Natural Lightscapes Overview

What Is Light Pollution?

It is difficult to believe at first— how could something so simple and harmless as light be considered a pollutant? After all, the day is full of light, so how could a little light at night be so bad? The answer is simply that artificial light at night is out of place, so even a small amount of light can greatly change the essence of night. Artificial light is a powerful tool that humans wield, only being common with the invention of the light bulb in 1879. From space, the first environmental change on our planet that one might see is the light from cities at night.

The global problem of light pollution is easily seen from space, as in this composite satellite view. C. Mayhew & R. Simmon (NASA/GSFC), NOAA/NGDC, DMSP Digital Archive

Astronomers were first to sound the alarm as the view of the night sky through telescopes and unaided eyes literally disappeared while city lights grew brighter. Stray light increases the brightness of the night sky making space appear light gray or pale yellow, and causing stars and faint objects to be lost by reduced contrast. Light pollution also prevents the human eye from fully dark–adapting, whether you are outside looking at the night sky or trying to get sleep with a light shining in your window.

For nocturnal animals stray light means the disruption of habitat. Animals often depend on darkness in order to hunt, conceal their location, navigate, or reproduce. This is made worse by the fact that many species have vision far more sensitive than human vision. Plants too can be affected by artificial light— you may have noticed that a tree beneath a bright streetlight will lose its leaves in autumn after the other nearby trees have.

What Causes Light Pollution?

The primary cause of light pollution is outdoor lights that are aimed upwards or sideways. Any light that escapes upward, except where a tree or building may be blocking it, will scatter throughout the atmosphere and brighten the night sky, thereby diminishing the view of it. Air pollution particles will also increase the scattering of light at night, just as it impacts visibility in the daytime.

This diagram shows a lighting fixture that throws light upwards and sideways.

This diagram shows a lighting fixture that directs light downward.

Light fixtures that direct all light downward greatly reduce the amount of light pollution. These are referred to as Shielded Lights, or the engineering term Full Cut-off. Some light does reflect off the ground and scatter through the sky, but this has a smaller effect than light that shines directly upward. Light pollution can be further reduced by utilizing a less powerful lamp or bulb. These modern improvements in lighting not only greatly improve the quality of the night sky, but have other benefits as well. These include better visibility, improved safety, less energy use, and better aesthetics.

Even a single light can be disruptive to the natural lightscape. While one light may not affect the entire night sky, the bright point source of light can annoy neighbors (a problem called Light Trespass), reduce perceptions of solitude and naturalness, confuse nocturnal animals and migrating birds, and spoil cultural landscapes.

Protecting and Restoring Dark Night Skies

Light pollution is not the inevitable side effect of progress, but is instead indicative of wasteful and inefficient outdoor lighting. The loss of the night sky is unnecessary. Protecting dark skies doesn't mean throwing civilization back into the dark ages; it simply requires that outdoor lights be used judiciously, respecting our human environment, wildlife, and the night sky that belongs to us all.

National parks and cities large and small are taking steps toward improving outdoor lighting and protecting night skies. If every outdoor light across America was made to be night sky friendly, it would save between $2 billion and $5 billion dollars annually in electrical usage. In most cases, upgrading lighting pays for itself in a few short years.

Natural Lightscapes and National Parks

The Milky Way loops from east to west in this full panoramic image from Death Valley's Racetrack Playa, one of the best night skies in the country.

National parks are protective harbors for some of the last remaining dark skies in this country. If you've seen a really outstanding night sky recently, chances are you saw it within a national park. The NPS has come to embrace night skies as one of the many scenic vistas the agency is a steward of. It is essential to keeping a park whole and touches on almost every aspect that is important to us- from sustainability to stargazers, and animals to ancient ruins.

Light can travel a long way, so even remote national parks aren't safe from the effects of light pollution. The Night Sky Team has documented light from distant cities affecting night skies over 200 miles away. In fact, almost every park that has been surveyed has noticeable light pollution. It is important for visitors and communities alike to understand how light pollution can cross boundaries and degrade the natural lightscape that has been a common heritage for millennia.

National parks are expressions of our values. They tell stories of the strength of our nation and its peoples, the American frontier wilderness, and the beauty and wonder of nature. Parks are places we can go to for rejuvenation, inspiration, and to delve into the larger world within which we live. But how could a park ever completely succeed if we cannot gaze upon the cosmos from which we came, that has been mankind's companion for so long.