The sky — astronomy

Light Pollution and Dark Skies

Light pollution is excessive or poorly directed artificial light that brightens the night sky and washes out faint stars. That is why a city shows only the brightest objects. To see a truly dark sky, travel well away from large towns; the nine-class Bortle scale tells you how dark a site is.

What Is Light Pollution?

Light pollution is excessive, unnecessary or poorly directed artificial light at night. Its best-known form is skyglow, the dome of brightness over towns and cities. Street lamps, signs and building lights send some light upward, and air molecules, moisture and dust scatter it, giving the whole sky a grey or orange tint.

The stars are still there, but faint ones are lost against the bright background. The eye can pick out an object only if it stands out enough from the sky behind it. Under a dark sky, the unaided eye can see stars down to about magnitude 6 (a brightness scale in which larger numbers mean fainter objects). In the middle of a big city, only a few dozen stars remain visible across the whole sky.

Researchers usually describe several components:

• skyglow: the general brightening of the night sky;
• glare: light so bright it hampers vision;
• light trespass: light that falls where it is not wanted, such as a bedroom window;
• clutter: confusing, excessive groupings of lights.

A global atlas of sky brightness published in 2016 in the journal Science Advances estimated that more than 80% of the world’s population lives under light-polluted skies. In the United States and Europe the figure is above 99%. The same study found that more than a third of humanity can no longer see the Milky Way.

The Bortle Scale

The Bortle scale was proposed by amateur astronomer John Bortle and published in Sky & Telescope magazine in 2001. It has nine classes and describes what you can actually see.

• Class 1: truly dark sky. The Milky Way casts a noticeable glow, and zodiacal light (a faint glow along the ecliptic) is obvious.
• Classes 2–3: rural skies. The Milky Way is rich in detail and many faint stars show.
• Class 4: a rural-to-suburban transition, with light domes visible on the horizon.
• Classes 5–6: suburban sky. The Milky Way is hard to see, and at class 6 only near the zenith.
• Class 7: suburban-to-urban transition. The Milky Way is barely noticeable.
• Classes 8–9: city sky and inner-city sky. Mostly the Moon, planets and the brightest stars remain.

The scale is descriptive and approximate rather than a precision instrument, but it is handy because anyone can estimate their class just by looking up. Accurate measurements are made with sky quality meters, which record the brightness of the sky.

Effects on Nature and Observing

For astronomy the effect is direct. Faint galaxies, nebulae and the streaks of meteor showers get lost against a bright background. Even a large telescope cannot restore contrast that has already been lost. Professional observatories are therefore built far from cities, often at high altitude.

Light pollution also affects wildlife. Scientific studies describe how artificial light can disorient insects, migrating birds and newly hatched sea turtles, which orient by natural light. It can also alter the activity rhythms of many animals. Research on effects on people is ongoing; the safest conclusion is simply that night lighting deserves thoughtful use. For health questions, consult a doctor.

The good news is that light pollution is among the most reversible forms of pollution: switch the light off or aim it better, and the effect disappears. Dark-sky advocates generally recommend:

• light only where it is needed;
• shielded fixtures that point downward;
• lower brightness where possible;
• warmer colours with less blue light, since blue scatters most strongly in the atmosphere;
• turning lights off or dimming them when they are not needed.

How to Find a Dark Place

You do not have to travel to a desert to find a good sky. A few practical steps help:

1. Leave the city. Even 30–60 kilometres (about 20–40 miles) makes a big difference, especially if a hill or mountain lies between you and the glow.
2. Use a light pollution map. Free online maps built from satellite data show where skies are darkest.
3. Pick an open horizon. A meadow, hilltop or mountain pass away from street lamps and car headlights.
4. Watch the Moon. A full Moon brightens the whole sky. The best nights are around new Moon (see the Moon).
5. Give your eyes time. Full dark adaptation takes about 20–30 minutes. Use a red torch, which preserves night vision, and avoid screens.
6. Check the weather. Clear, dry air improves transparency.

DarkSky International (founded in 1988 as the International Dark-Sky Association) recognises places with high-quality night skies: parks, reserves, sanctuaries and communities. A dark sky reserve is typically a larger protected area with a dark core and a surrounding zone where lighting is controlled. Such places exist around the world, and often closer to you than you might expect.

Once you are under a dark sky, try to find the Milky Way, and see more tips in How to Watch the Sky. The first night under a truly dark sky is often unforgettable, because there are far more stars than you imagined.

Positions in Astronativa are astronomically exact. The interpretation is for inspiration. · Glossary · Terms · Privacy