Dark Sky Tutorial

As you get more involved in astronomy, you'll realize that the ability to faint objects, such as galaxies, nebulas, and comets, requires a high contrast between the object and the background sky.  In other words, a faint fuzzy shows up best against a dark sky.  This has become more difficult because poorly-designed and/or poorly-aimed lighting fixtures cast much of their illumination into the sky instead of downward where it is needed.  This waste of energy, fossil fuels, dollars, and the night sky is known as "light pollution," and it is the bane of stargazers everywhere, in addition to being a serious driving hazard due to the side glare these fixtures create.  Notorious light polluters in Stark County are billboards that are illuminated from the bottom (much of which misses the sign completely), the dusk-to-dawn unshielded barnyard "security" light (which hardly provides security), unshielded "wall mount" lights on buildings, and, especially, the "acorn" lights, such as those in the center of Canal Fulton.  To get into "dark sky country," you have to get away from all of these.  But finding as dark a sky as possible is not as easy as one might initially think.  Even small towns can contribute a reflected "sky glow" as their lights spill outward and upward.  To find a good dark-sky site, it is necessary to get as far away as one can from all sources of light pollution.  The challenge lies in finding a dark-sky site that is both remote, yet a convenient distance away.

How do we stack up dark-sky sites against each other?  The answer lies in what astronomers call the magnitude system.  The higher the magnitude number, the fainter the star, so a 1st-magnitude star is brighter than a 2nd-magnitude star, which is brighter than a 3rd-magnitude star, and so on.  Some objects like Venus, the Full Moon, and the Sun are so bright they have NEGATIVE magnitudes (about -4, -13, and -27, respectively).  Each magnitude represents a difference in brightness of 2.5 times.

The stars of the Big Dipper are 2nd- and 3rd-magnitude stars, while the stars of the Little Dipper range from magnitude 2.0 to 5.0.  For the purposes of dark-sky searching, the faintest stars that are visible to the unaided-eye at the overhead point are what is known as the "unaided-eye limiting magnitude," or NLM, for short (see note 1 below). For example, on a clear, moonless night at TWC, our NLM is around 6.0, provided there is no haze.

Skyglow extends far beyond its city of origin.  For instance, in a typical suburban sky that is not directly impacted by lighting, a NLM of about 5.0 is common.  This means that at TWC, the faintest stars you could see would be 1.0 magnitude fainter--but you would see 2.5 TIMES as many stars!  Sadly, TWC is not as dark as it once was, thanks to the effects of light pollution, chiefly from Massillon and Canton, but it is still a vast improvement over the more populated areas of Stark and Wayne County.

What do dark-sky sites have in common?  For the most part, they are remote, typically well away from any towns.  Elevated sites are preferred, both in terms of horizons and limiting the amount of dew or fog that could form as the night goes on.  Land that was formerly strip mine land is ideal in this regard.  Preferably the site is away from the road (oncoming headlights tend to ruin your night vision), accessible in a variety of conditions, and at least 500 yards from the nearest dwelling (again, the notion that the steady glow of a dusk-to-dawn "security" light will somehow frighten an intruder off is a popular one even in the countryside).  Even if all of the above apply, make sure you are at least several miles from any prisons or ski slopes; since they are usually the worst rural light polluters.   Obviously, the darker the sky, the better, but the tradeoff is typically a longer drive.

WHAT a dark-sky site is like is one thing.  WHERE to find them is the challenge.  The Dark Sky Search Tips was created solely for this purpose.  We have selected several regions and list the possible advantages and disadvantages of the sites.  Notice that we selected dark sky REGIONS.  One still has to find an individual SITE.  To this end, there are some useful resources.  The first, of course, is us!  Many WCAC members know of the location of dark-sky sites and appreciate the company.  Second, if you find an area that looks remote, try http://terraserver.microsoft.com.  This site provides on-line topographic maps and aerial photographs for the United States.  As always, if you find a spot, make sure you have permission to use the site, whether it is a park, public land, or private property.

Happy Searching!

Phil Creed
4/1/2000

Web master's Note 1) I replaced the term common to many amateurs N@$#D-EYE with UNAIDED EYE due to children's Web filter software that will not allow pages with that N word to be displayed. (If you have any questions email Bill Castro.) (return to text.)
 


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