Topic 1.6: Light Pollution and Dark Skies
A century ago, nearly everyone on Earth could look up on a clear night and see the Milky Way. Today, more than a third of humanity (and about 80 percent of people in North America) live where it is hidden. The reason is light pollution.
What Is Light Pollution?
Light pollution is artificial light that shines where it is not needed, or more than it is needed. It comes in several kinds:
- Skyglow: the bright dome of light over a town. It comes from lights that shine upward or sideways, and the air scatters it back down.
- Glare: bright light that hurts your eyes (an unshielded street lamp).
- Light trespass: light that spills onto a place you did not want lit, like your bedroom window.
- Clutter: too many bright lights in a confusing group.
Seeing What Is Lost
Astronomers measure how dark a sky is by the faintest star you can see. In the middle of a big city you might see only a few dozen stars. In a dark rural place you may see 2,000 or more. The 1-to-9 Bortle scale ranks sites from the darkest skies (1) to inner-city skies (9).
Why It Matters
- Astronomy. Telescopes near cities are almost useless for faint objects.
- Wildlife. Sea turtle hatchlings find the ocean by heading toward the brightest horizon. On Alabama's Gulf beaches, lights from buildings can lead them the wrong way. For that reason, Gulf Shores and Orange Beach have sea turtle lighting rules that limit beachfront lights during nesting season. Many insects, birds and bats are affected too.
- Sleep and health. Bright light at night disrupts the body's sleep rhythms.
- Energy and money. Light that goes up into the sky is energy wasted. Poorly aimed lights cost cities millions of dollars every year.
What You Can Do
- Shield lights so they point down, not out.
- Use only as much light as you need, and use motion sensors or timers.
- Choose warm colors (a lower color temperature, about 3,000 K or less). Blue-white light scatters more.
- Turn off lights you are not using.
- Find a dark place. Even 30 minutes' drive from a city can change the sky completely. State parks and rural fields are good choices.
Worth a Pause
The first day's work in Genesis is a separation: "God divided the light from the darkness. And God called the light Day, and the darkness he called Night" (Genesis 1:4-5). Night was made on purpose. We tend to think that more light is always better, but the dark is a gift, and it is where the stars appear. Caring for it is a small form of caring for creation.
Try It: Sky Navigator. Find stars and planets by altitude and azimuth, then use the Big Dipper to locate Polaris. Play Sky Navigator »
Practice Problems
- Define light pollution and name two of its types.
- About what fraction of North Americans cannot see the Milky Way from where they live?
- How does light pollution change the faintest star you can see?
- Why are sea turtle hatchlings in trouble when lights shine near a beach?
- Give three ways a family can reduce light pollution at home.
- Why is warmer (more orange) outdoor lighting better for the night sky than cool blue-white lighting?