The Bortle scale is a nine-level numerical scale that measures the brightness of the night sky in a given location.
It quantifies the level of astronomical observability of celestial objects and the annoyance caused by light pollution.
Created by John E. Bortle and published in the February 2001 issue of Sky & Telescope magazine, it is first and foremost a description of " darkness " and an index of atmospheric sky purity.
It is one of the indices of environmental quality of interest to astronomers, ecologists and others involved in the protection, management or restoration of biodiversity.
Amateur astronomers generally judge their sky by the magnitude of the faintest star visible to the naked eye. The magnitude limit of visibility to the naked eye is a limited criterion. It depends as much on the individual's visual acuity as on the effort and time required to detect the faintest star.
The 9 grades :
Grade 1: Excellent site The zodiacal band is visible and crosses the entire sky. Even in direct vision, the galaxy M33 is an obvious object to the naked eye.
Grade 2: Really dark site The summer Milky Way is highly structured to the naked eye, and its brightest parts appear as mottled with ordinary binoculars.
Grade 3: Rural sky Some signs of light pollution are evident in certain directions on the horizon. Clouds appear dimly lit, but remain black as they leave the horizon.
Grade 4: Rural/urban transition In several directions, domes of light pollution appear clearly above built-up areas.
Grade 5: Peri-urban sky The Milky Way is very faint or invisible as it approaches the horizon, and appears washed out beyond it. Light sources are evident in almost if not all directions.
Grade 6: Suburban sky The presence of the Milky Way is only apparent towards the zenith. The sky up to 35° above the horizon emits an orange-gray light. Clouds all over the sky are luminous.
Grade 7: Suburban/City Transition The background of the entire sky has a vague orange-gray hue. Powerful sources of light are evident in all directions.
Grade 8: City sky The sky is orange, and newspaper headlines can be read without difficulty.
Grade 9: Downtown sky The whole sky is illuminated, even at the zenith. Many of the stars that form the constellations are invisible.
