Gone Missing
How the Bortle Scale Will Break Your Heart
The Bortle Dark-Sky Scale, introduced by amateur astronomer John Bortle in the pages of Sky & Telescope Magazine over twenty years ago, is a tool that scientists use to measure the changing state of the starry night sky. The Bortle Scale compares what is to what it might be, and ranges from Bortle Class 1 to Class 9. It relies upon the naked eye to assess what can still be seen in the heavens, unaided.
Centennial, Arizona Bortle Class 2 Night Sky | Photo by Jim Vajda
“Still” is an important qualifier. Because, like beaches around the globe, which are disappearing as the oceans rise, and like glaciers, which are shrinking as they melt from below, our starry night sky is simply fading from view.
The Bortle Dark-Sky Scale attempts to quantify that absence (missing stars, globular clusters, whole galaxies) by measuring the presence of artificial light at night (ALAN), and the intensity of that pollution. The Scale is additive, climbing from a Class 1, which describes perfect night sky seeing conditions that peer into infinity, to a Class 9, where the low, dim lid of the sky permits a view of the Moon, but little besides. The brighter the night sky, the larger the class, and the less that can be seen within the heavens.
So why measure what’s missing? Because these metrics help us mark decline, and assess the rate of change over time. Like explosive cholesterol numbers that prompt your doctor to advise the lifestyle changes that will keep you alive, there are things we can do to keep our Bortle Scale numbers in check – and our starry night skies visible. While we know, from measurements collected by citizen scientists, that “sky brightness resulting from artificial light has doubled in less than eight years” and continues to increase “exponentially in the world [at] an alarming average of 10% each year” (Science), we also know that ALAN pollution can be quickly reduced, if efforts to do so are well-coordinated and far reaching. Understanding the extent of the problem is the first step for any coordinated effort – becoming fluent in the Bortle Scale helps those efforts.
So where does your neighborhood sit on the Bortle Scale? Cities that are densely populated and industrialized, and don’t subscribe to dark-sky friendly lighting ordinances, are often the greatest producers of ALAN pollution. In these cities and neighborhoods it’s not uncommon for outdoor lighting to spill beyond the area of intended illumination and degrade the surrounding property and airspace with unnecessary light. Cities and suburban transitional spaces often rate a 7 to 9 on the Bortle Scale because they’re plagued by skyglow – the diffuse light that fills city skies at night and washes out the starscape with a grayish-white or orange glow.
That cloudy night sky? Is probably not clouds at all, but skyglow.
Under these conditions the Milky Way is no longer visible from the ground (Bortle Class 7 Suburban/Urban Transition Sky), and any clouds that are present in the sky will be brilliantly lit from below. Constellations – like the Big Dipper and Orion – become difficult to see at all under a Class 7 sky; Under a Class 8 City Sky, as more and more light at night is introduced and the star-seeing conditions degrade further, the skyglow is bright enough to read words off a page. Of the Messier Objects[ 1 ], only the Pleiades remains visible under a city sky. Move inward, to the heart of the city, where Class 9 Inner City Skies are the norm, and only the largest, brightest objects in the night sky can be seen. That includes objects like the Moon, and planets like Jupiter and Venus.
Chicago Bortle Class 9 Night Sky | Photo by George Livingston
In Illinois, Chicago is a Bortle Class 9 Inner City Sky; Downers Grove, in the southwest suburbs, is a Class 8 City Sky; and the city of Waukegan, an hour north of Chicago, is a Class 7 Suburban/Urban transitional sky. Very few stars can be seen over our densely populated Chicagoland.
Contrast these conditions to those of a Class 1 or 2 dark sky in a place like Yellowstone or Yosemite National Park, where one might stand under a night sky where the stars of the Zodiac and the Milky Way are bright enough to cast shadows on the ground, just as the Sun does on a cloudless day. The Milky Way – which is visible as a smoky wisp in the sky under some Class 3 and 4 skies – shines blue-white under true Class 1 and Class 2 skies, and stands in stark contrast to the neighboring stars, which glow yellow, like our Sun. Also visible under Class 1 and 2 conditions: The rich complexity of the Milky Way is easily in evidence and, with binoculars, you can make out intricately veined, marbled detail in our neighboring galaxy. The Milky Way appears limitless in Class 1 and 2 conditions, as do the heavens surrounding it. If you are lucky enough to share the experience of a truly dark sky with friends, they will be largely invisible to you, except where they (and you) are illuminated by starlight. (Give your eyes a minute or two to adjust to the dark.)
Here’s the heartbreak for folks in the Chicagoland area: There are no known Class 1, 2 or 3 dark sky sites in the state of Illinois. We’ve obscured them all, with our outdoor lights.
Bortle Scale Classes as described by John Bortle in Sky & Telescope Magazine, Feb 2001
You can experience a Class 3 Rural Sky in Michigan, at the Keweenaw Dark Sky Park, eight hours north of Chicago, where the Keweenaw Peninsula extends into Lake Superior. Although there is still some light pollution in evidence, at Keweenaw, Messier objects M4, M5, M15 and M22 are visible to the naked eye. Class 3 skies are dark enough that objects within twenty to thirty feet from where you stand, under that starry dome, will be obscured.
The Bortle Scale is rated against whether or not you can spot celestial objects, like the ones described above, with the naked eye. But how are you to find those objects if you don’t even know where to look for the Triangulum Galaxy, let alone the Milky Way? Several apps make this easy: The SkyView LiteⓇ mobile app for Android OS and Apple iOS helps you scan and search the skies via an augmented reality interface that adjusts to your location to help you locate the precise position of objects in the night sky. Most Zodiac constellations can be searched for in the app by name, as can the Milky Way. To find Messier Objects via the SkyView Lite app you’ll need to search for the associated NGC[2] number of that object:
Another useful tool for surveying a night sky remotely is the Clear Outside website, which was “designed for astronomers by astronomers.” Clear Outside provides hourly cloud and weather forecasts that include the Bortle Class rating for any location queried, and can also be accessed as Android and Apple iOS mobile apps.
The stars haven’t completely vanished over the state of Illinois. Over the Starved Rock campground, on the Illinois River, you can still experience a Class 4 rural/suburban transitional sky where, even though light pollution domes radiating from nearby Ottawa and Marseilles are visible, the Milky Way is still available to the eye.
But hurry if you want to catch it, because at the current rate the stars over that campground will soon disappear – that is, unless we do the work we need to do, as citizens and neighbors, to manage outdoor lighting. The work is simple: Wherever outdoor light appears, ensure that it is no brighter than absolutely necessary, is targeted to reduce light trespass, and is controlled to go off when it’s no longer needed. And tell your neighbors to do the same.
How you can help
Every month Globe at Night collects Bortle Scale data from around the world to contribute to critical ALAN research. It’s easy to add to the data set. Simply:
Download and install the SkyView LiteⓇ mobile app for Android OS or Apple iOS
On the dates designated by Globe at Night, locate the constellations requested—often Orion or Gemini or Leo. You’ll find the calendar and constellations for 2023 here
Follow the instructions on the Globe at Night website and enter the information requested
Ask three friends to do the same
Repeat every month
The constellation Orion in the night sky | Photo by Jim Vadje
[1] The Messier Objects are an assortment of 110 astronomical objects, including galaxies and globular star clusters, which were identified by Astronomer Charles Messier in 1774. Messier identified the objects to differentiate them from the comets that he was attempting to find in the night sky.
[2] NGC = New General Catalog of Nebulae and Clusters of Stars
