The Milky Way near the Grand Canyon, coincidentally the first place I myself ever saw the Milky Way, which didn’t happen until my 20s, as I grew up in urban areas. Image credit: Bureau of Land Management, under a cc-by-2.0 license, via https://www.flickr.com/photos/mypubliclands/16353173238.

What light pollution costs us every night

If you don’t have pristine, dark skies, you might never connect to the Universe. But there’s hope.


“Before we devised artificial lights and atmospheric pollution and modern forms of nocturnal entertainment we watched the stars. There were practical calendar reasons of course but there was more to it than that. Even today the most jaded city dweller can be unexpectedly moved upon encountering a clear night sky studded with thousands of twinkling stars. When it happens to me after all these years it still takes my breath away.” -Carl Sagan

Human vision is ill-adapted to true darkness, but our eyes can provide us with stellar views of the night sky.

Notre Dame de Paris, by night time, seen from the east side. Note how few stars are visible behind it. Image credit: Wikimedia Commons user Atoma.

Since the invention of artificial lighting, however, our views of those natural wonders have diminished precipitously.

A composite image of the Earth at night, with data from 1994/1995. Image credit: Craig Mayhew and Robert Simmon, NASA GSFC, with data from Marc Imhoff of NASA GSFC and Christopher Elvidge of NOAA NGDC.

A view of the Earth at night shows how brightly lit our planet is.

The Bortle Dark Sky Scale, from 1–9, illustrating urban to pristine skies. A full Moon, incidentally, is bright enough that it can turn even a ‘1’ into a 6 (away from the Moon) to an 8 (nearby it) on its own. Image credit: International Dark Sky Association, 2008, using the free software Stellarium.

While this severe lighting negatively impacts wildlife — flora and fauna — the greatest loss to humanity has been our connection to the Universe.

What a digital camera (top) and the human eye (bottom) sees from dark sky locations rating a 4, 6 and 9 on the Bortle scale, respectively. Image credit: Tony Flanders of Cloudy Nights, via http://www.cloudynights.com/topic/306632-illustrated-with-real-images-bortle-scale/.

The amount of light pollution is quantified by the Bortle Dark-Sky Scale, where lower numbers represent the most pristine conditions.

The Bortle Dark-Sky Scale was begun by amateur astronomers in 2001, but is now used as a universal standard for light pollution. Image credit: The Bortle Dark Sky Scale according to the Big Sky Astronomy Club, via http://www.bigskyastroclub.org/lp_bortle.html.

One is where the ground brightness is below 1% of the natural dark sky; 4/5 occurs when the ground brightness equals the sky; a 9, hundreds of times as bright, will erase nearly every star.

Under truly dark conditions (L), clouds appear dark against the night sky. If the ground lighting is too great, clouds themselves will be brighter than the sky (R). Images credit: Christopher Kyba and Ray Stinson under c.c.-by-s.a. 3.0, via http://userpage.fu-berlin.de/~kyba/images/night_cloud_comparison.html.

When the ground’s brightness surpasses the sky’s, clouds appear bright against the night, rather than as dark silhouettes.

A view of a pristine night sky, edited to reflect the full extent of what the human eye can see, from the Grand Canyon’s North Rim. Image credit: Richard Ryer of Panoramio, via http://www.panoramio.com/user/87752.

While cameras reveal many more stars than the unaided eye can, a dark, night sky offers spectacular views to humanity.

The increase in artificial night sky brightness in North America, including an extrapolated prediction for light pollution levels in 2025. Maps created by P. Cinzano, F. Falchi, and C. D. Elvidge.

As urbanization increases, dark skies become rarer and less pristine.

Unless we take precautions, the only remaining dark skies will be found in space.


Mostly Mute Monday showcases an astronomical phenomenon, event or object in pictures, visuals and no more than 200 words.

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