Light pollution
Light pollution refers to the overall negative effects of excessive or poorly directed artificial lighting on the nighttime environment. For astronomers, it results in a diffuse brightness in the sky that obscures stars and faint objects. Today, more than one-third of humanity can no longer see the Milky Way with the naked eye from where they live.
Mechanisms of Light Pollution
Artificial light emitted upward or in an undirected manner is scattered by air molecules, dust, andatmospheric humidity. It creates a luminous background in the sky— the luminous halo visible above cities—which reduces the contrast between celestial objects and the sky background. The brighter the sky, the harder it is to see faint objects.
The Bortle Scale
The Bortle Scale, established in 2001, classifies the quality of the night sky into nine levels, from Class 1 (unspoiled sky, with no light pollution) to Class 9 (the center of a large city, with only a few dozen stars visible). In France, the vast majority of urban areas fall between Classes 7 and 9. A few protected areas, such as dark sky reserves, reach classes 2 or 3.
Impact on Astronomy and Beyond
Light pollution doesn’t just harm astronomers. It disrupts the biological cycles of many animal species—insects, migratory birds, sea turtles—that rely on natural light for navigation or reproduction. In humans, exposure to artificial light at night disrupts melatonin production and circadian rhythms, with consequences for health.
A concrete example
The Pic du Midi International Dark Sky Reserve, in the Pyrenees, isone of the few places in France where you can still observe the Milky Way with the naked eye in all its splendor. Efforts to reduce street lighting in the surrounding towns have helped maintain a Class 3 sky over several hundred square kilometers.
Frequently Asked Questions
Can you see stars from a city?
Yes, but only to a very limited extent. The brightest stars, the planets, and the Moon remain visible from cities. However, faint objects such as galaxies, nebulae, or the Milky Way require a trip to areas with low light pollution.
Can filters compensate for light pollution?
Partially. Specific filters, such as UHC or OIII filters, block light from streetlights while allowing certain wavelengths emitted by nebulae to pass through. They improve contrast on certain objects but do not replace a dark sky.
How can light pollution be reduced?
By using only downward-facing lighting, reducing light intensity at night, favoring(amber rather than cool white), and by turning off unnecessary lights. Local regulations exist in some regions to protect areas used for astronomical observation.