of Your Sky
From your location, you can employ NELM to figure out the faintest star that you can see without equipment. The best place to do this is the Great Square of Pegasus — the asterism formed by four bright stars in the constellation of Pegasus.
Inside the square are 26 stars brighter than magnitude 6.5. How many you can count gives a good indication of your naked eye limiting magnitude:
- At magnitude 4 = Zero stars visible inside the square
- Mag. 4.5 = 1 star, Upsilon Pegasi
- Mag. 5.0 = 4 stars
- Mag. 5.5 = 7 stars
- Mag. 6.0 = 10 stars
- Mag. 6.1 = 14 stars
- Mag. 6.2 = 18 stars
- Mag. 6.3 = 21 stars
- Mag. 6.4 = 26 stars
The star charts below, which are inverse monochrome (black on white) charts from SkySafari 6, show the number of stars visible within the Great Square of Pegasus at various naked-eye-limiting magnitudes.
Click on them for full-screen versions which you can print off and use to test under your sky.
You will have no problem finding Upsilon Pegasi, but there are no guarantees on the rest if you live in a light-polluted area.
The number of stars that you can see within the Great Square of Pegasus will tell you about how much light pollution exists in your area, thus giving a good guide as to where you sit on the Bortle Scale.
Summary
Light pollution is the worst enemy of stargazing.
We can avoid it better by understanding the Bortle scale to learn where the darkest skies are and where our observing location ranks on the scale.
Use the information in this article to plan your next dark-sky adventure!
