How to measure the missing stars over your suburban backyard

How dark is your night sky? In Illinois, there are no known Class 1, 2 or 3 dark sky sites. We've obscured them all with our outdoor lights. The Chicago Chapter of the International Dark-Sky Association is encouraging residents to measure the presence of artificial light at night and the intensity of that light in your neighborhood.

The Bortle Dark-Sky Scale 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.

"Still" is an important qualifier, because the starry night sky is simply fading from view.

The Bortle Dark-Sky Scale attempts to quantify that absence (the missing stars, globular clusters, whole galaxies) by measuring the presence of artificial light at night (ALAN), and the intensity of that light.

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 Bortle 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. 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 and reported by Science Magazine, that sky brightness resulting from artificial light has doubled in less than eight years and continues to increase exponentially at 10% each year, we also know that ALAN 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.

Cities that are densely populated and industrialized, and don't subscribe to dark-sky friendly lighting ordinances, are often the greatest producers of artificial light at night.

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, 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 star-seeing conditions degrade further, the skyglow is bright enough that you could read a paper copy of the Daily Herald, outside, without any additional light.

You might be able to see the Pleiades under a Class 8 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, like the Moon, and planets like Jupiter and Venus.

In Illinois, Chicago is a Bortle Class 9 Inner City Sky; Downers Grove, in the southwest suburbs, is a Class 8 City Sky; and Naperville and Waukegan are Class 7 Suburban/Urban transitional skies. Very few stars can be seen over the densely populated Chicago area.

Contrast these conditions to those of a Class 1 or 2 dark sky in a place like Yellowstone or Yosemite National Park, where the stars 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, your friends 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.)

You can experience a Class 3 Rural Sky in Michigan, at the Keweenaw Dark Sky Park (www.darksky.org), eight hours north of Chicago, where the Keweenaw Peninsula extends into Lake Superior.

Although there is still some light pollution in evidence, the Keweenaw Class 3 sky is dark enough that objects within 20 to 30 feet from where you stand, under that starry dome, will be obscured by deep dark.

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 that radiate from nearby Ottawa and Marseilles are visible, the Milky Way is still visible to the eye.

But hurry if you want to catch it, because at the current rate the stars over that campground will soon disappear unless we start managing outdoor lighting.

That 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 by timers or motion sensors to turn off when it's no longer needed. And tell your neighbors to do the same.

Learn more at darksky.org.

For more on the Chicago Chapter of the International Dark-Sky Association and the Bortle Dark-Sky Scale, visit www.idachicago.org/resources/bortle-scale.

Bringing back starry night skies

Every month Globe at Night (<a href="https://globeatnight.org/">globeatnight.org</a>) collects Bortle Scale data from around the world to contribute to critical ALAN research.

It's easy to add to the data set.

• Download and install the SkyView Lite mobile app for <a href="https://play.google.com/store/apps/details?id=com.t11.skyviewfree&hl=en_US&gl=US">Android OS</a> or <a href="https://apps.apple.com/us/app/skyview-lite/id413936865">Apple iOS</a>.

• On the dates designated by Globe at Night, locate the constellations that they've requested: You'll find the calendar and constellations for 2023 at <a href="https://globeatnight.org/campaigns/">globeatnight.org/campaigns/</a>.

• Follow the instructions on the Globe at Night <a href="">website</a> and enter the information requested

• Ask three friends to do the same

• Repeat monthly