Sky glow is one of the most significant consequences of light pollution, which not only disrupts the work of astronomers but also deprives city dwellers of the possibility of enjoying the beautiful night sky and nocturnal landscapes. Unfortunately, one must now travel several hundreds of kilometers from an urban centre to clearly see the Milky Way. Indeed, more than two thirds of Americans are not able to make out the Milky Way with the naked eye from their home. Almost 97% of stars are no longer visible from the major cities of the world. In the region of Montreal, one can see only about 100 stars, while it is possible to witness over 3,000 stars with the naked eye in the region of Mt. Megantic.

Sky glow is caused by artificial light emitted toward the sky, either directly from luminaires or reflecting from the ground. Light cast upwards is visible (and thus harmful) due to reflection off airborne particles (dust, aerosols, humidity) and intrinsic diffusion of the atmosphere. White light, because it is more diffused by the atmosphere, contributes more to sky glow than a yellow light.

A dark sky is essential to astrophysicists for the observation of celestial objects, particularly those of faint luminosity. Increasing light pollution seriously threatens studies at many major observatories, since artificial brightness in the sky has the same effect as reducing the size of the telescope. The largest telescope on Canadian soil, the David Dunlap Observatory, was forced to shut down recently because excessive sky glow over Toronto made accurate observations impossible. Research at two other observatories in large American suburbs is also compromised for the same reason. These observatories are at Harvard University in Massachusetts and on Palomar Mountain in California.