In order to really see what the night sky has to offer, after you've waited for your eyes to fully adapt to the dark and there's no moon, you need to have cloudless skies and be away from city lights, including lights from housing developments, highways, rural strip malls, and industrial parks. Who came up with the term "industrial park?" In the early 1800s, the view from a city park or even from a window in a row house was almost as dark as the view from the countryside and certainly darker than any city or suburban location today. However, later in the 19th century, once gaslights and electricity were utilized for lighting purposes, cities and towns began to glow, resembling fireflies in a darkened field.

Today, urban areas are awash in light. Light pollution is a term that was introduced only in the last 40 years. For many urban dwellers, with the exception of a few of the brightest stars and planets, the stars don't really come out at night anymore. Some sky glow is natural, for instance, the faint glow from aurora and other upper atmospheric sources (airglow), the weak glow from starlight that's scattered by the atmosphere and the glow from sunlight scattered off interplanetary dust particles (zodiacal light). But these natural sources of sky glow are rendered invisible by a couple of floodlights in a neighbors yard or even a lamp post or two along your driveway. In the suburbs today, the night sky directly overhead is about 5 to 10 times brighter than the natural sky glow, and overhead in a large city, the sky may be 25 to 50 times brighter than the sky in a remote area unsullied by light pollution! As a result, inhabitants of urban areas are essentially being deprived of the wonders of the night sky.

Light pollution that comes from a horizontal direction is worse than the light that's directed upward. Light aimed straight up generally impacts those people within the immediate vicinity of the source itself, but light that comes from a sideways direction (glare) passes through more of the atmosphere, and thus pollutes more of the night sky.

You've probably seen those nighttime visible satellite images of the US and the world that show the presence of city lights. These images are being used to show where urban sprawl is increasing most rapidly. Actually, these images look more ominous, as sources of light pollution, than they are in reality. The sensors that make these images use a type of photomultiplier tube that's especially sensitive to the brightest lights. So a satellite pixel can be saturated with light when, on the ground, the light pollution may, in fact, be less severe. Nevertheless, by any standard the nighttime skies over certain parts of the US are simply too bright to adequately observe what's there to see. Whereas 30 or 40 years ago, you could drive an hour away from most cities and still find a good dark sky, nowadays, in the northeastern US, a two to three hour drive may be required.

One litmus test for deciding if the night sky suffers from too much light pollution is whether or not you can clearly see the Milky Way - the milky band traversing the sky consisting of the hundreds of millions of stars that make up our own spiral galaxy. A more detailed test for determining the darkness of the night sky was formulated by amateur astronomer John Bortle.

The Bortle Dark-Sky Scale was developed to rate the night sky conditions for a particular observing site. A 9 level scale is used to judge the truly darkness of the sky. Level 1 represents the best viewing conditions on Earth. It's so dark that nearby objects on the ground, such as a telescope are almost invisible. With the unaided eye, you're able to detect the very faint glow of a handful of galaxies, and the Milky Way can be seen stretching all of the way across the sky. Level 5 represents a typical suburban sky. The glow of the Milky Way is rather weak overhead and can't be seen at all near the horizon. Light sources are obvious in most all directions, and if clouds are present, they're brighter than the sky itself. Level 9 represents what you can see from the "inner city" of an urban complex. Even overhead the sky is brightly lit, and only the brightest stars (magnitude 2 or brighter) and most obvious constellations (Orion and the Big Dipper, for instance) can be made out. With a telescope, only the moon and a few planets are interesting.

So, how many of us can clearly see the Milky Way on a clear, moonless night? According to astronomers who recently put together the first World Atlas of Artificial Sky Glow , it was estimated that about 1/2 of the people living in Europe and approximately 2/3 of the people here in the US cannot see the Milky Way from their home. World-wide, perhaps 1 out 5 people live in a place where the night sky is too bright to identify the Milky Way. Even sadder, the astronomers found that 99% of the US and European population live in areas considered light polluted!

Fortunately, sky glow is not necessarily a permanent problem. Light pollution can be drastically reduced by eliminating the wasted light that's directed upward and using bulbs that reduce glare. It's believed that 30% of all lighting in the US is wasted light, and wasted light is wasted energy. As an example of what can be done to lessen the effects of sky glow, light pollution activists in Connecticut helped convince the state and county governments to replace more than 180,000 streetlights with glare-free light fixtures as the old lights wore out.

For more about this see - Sky Maps at http://www.skymaps.com