The bright glow of cities is a testament to human ingenuity. In less than 150 years, we've gone from the first light bulb to a carpet of electric lights visible from space. But all that light has a dark side to it -- it's blotting out the stars, confusing animals and endangering ecosystems.

Light pollution is the subject of a new documentary film premiering Thursday on PBS, The City Dark, by Ian Cheney, the co-creator of King Corn, a documentary exploring the corn industry. The filmmaker tracked down ecologists, astronomers and cancer researchers in order to gain perspective on what we lose in all that brightness.

Stargazing

A lot of the lights in cities are unshielded, casting light in all directions. That means a good portion of light is shooting straight up into the sky, making it difficult for astronomers to get a glimpse of faint objects that are swallowed up by the urban glare.

City lights also interfere with another tool of astronomy called spectroscopy, in which a scientists splits the light entering the telescope into all the different component colors. Spectroscopy allows astronomers to figure out lots of characteristics of objects in space: chemical composition, temperature, even how fast the object is moving.

But if city light invades the telescope as well, astronomers have no way to separate out this light from the light generated by stars and galaxies they're trying to observe. The encroachment of cities and the spread of light pollution has forced many telescope jockeys to flee far from civilization -- to observatories on islands, mountaintops, deserts and Antarctica.

Not only does light pollution make it harder for dedicated astronomers to do their jobs, it means most city- and suburban-dwelling people have become astonishingly unfamiliar with the night sky.

In a 2009 piece for the journal Environmental Health Perspectives, writer Ron Chepesiuk recalls how frantic Los Angelenos called emergency services after a 1994 blackout. The callers were anxious about a strange, vast silvery cloud in the night sky.

What they were really seeing for the first time was the Milky Way, long obliterated by the urban sky glow, Chepesiuk wrote.

Animal Behavior

Light pollution also has deleterious effects closer to the ground.

While some species may benefit from an artificially longer day that gives them increased time for foraging, there's plenty of scientific evidence showing that a lot of species are being harmed by the constant barrage of light.

For generations, newly hatched sea turtles have made their way to the sea instinctively by heading away from the unbroken darkness of land. But now, with artificial light invading many beaches and hatching grounds, the newborn turtles will head towards the light, often to their deaths.

Bright lights also disrupt the navigation systems of migrating birds, many of whom calibrate their flight patterns based on the moon and stars. When passing through brightly lit areas where those landmarks are blotted out, the birds can become disoriented and may even crash into illuminated towers or buildings. The advocacy group Fatal Light Awareness Program, or FLAP, estimates that between100 million and 1 billion birds are killed by man-made structures each year in North America.

Light-based Patterns

Constant light exposure also confuses plants, many of which calibrate their life cycle -- how they grow, when they flower -- based on the photoperiod, or day length.

Artificial lighting ... extends the day length and can change flowering patterns and, most importantly, promote continued growth, thereby preventing trees from developing dormancy that allows them to survive the rigors of winter weather, Purdue University urban forester William R. Chaney wrote in 2002.

Humans can also feel the effects of all that light -- it's thought that artificial light upsets our natural circadian rhythm, contributing to a range of sleep disorders. Scholars from the University of Aberdeen wrote in a 2011 paper in the journal Medical Hypotheses that this circadian imbalance may contribute to metabolic dysfunction -- in short, that light pollution is one of the contributing causes to the obesity epidemic.

What can be done to turn on the dark? The International Dark-Sky Association, which has recently had success pushing for outdoor nighttime lighting regulations in Hawaii, New Hampshire and Italy, recommends that people turn off outdoor lights when not in use and use shielded fixtures that direct light towards the ground.