How Dark is Your Sky?

Have you ever seen a truly dark sky before?  Have you gazed up at the night sky and seen the Milk Way in all its dazzling beauty?  Have you seen the staggering splendor of the zodiacal light after twilight?  Have you seen the magnificent Messier galaxies and clusters with your naked eye on the clearest of nights?  If you are younger than 60 and live within 150 miles of a major city, chances are you answered “No” to each of those questions.  That is because of the effects of light pollution, specifically skyglow.  Skyglow is the unmistakable “glow” visible in a dome shape that seems to hover above and around cities engulfed in the effulgent glow of electricity.  It is remarkable how the tiniest amount of skyglow can render invisible a large amount of stars at night leaving the brightest objects illuminated as mere ghosts of their true beauty.

This type of light pollution drives astronomers, professional and amateur alike, insane!  It hinders the effectiveness of not only the naked eye, but the most sophisticated telescope, it masks the fierce beauty of the night sky, and hampers the scientific pursuits of the amateur star-watcher.

Aside from its adverse effects on astronomy, light pollution also is a huge energy waster!  I touched on this earlier in my post on Dark Skies, but it’s worth mentioning again that over illumination in the United States is responsible for approximately 4-5 million barrels of petroleum per day in energy wasted!  Our light-loving lifestyle is highly inefficient and wasteful and is damaging our health and the environment as well.  If you are passionate about night sky preservation visit the folks over at the International Dark-Sky Association.  They have many resources available to make efficient and night sky-preserving lighting possible.

If you want to know the extent of the light pollution where you live you can obviously just look outside on a clear night.  You can also use this chart published by Sky & Telescope Magazine in 2001.  The Bortle Dark-Sky Scale ranks the varying levels of dark sky based on the ability to see certain celestial objects and the visible magnitude of those objects.  This chart describe the classes of dark sky as outlined by John Bortle.  (**Note that this chart is from Wikipedia; I would have linked to it here but as Wikipedia will be blacked out globally tonight to protest the SOPA and PIPA acts in the U.S. I’ve provided here.  This graphic belongs to Wikipedia, I do not own this graphic nor have I had anything to do with its creation).  Sorry if the chart doesn’t quite fit on the page, here’s a link to the original article in Sky & Telescope.

That being said, go ahead and use this chart to find out which class your home falls in.  If it falls in classes 1-4 get out there tonight and do some stargazing!  If you’re like me and most likely live in a class 5 or 6, well my binoculars are easy to transport to someplace darker which is quite often the case when I use them.  Please feel free to comment on your night sky conditions where you live so we can all come over your house and have a star party! (only if you live in a 4 or above ;) )  Enjoy!

 

Class Title Color key Naked-eyelimiting magnitude Stellar limiting magnitude (with 12.5″ reflector) Description
1 Excellent dark-sky site
black
7.6–8.0 19 at best Zodiacal lightgegenschein, zodiacal band visible; M33 direct vision naked-eye object; Scorpius and Sagittarius regions of the Milky Way cast obvious shadows on the ground; airglowis readily visible; Jupiter and Venus affect dark adaptation; surroundings basically invisible.
2 Typical truly dark site
gray
7.1–7.5 17 at best Airglow weakly visible near horizon; M33 easily seen with naked eye; highly structured summer Milky Way; distinctly yellowish zodiacal light bright enough to cast shadows at dusk and dawn; clouds only visible as dark holes; surroundings still only barely visible silhouetted against the sky; many Messier globular clusters still distinct naked-eye objects.
3 Rural sky
blue
6.6–7.0 16 at best Some light pollution evident at the horizon; clouds illuminated near horizon, dark overhead; Milky Way still appears complex; M15M4M5, and M22 distinct naked-eye objects; M33 easily visible with averted vision; zodiacal light striking in spring and autumn, color still visible; nearer surroundings vaguely visible.
4 Rural/suburban transition
green
6.1–6.5 15.5 at best Light pollution domes visible in various directions over the horizon; zodiacal light is still visible, but not even halfway extending to the zenith at dusk or dawn; Milky Way above the horizon still impressive, but lacks most of the finer details; M33 a difficult averted vision object, only visible when higher than 55°; clouds illuminated in the directions of the light sources, but still dark overhead; surroundings clearly visible, even at a distance.
yellow
5 Suburban sky
orange
5.6–6.0 15 at best Only hints of zodiacal light are seen on the best nights in autumn and spring; Milky Way is very weak or invisible near the horizon and looks washed out overhead; light sources visible in most, if not all, directions; clouds are noticeably brighter than the sky.
6 Bright suburban sky
red
5.1–5.5 14.5 at best Zodiacal light is invisible; Milky Way only visible near the zenith; sky within 35° from the horizon glows grayish white; clouds anywhere in the sky appear fairly bright; surroundings easily visible; M33 is impossible to see without at least binocularsM31 is modestly apparent to the unaided eye.
7 Suburban/urban transition or Full Moon
red
4.6–5.0 14 at best Entire sky has a grayish-white hue; strong light sources evident in all directions; Milky Way invisible; M31 and M44 may be glimpsed with the naked eye, but are very indistinct; clouds are brightly lit; even in moderate-sized telescopes the brightest Messier objects are only ghosts of their true selves.At a full moon night the sky is not better than this rating even at the darkest locations with the difference that the sky appears more blue than orangish white at otherwise dark locations.
8 City sky
white
4.1–4.5 13.5 at best Sky glows white or orange—one can easily read; M31 and M44 are barely glimpsed by an experienced observer on good nights; even with telescope, only bright Messier objects can be detected; stars forming familiar constellation patterns may be weak or completely invisible.
9 Inner-city sky
white
4.0 at best 13 at best Sky is brilliantly lit, with many stars forming constellations invisible and many weaker constellations invisible; aside from Pleiades, no Messier object is visible to the naked eye; only objects to provide fairly pleasant views are the Moon, the planets, and a few of the brightest star clusters.

About Tim

My name is Tim Phelan. I am a nerd, amateur astronomer, sports nut, and follower of Jesus. I live in Baltimore, MD where the skies are oh so polluted with light. This is Ravens Country, Birdland, and the City that Reads, or whatever. Follow me on acrosstheuniverseinnotime.com and tphelan.wordpress.com

Posted on January 17, 2012, in Night Sky and tagged , , , , . Bookmark the permalink. 5 Comments.

  1. HOW DO I KNOW WHICH CATEGORY MY HOM E IS IN?

    • Hi Dan,

      The best way to find out which category your sky is would be to use a magnitude chart. They show you the stars visible in each category in an easily recognized constellation. Globe at Night has a website where you can see a visual representation of each category by looking at the constellations Orion or Leo. Check it out here: http://www.globeatnight.org/observe_magnitude_orion.html Best of luck!

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