Light at night

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
Kate sets up night sky monitoring equipment.
Kate sets up night sky monitoring equipment on the roof of Clingmans Dome tower.

NPS photo.

Light Nights in the National Parks

The dark skies of many national parks are disappearing. Monitoring helps us keep an eye on how light our skies are becoming with surrounding human development, how animal behavior changes with more light, and how we can plan to keep our dark skies dark in the future.

The program to monitor our night skies began in Utah and is now based in Ft Collins at Colorado State University. This program is the first in the country to measure light from the ground. Traditionally, scientists measure light from space using satellite imagery. But this doesn’t take into account perspectives from certain places on the ground, which is how people and other animals experience the light.


Night Skies Program technicians use a CCD camera, a sturdier field version of what astronomers use in a planetarium. This digital camera helps scientists estimate how much light hits each pixel on the image of the sky. The light is measured in magnitudes (intensity of the light’s brightness) per arcsecond (a sliver of space cut out of the circular “pie” of the sky, looking up).

As the Night Sky Monitoring technician, Kate visits about 30 of the 50 national parks enrolled in the Night Sky Monitoring Program each year. While more Western parks are represented now, the program is adding Eastern units where there are more bright cities close to parks.

At Yosemite, we can see the Milky Way and an almost pristine night sky.
At Yosemite National Park, we can see the Milky Way in an almost pristine, dark night sky.

NPS photo.

Which parks are the darkest?
If you’re looking for stars, head out West to parks far from population centers. Keep in mind that landforms can affect how much you can see the stars, too: snow, rock, and ocean can reflect the moon.

Out of 112 parks measured, 29 were tied for the darkest. These included Badlands National Park, Big Bend National Park, Canyonlands National Park, Chiricahua National Monument, Glacier National Park, Isle Royale National Park; Lassen Volcanic National Park, and Death Valley National Park.

Park units closest to—sometimes in—cities are the brightest. They include the Statue of Liberty National Monument, Rock Creek Park, Muir Woods National Monument, the Presidio of San Francisco, and Independence National Historical Park.

An image of the night sky from Clingmans Dome.
The CCD camera takes an image of the sky looking straight up. Dark blue areas are dark skies; red, pink, and white show light pollution.

NPS image.

How dark are the Smokies? The Smokies are fairly dark, probably because the nearest large cities are 40 to 100 miles away. The Smokies had similar darkness measures as Bandelier National Monument in New Mexico, Assateague Island in Maryland/Virginia, and Rocky Mountain National Park in Colorado.

In the Smokies, we can see bright (shown in white) light coming from nearby Knoxville, Maryville, Pigeon Forge and development in Sevier County, and from more distant Waynesville and Asheville in North Carolina, and Atlanta about 140 miles (as the crow flies, or, in this case, as light shines) in Georgia. This light can have complex changes on the animals—and people—that as we know them in the Great Smoky Mountains.

The Great Smoky Mountains National Park offers the best view of the night sky in the area. The parks vastness, high peaks and numerous scenic vistas offer visitors a unique opportunity to observe the landscape above unencumbered by artificial light. To illustrate how the parks night sky viewshed differs from surrounding urban areas, the map below depicts artificial radiance measured by the Visible Infrared Imaging Radimeter Suite (VIIRS) in 2012.
Articial radiance map legend
Map Legend


The map depicts artifical light as units of radiance, or nanowatts/square centimeter/square radian. A typical flashlight emits .5 w/cm2/sr. Compare the 2012 map above to the 2016 map, below. The map is interactive so you can zoom and pan to explore areas of artificial radiance around the park.

How Do We Reduce Light Pollution?

We can immediately fix light pollution by turning unused lights off, adding a light shade outdoors, and adding motion sensors to lights so they only shine when they're needed.

Stars emerge during a blackout.
During a blackout, the Milky Way & stars of the night sky emerge in a city.

Photo (C) Todd Carlson. Used with permission.

Reducing light pollution in our communities will help us:

  • Make our neighborhoods and roads safer: Light applied correctly—with hoods so they point down instead of up or across—apply an even, low light at night, and don’t cast deep shadows or blind you. Thinking about how light is applied is most important for safety.
  • Make our city budgets stretch farther: Turning off lights, changing intensity of light, and changing the types of light fixtures we use will cut budgets. Currently, 3-5% of all money spent on energy funds exterior lights; one third of this powers residential lights. Much of this light is wasted: any light you can see from a distance or from space is not illuminating what it was meant to. The waste adds up: we spend $10 billion lighting the sky each year.
  • Make us healthier: Our circadian rhythms are regulated by light. Changing light amounts and timing changes these rhythms, and, scientists are learning, disrupts hormone production. We usually make melatonin, a hormone that regulates normal cell replication, while we sleep in the dark. Exposure to light at night has been linked in some studies to an increase in breast cancer in women. In one study, researchers found women who live in constantly lit urban areas had a significantly higher risk of breast cancer than did women who live in rural, darker areas. Reducing overall light will help cut those risks.
A brightly lit city sky.
The same neighborhood after the blackout: city lights shine into the sky, obscuring the Milky Way & most stars.

Photo (C) Todd Carlson. Used with permission.

  • Help slow global climate change: Wasted light means wasted energy to create that light. Turning off lights that aren’t needed, replacing constant lights with motion-sensor lights, and retrofitting globe and partial-cover lights with full-cover lights are all ways to use light efficiently.
  • Keep animals safe: Sea turtle hatchlings that crawl the wrong way—into roads and resorts rather than into the sea--aren't the only animals impacted by artificial light. It also attracts birds that migrate at night. The birds sometimes circle lit towers or buildings until they collide with other birds, hit windows, or drop from exhaustion. Scientists have noticed other behavior changes, as well. City lights attract insects, which bring more bats into areas where they can be killed by cars or pets. Lights may confuse or discourage some large animals from crossing areas or using their normal travel corridors; lit roads far from town may scare mountain lions or other hunters, which can lead to changes in the whole food chain.

Initiatives such as the National Park service’s Night Skies Program will help us understand how much artificial light parks receive, and help us preserve unimpaired dark skies for the future.

Return to the Air above and beyond the Smokies page.

Return to Air Quality main page.

Last updated: March 8, 2017

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