Light at night

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Clingmans Dome.

Night sky monitoring takes place on the roof of Clingmans Dome, the highest point in Tennessee.

NPS photo.

It’s the middle of a frigid October night, and we’ve climbed on the roof of Clingmans Dome. Fog swirls tight around us, nosing its way through our jackets and hiding the sudden edges of our perch. We hurriedly open equipment cases and hand sensors, cameras, and tripods to Kate Magargal, the technician from the NPS Night Sky Program. When she’s done assembling her instruments, we stand and shiver and wait for the fog to clear, so we can take images of how dark the night sky is—or isn’t—over Great Smoky Mountains National Park.

When Thomas Edison invented the light bulb, he probably could not have imagined the pulsing glowing cities and veins of highways lit with headlights connecting them. And few people would have imagined a downside to more light. Yet the constant shine of light over much of the globe had has consequences, both visible and invisible. On humid nights, we can see a tangerine glow in the sky rather than stars. City and traffic lights illuminate the night sky, masking the light from stars and moon and limiting our ability to see safely. These are tangible results of light, and are so common we hardly remark upon them, except in their absence.

Lights all over the world glow into the sky at night.

Much of our light at night spills into the sky, wasting money and altering animal behavior.

C. Mayhew & R. Simmon, NASA, NOAA, NGDC.


Light at night has invisible impacts, too. All over the world, millions of artificial lights spilling into the sky change the way animals—including humans—behave and how healthy they are, both on the individual and population level. Biologists call the extra light that spills into the sky ecological light pollution. Monitoring light as part of a “threat assessment” program is a relatively new idea, because we’re just realizing all of the ecological impacts that artificial light at night can have.

A new discipline—Scotobiology—is the study of light as related to biological processes. Scotobiological studies show that artificial light at night has impacts on

  • Predator/prey relationships, which are changed when predators can see more, and prey can’t feed, breed, or move in the night light
  • Migratory animals such as birds, insects, bats, and others that navigate by light
  • Urban & suburban animals that are attracted to dangerous roads and windows by light
  • Sea turtle hatchlings that mistake land lights for a moonlit ocean
  • Tropical species, which are used to 12 hours of dark and light
  • Humans, whose exposure to light during the night disrupts circadian rhythms, which in turn alters melatonin levels and is linked to cancers
  • City budgets, which nationwide waste $10 billion casting extra light into the sky

What’s the night sky like in national parks? Go to page 2: Light nights in the national parks.

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