Light Pollution Map

The Bortle scale was published by John E. Bortle in 2001 as a nine-level numeric scale for measuring the naked-eye and stellar limiting magnitude of the night sky in a given location. Class 1 represents the darkest skies on Earth; Class 9 represents the severely light-polluted sky of an inner city.

Sky quality is often measured with a Sky Quality Meter (SQM), which gives a reading in magnitudes per square arcsecond. A Bortle 1 site typically reads above 21.99 mag/arcsec; a Bortle 9 city reads below 17.80.

Light pollution comes primarily from streetlights, commercial signs, and buildings that scatter light upward into the sky, preventing it from reaching the ground as darkness.

At Bortle 1-2, the Milky Way is so bright it casts visible shadows on white paper. The Gegenschein (a faint glow opposite the Sun) is clearly seen, and thousands of stars crowd the sky.

By Bortle 5 (suburban), the Milky Way is washed out near the horizon and barely visible overhead. Familiar constellations still stand out but the background sky lacks the grainy texture of countless faint stars.

At Bortle 7 and above, the Milky Way is completely invisible. Only the brightest stars, planets, and the Moon remain clear. Most people living in cities have never seen the Milky Way.

The International Dark-Sky Association (IDA) certifies Dark Sky Parks, Dark Sky Reserves, and Dark Sky Communities. These designations recognize places with exceptional starry skies protected through local ordinances limiting artificial light.

Famous sites include the Atacama Desert in Chile (home to ESO and ALMA observatories), Mauna Kea in Hawaii, and Roque de los Muchachos in the Canary Islands - all rated Bortle 1.

The NamibRand Nature Reserve in Namibia was the first Dark Sky Reserve in Africa. In Europe, the Aoraki Mackenzie reserve in New Zealand is the largest designated dark sky reserve on Earth.

Light pollution has grown roughly 2% per year globally. Studies show it disrupts wildlife migrations, insect navigation, and human circadian rhythms. It also wastes an estimated 33% of outdoor lighting energy.

Effective mitigation strategies include shielded "full cutoff" luminaires that direct light downward, amber LEDs with reduced blue-light content, and motion-activated or timer-controlled lighting.

Flagstaff, Arizona was the world's first IDA Dark Sky City. Its ordinances limiting unshielded lights have kept its skies at Bortle 5 despite the city growing to 75,000 residents, demonstrating that lighting policy can meaningfully protect the night sky.

High-altitude sites benefit astronomers in two ways: the atmosphere is thinner above more water vapor and dust, and city light domes are more distant or blocked by terrain.

Even under suburban skies (Bortle 5-6), the Moon, planets, bright star clusters, and double stars are excellent telescope targets. Light pollution primarily affects extended objects like nebulae and galaxies.

A 1-hour drive from most cities can improve your sky by 2-3 Bortle classes. The gain is dramatic: going from Bortle 7 to Bortle 4 nearly triples the number of naked-eye stars visible and reveals the Milky Way.

Amateur astronomers use apps like Light Pollution Map and Clear Outside to find dark sky locations nearby. A sky quality meter (SQM-L) can objectively measure your site before setting up equipment for a night's observing.