Practical Astronomy: Figure Out How Dark Your Sky Is

Those who are involved in popularizing astronomy are asked certain questions time and again.  One of these is "How many stars are there?" Only when one is blessed with a rare glimpse of a star-spangled sky with its myriad luminaries, can we understand why this question occurs.  Special computer analysis can readily answer the question. 

Most people probably want to know how many stars they can see with their unaided eyes.  Unfortunately, few enjoy unspoiled, pollution-free skies.  But for those who do, it is often said that the average naked-eye limit is magnitude 6.5. Over the entire celestial sphere there are 8,479 stars within that magnitude limit. 

(On this scale, larger numbers represenent dimmer objects, and the brightest objects have negative magnitudes. The brightest star, Sirius, shines at magintude -1.42.)

Of course, we couldn't see them all at once since half of these available stars are always below the horizon.  There is also the factor of atmospheric absorption, which severely reduces the number of stars visible near the horizon, even under ideal skies. 

Because of all of this, the total number we can see at any given moment-under perfectly dark and clear conditions-is close to 2,500. In a city like New York, with all the local glare, the number can dwindle to just 15 or so. So, poets who talk of millions of stars are either using a telescope or they exaggerate.

• See Also - Our Tiny Universe: What's Really Visible at Night

Your neighborhood

So, just how dark are the night skies in your neighborhood? 

An easy way to make a quick determination is to seek out the constellation Ursa Minor, the Little Bear, popularly known as the Little Dipper.

In Ursa Major, the Big Bear, we have the bright and familiar stars of the Big Dipper.  In contrast, the stars of the Little Dipper are rather faint, except for Polaris, the North Star at the end of the handle and the two stars in front of its bowl.  These stars, Kochab and Pherkad have been called Guardians of the Pole because they march around Polaris like sentries.

The four stars in the bowl of the Little Dipper are composed of stars of magnitude 2, 3, 4 and 5.  So, if you can see all four stars in the bowl, you have access to a good, dark sky. 

If, however, you can only see the Guardians, your sky quality can be considered fair-to-poor. 

• Make Your Own Sky Maps Online

Pollution effects

Sadly, the increase in light pollution -- defined simply as excessive or misdirected outdoor lighting -- especially over the last quarter-century has made finding such dark skies more difficult.  David Crawford of the International Dark Sky Association notes: "Few members of the general public have ever seen a prime dark sky.  For urban dwellers, star studded nights are limited to planetarium simulations." 

Dazzling, unshielded light fixtures-the kind that waste illumination by sending it directly up into the sky-is also a very real hazard to motorists by sending vision-impairing glare directly into their eyes. In fact, some years ago in the New York Driver's Handbook, under the heading "Night Driving," it was advised that drivers "adjust sun visors to reduce glare from overhead (street) lights."  Properly shielded lighting not only minimizes light pollution but also avoids such hazardous glare (and saves energy to boot). 

In a land where auto use is so widespread, accident rates high and insurance costs soaring, we have a very good case that the worst type of lighting for astronomy can also be life threatening to everyone else.

• Disappearing Stars: How 2,500 Points of Light have Dwindled to 15

Party on

From now through March 21, you are invited to join thousands of other students, families, educators, and citizen-scientists by participating in "GLOBE at Night." GLOBE stands for Global Learning and Observations to Benefit the Environment sponsored by a worldwide hands-on, primary and secondary school-based education and science program sponsored by NASA, the National Science Foundation and others. There is no cost to participate. 

GLOBE is designed to observe and record how the constellation Orion appears from different locations as a means of measuring the brightness of the sky at a variety of urban and rural sites.

Prospective observers can report their results online by comparing the number of stars they could see in Orion with a set of template images on the program's Web site, which depict the number of stars in Orion for a range from bright skies to very dark.

Participation is open to anyone-anywhere in the world-who can get outside and look skyward in the evening from now through March 21.  This time frame was chosen because the Moon would provide little or no interference with observations. 

Clear skies and good luck!

• Make Your Own Sky Maps Online
• Our Tiny Universe: What's Really Visible at Night
• Disappearing Stars: How 2,500 Points of Light have Dwindled to 15
• The 10 Brightest Stars

Joe Rao serves as an instructor and guest lecturer at New York's Hayden Planetarium. He writes about astronomy for The New York Times and other publications, and he is also an on-camera meteorologist for News 12 Westchester, New York.