One hundred years ago, on a clear night, you could see the Milky Way from almost any location in the United Sates. Today, most people cannot. With the widespread use of artificial light at night, we are losing the spectacular starry night sky. As an astronomer, this greatly concerns me.

To see what I mean, look at the moonless night sky in town and then drive 5 or 6 miles out of town to a dark site. You will notice a dramatic difference.

Most of us are familiar with air, water and land pollution, but the inappropriate or excessive use of artificial light can also create pollution — light pollution. Light pollution not only destroys our view of the universe, but it can have serious environmental consequences for humans, wildlife and our climate.

There are several sources of light pollution, including exterior and interior building lighting, advertising, commercial properties, offices, factories, streetlights, and illuminated sports centers.

We need much of this lighting, and I am not suggesting we need to remove all artificial lights. But much of the outdoor lighting is inefficient, overly bright, poorly targeted, improperly shielded and sometimes completely unnecessary.

Light that is not focused on the actual objects and areas that need to be illuminated is unnecessary. It is spilling into the sky, and the electricity used to create it is being wasted.

This is not just an astronomical problem. Plants and animals, including humans, depend on the daily cycles of light and dark rhythm to govern behaviors involved in reproduction, nourishment, sleep and protection from predators. For example, day length is one of the factors that determines when flowers bloom.

Nocturnal animals normally sleep during the day and are active at night. Many predators need light to hunt, so prey use darkness as cover. Light pollution radically alters this nighttime environment by turning night into day.

Frogs and toads use nighttime croaking as part of their breeding ritual. Artificial lights disrupt this nocturnal activity, interfering with reproduction. It appears the day-night cycle is encoded in the DNA of many plants and animals.

Humans also adhere to a circadian rhythm, our biological clock — a sleep-wake pattern governed by the day-night cycle. We produce the hormone melatonin in response to circadian rhythm. Melatonin induces sleep, boosts the immune system, lowers cholesterol and helps the functioning of the thyroid, pancreas, ovaries, testes and adrenal glands. Nighttime exposure to artificial light suppresses melatonin production. This nighttime exposure may increase the risk for obesity, depression, sleep disorders, diabetes, breast cancer and more.

The good news is that light pollution, unlike many other forms of pollution, is easy to reduce. Modern society requires outdoor lighting for a variety of reasons, including safety and commerce, but we need to wisely use this required lighting. Here are some basic rules for reducing light pollution:

First, only light the areas that need it, and only turn the lights on when needed. Install motion detectors and timers to control when the lights are on.

Second, lighting should be no brighter than needed and should always point downward. Avoid outside lights that point toward the sky. If you need to light a sign or other object, light it from above so the light is pointed toward the ground.

Third, properly shield all outdoor lights. Light should be focused toward the ground and cover an area below the fixture. Several low-output, down-facing lights are better than one floodlight that tries to cover a large area. Keep your blinds drawn to keep your indoor light indoors.

There are three main types of outdoor lighting: low-pressure sodium (LPS), high-pressure sodium (HPS), and light-emitting diodes (LEDs). LPS is very energy-efficient and emits only a narrow spectrum of light. This narrow band of light can be removed by using a filter, so LPS is an excellent choice for lighting near astronomical observatories.

An advantage of LED lighting is that it is dimmable. This means that an empty street or parking lot can be lit with dimmer light and only brought back to full brightness as necessary. This saves energy and reduces light pollution.

We can start by minimizing the light from our own homes and then help educate others to do the same. If we all work together, we can save our wonderful night sky for future generations.

Marty Scott is a resident astronomer at Pacific Northwest Regional Observatory. He designs, builds and maintains astronomical equipment. He also formerly taught astronomy at Walla Walla University. He can be reached at marty.scott@wallawalla.edu.

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