Bortle scale

The Bortle scale ranks night skies from class 1, exceptionally dark, to class 9, inner-city sky, using visible celestial objects and light-pollution effects.

Limiting magnitude: The faintest apparent magnitude of an object detectable under specified observing conditions. The faintest stars visible are a central clue for assigning a Bortle class.
Amateur astronomy: Astronomical observation and study undertaken outside professional employment. Amateur observers use Bortle classes to judge whether a site suits their targets.
John E. Bortle: An American amateur astronomer who devised the nine-class Bortle scale. He created the classification and published it for observers to apply in the field.
Sky Quality Meter: A handheld photometer that estimates night-sky brightness, commonly in magnitudes per square arcsecond. It provides an instrumental brightness reading rather than an overall visual class.
Skyglow: Diffuse brightening of the night sky caused by natural and artificial light scattered in the atmosphere. Its intensity and extent distinguish dark rural skies from light-polluted ones.
Astrophotography: The practice of photographing astronomical objects and phenomena. A darker Bortle class generally makes faint, extended targets easier to capture.
Sky & Telescope: An American astronomy magazine founded in 1941 for amateur and professional astronomy audiences. The Bortle scale was introduced to a broad audience in the magazine in 2001.
SQM-L: A narrow-field version of the Sky Quality Meter designed to measure sky brightness in a smaller region. Its directional reading can reveal local variation that a Bortle label conceals.
Zodiacal light: A faint, triangular glow along the ecliptic, produced by sunlight scattered by interplanetary dust. Its visibility is one of the scale’s indicators of a very dark sky.
Deep-sky object: An astronomical object beyond the Solar System, such as a nebula, star cluster, or galaxy. Its visibility is especially sensitive to the sky brightness summarized by the scale.

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