The Bortle Scale: Light Pollution Rating System Explained
Last Updated: November 9, 2023
You may know that observing stars in a big city is harder than when out in the country, but how can we measure the impact of city lights on the night sky and how can you determine your chance of seeing stars or deep sky objects in your current location?
Let’s dive into light pollution, how it’s measured with the Bortle Scale, why it’s an issue, and how it affects us and other life forms.
Light Pollution Introduction: What is it, how it affects us, and what’s the concern?
Artificial light helps us see at night, but it also overpowers the light from stars and to a degree, even the moon, creating light pollution. In particular, light pollution refers to unwanted, inappropriate, or excessive artificial lighting which sends light out everywhere, brightening the night sky and disrupting nighttime activities.
Many animals are guided by the light of the stars and moon for seasonal migrations and other various navigation such as baby turtles trying to find the sea or animals knowing when mating season has begun. Light pollution, therefore, affects their ability to perform these vital activities safely and efficiently.
As humans, we can also be affected by light pollution since it counteracts our bodies’ natural wake and sleep patterns. Anyone who has lived in a busy city with neon lights or even simply poorly designed lighting on nearby buildings can attest to issues with falling and staying asleep without the aid of blackout curtains and other ways of blocking light.
Light pollution can also make stargazing and other astronomical observations quite difficult as it overpowers the natural light from these sources. Researchers will have trouble finding and observing in detail their targets in areas with higher light pollution. Amateur astronomers will struggle to find and image various targets. If they are just starting out, they may struggle to find objects of interest, even the “go-to” brighter ones.
Knowing how much light pollution is in a given area can help humans create more effective methods of dealing with light pollution’s effects on their lives and the lives of local animals. This can also help astronomers know their chances of finding various astronomical objects.
Just thirty years ago, truly dark skies could be found within an hour’s drive of major population centers, but today you often need to travel 150 miles or more from these centers. Therefore, many of us have never been under a truly dark sky, making these assessments of what is considered a dark sky and what can likely be observed difficult.
Historically, amateur astronomers used the naked-eye limiting magnitude (NELM) technique to judge the level of light pollution in their sky by figuring out the magnitude of the faintest star visible to their naked eye. Magnitude is a scale developed by the ancient Greeks that categorizes how bright stars appear from Earth with the brightest stars in the sky having a magnitude of 1 and the faintest receiving a magnitude of 6.
A magnitude 1 star is 100 times brighter than a magnitude six star. Since this scale was developed with interstellar stars as the basis, it is important to note that many of the brighter objects within our solar system will have a negative magnitude. In addition, this scale has been expanded past 6 as our ability to observe with instruments has increased.
So, using the naked-eye limiting magnitude (NELM) technique, if the faintest star you could see was 47 Ursae Majoris (47 UMa) in the Ursa Major constellation below Dubhe and Merak in the Big Dipper asterism (located in the top center of the map below), then your sky would have a naked-eye limiting magnitude of 5 since 5 is the faintest magnitude you can see.
47 UMa-starmap By Tomruen – Own work, CC BY-SA 4.0
But this technique depends on a number of factors including the observer’s eyesight and visual acuity (how easily they can pick out details or how “sharp” their eyesight is) as well as how long and hard they looked to find the faintest star. This means one person could classify a location’s sky as having a 5.5 magnitude visibility while others could classify it as a 6.3 magnitude sky, a 4.7 magnitude sky, anything between, or even more on either side. It also creates problems when observing objects other than stars that are affected even more by light pollution such as comets, nebulae, and galaxies.
The Bortle Scale
To address this issue, astronomer John E. Bortle created the Bortle Scale and published it in the February 2001 edition of Sky & Telescope to help amateur astronomers know exactly the level of their sky’s visibility and provide a standard for astronomers based on his 50 years of observing experience.
The Bortle Scale also helps professional astronomers, particularly when fact-checking reported observations because they can check to see how likely it is that someone could have observed that type of astronomical object from that location as part of their verification processes.
Maps have been created using this scale to make it even easier for someone to know their sky’s Bortle Scale Rating at a glance. The Bortle Scale ranges from Class 1, the darkest skies on Earth, to Class 9, inner-city skies.
First, let’s define a few astronomical terms that are often used to categorize these levels:
- Zodiacal light: a faint glow of diffuse sunlight scattered by interplanetary dust that appears to extend from the Sun’s direction in a roughly triangular shape along the line of zodiac constellations.
- Zodiacal band: the specific portion of Zodiacal light along the ecliptic (the path of the planets and moon across the sky as it is the orbital plane around the Sun) where the zodiac constellations are located.
- Gegenschein/counterglow: a faintly bright spot/ optical phenomenon in the night sky centered at the point directly opposite from the Sun created by the backscatter of sunlight by interplanetary dust.
- Airglow/ nightglow: a faint emission of light by Earth’s atmosphere, causing the night sky to never be completely dark.
- Direct vision naked-eye object: an object that can be viewed with the naked eye straight on as opposed to in peripheral vision; many deep sky objects are so faint that you have to look in your peripheral vision to see them in higher classes of the Bortle Scale.
|Faintest Apparent Magnitude/ NELM
Class 1: Excellent Dark Sky Site
The darkest level achievable on Earth
7.6-8.0 (with effort)
Jupiter or Venus can limit this
Milky Way is visible in its entirety
All Zodiac constellations are visible
Many Messier objects and globular clusters are visible with the naked eye
M33, the Triangulum Galaxy, is an obvious direct vision naked-eye object
Faintest objects in the sky are visible
Many constellations are difficult to spot because of the presence of much fainter stars that are not normally visible
Zodiacal light, Gegenschein, and airglow easily visible
Zodiacal band visible and reaches across the entire length of the sky
Scorpius and Sagittarius regions of the Milky Way cast diffuse shadows on the ground
A 32-centimeter (12½-inch) scope can detect stars to magnitude 17.5 with effort; a 50-cm (20-inch) scope with moderate magnification can reach magnitude 19
Surroundings (and you) are practically invisible
Class 2: Typical Truly Dark Site
Very low light pollution and offers excellent viewing conditions
Summer Milky Way shows a lot of structure to naked eye; like veined marble with binoculars
Many Messier objects and globular clusters are direct vision naked-eye objects
M33 is fairly easy naked eye object with direct vision
Clouds appear as dark holes against the sky
Surroundings are barely visible against the night sky
Airglow weakly apparent along horizon
Zodiacal light casts weak shadows just before and after dusk with distinctly yellowish light
A 32-cm telescope reaches to magnitude 16 or 17
Class 3: Rural Sky
Some light pollution evident at the horizon from nearby towns or cities
Clouds are illuminated near the horizon, but dark overhead
Nearer surroundings are vaguely visible
Summer Milky Way appears fairly complex and detailed
Globular clusters M15, M4, M5, and M22 are direct vision naked-eye objects
M33 is easily visible in peripheral vision
Milky Way still appears complex
Zodiacal light is still striking in spring and autumn; color still visible at least weakly
A 32-cm reflector will reach to 16th magnitude
Class 4: Rural/ Suburban Transition or Brighter Rural
Sometimes broken up to also include a Semi-Suburban Transition Sky within a lower NELM and generally lower observing conditions
Light pollution domes visible in several directions
Zodiacal light still visible, but not past halfway to the zenith at dusk or dawn
Clouds are illuminated in the directions of light sources, but dark overhead
Surroundings are clearly visible even at a distance
The Milky Way lacks detail and not dramatically visible in the winter
Only the brightest deep sky objects are visible
M33 is difficult to spot in peripheral vision and only visible when high in the sky
a 32-cm reflector with moderate magnification will reveal stars of magnitude 15.5
Class 5: Suburban Sky
Light pollution visible in most, even all directions
Only hints of zodiacal light on best autumn and spring nights
Clouds are noticeably brighter
Milky Way is invisible near the horizon and looks washed out overhead, even subtle in the winter
A 32-cm reflector will reach about magnitude 14.5 to 15
Class 6: Bright Suburban Sky
Light pollution makes everything within 35 degrees of the horizon glow grayish white
No zodiacal light
Clouds appear fairly bright
Surroundings easily visible
Milky Way only visible near the zenith
M33 invisible without binoculars, Andromeda Galaxy (M31) moderately visible to naked eye
A 32-cm telescope used at moderate powers will show stars at magnitude 14.0 to 14.5
Class 7: Suburban/Urban Transition
Light pollution makes the entire sky light gray
Strong light sources evident in all directions
Clouds brightly lit
Milky Way nearly or completely invisible
M44 and M31 may be visible with naked eye but indistinct
Through even a moderate sized telescope telescope, the brightest Messier objects show little to no detail
A 32-cm reflector will barely reach 14th magnitude
Class 8: City Sky
Sky is light gray or orange
You can easily read newspaper headlines thanks to light pollution
Some stars in familiar constellations are weak to invisible
Even with a telescope, only bright Messier objects can be detected while M31 and M44 may be spied by an experienced observer on good nights
A 32-cm reflector limit is about magnitude 13
Class 9: Inner-City Sky
Sky is full of artificial light, even at the zenith
Many stars in familiar constellations may be weak to invisible and most fainter constellations are invisible
The Pleiades is the only Messier object visible to the naked eye
The only objects to observe well in a telescope are the Moon, planets, bright satellites, and a few of the brightest star clusters on good nights
Many resources utilize the Bortle Scale to create dark sky maps like ClearDarkSky’s Light Pollution Map or expanded versions of it such as Dark Site Finder and Light Polltuion Map. You can also check out the certified International Dark Sky Parks for dark sky sites that meet a number of other qualifications.
With the ever-increasing number of city lights and other artificial lights, many of us are lucky if we live in class 4 or 5 locations with even a class 3 within an hour’s drive. Finding a class 2 site is becoming rare as cities expand and suburban areas become more and more developed.
Light pollution causes a number of issues for both animals and humans and is the bane of astronomers. However, the Bortle Scale and maps that utilize it help us to better understand the light pollution in a given area and therefore its effects on the environment, where the darkest skies are, and what we can reasonably observe in the night sky in these locations.
While the Bortle Scale is primarily utilized by amateur astronomers to help them plan their observing sessions, it is also used by professional observers as a common baseline for observing conditions and verifying reports. It has also created a standard that helps other researchers, activists, and concerned citizens create solutions to help people and animals better perform functions that are linked to the night sky.
The next time you are considering stargazing, consider checking out where your location lies on the Bortle Scale to help determine what you will see and where you should go.
Sarah Hoffschwelle is a freelance writer who covers a combination of topics including astronomy, general science and STEM, self-development, art, and societal commentary. In the past, Sarah worked in educational nonprofits providing free-choice learning experiences for audiences ages 2-99. As a lifelong space nerd, she loves sharing the universe with others through her words. She currently writes on Medium at https://medium.com/@sarah-marie and authors self-help and children’s books.