“What do we lose when we lose the night?” is the question asked by “The City Dark,” an informative, well-rounded look at light pollution told with a star-gazing enthusiast’s passion for his subject. Helmer Ian Cheney, best known as the co-creator of docu “King Corn,” explores the social, physical and artistic consequences of our not-so-dark night skies, divvying them up into six chapters that provide a wide-ranging discussion of this recent phenomenon. Though a 53-minute version is on offer, smallscreens could easily showcase the longer fest cut, a natural for PBS and environment-friendly channels worldwide.

Cheney lives in Gotham but grew up as an astronomy dweeb in rural Maine. An accomplished astrophotographer, he was struck by the difference in the night skies between the two places, and set out to investigate how light pollution is changing the world. Like all market-savvy docu helmers, he knows to offer a bit of hope by the pic’s end, though irreversible global industrialization won’t cede modernization to the benefits of a dimmer glow.

Two-thirds of the planet’s population lives in a “luminous fog,” meaning the constant aura of urban lights obscure starry skies. Via an impressive roster of scientists and amateur astronomers, Cheney addresses the problems this causes for the planet, such as the increased difficulty of detecting potentially catastrophic meteors as they approach Earth even with super-light-sensitive instruments.

All this extra light is compromising the natural world: Sea turtle hatchlings instinctively head to the protection of light-reflecting ocean waters, but are doomed when urban glow coaxes them in the opposite direction. As for humans, studies show that women who do night-shift work are significantly more at risk for breast cancer, probably due to melatonin levels that need the light/dark cycle to properly adjust; thus, a brighter nighttime for the population in general might suppress melatonin. However, it would have been useful had Cheney spoken with experts on such problems in the Arctic, whose populations annually go three months without seeing the sun.

In the last two sections, the helmer admits that city lights in dangerous neighborhoods offer reassurance and have been credited with a decrease in crime, but he also speaks with light designers such as Herve Descottes, who worked on Manhattan’s High Line Park, to prove that properly directed light can keep the night sky dark while offering sufficient illumination.

Impressive images of star-filled nights will remind auds of childhood trips to the planetarium, and few will fail to notice the difference when they next look up into the heavens. Clever animation helps keep things upbeat, much like the music, though some may find the tunes try too hard to buoy the spirit.