Scale for measuring the brightness of the night sky
"Bortle" redirects here. For the asteroid, see 4673 Bortle.
The Bortle dark-sky scale (usually referred to as simply the Bortle scale or sometimes Bottle scale) is a nine-level numeric scale that measures the night sky's brightness of a particular location. It quantifies the astronomical observability of celestial objects and the interference caused by light pollution. Amateur astronomerJohn E. Bortle created the scale and published it in the February 2001 edition of Sky & Telescope magazine to help skywatchers evaluate the darkness of an observing site, and secondarily, to compare the darkness of observing sites.
The scale ranges from Class 1, the darkest skies available on Earth, through to Class 9, inner-city skies. It gives several criteria for each level beyond naked-eyelimiting magnitude (NELM).[1] The accuracy and utility of the scale have been questioned in 2014 research.[2][why?] The table summarizes Bortle's descriptions of the classes. For some classes, there can be drastic differences from one class to the next, e.g, Bortle 4 to 5.[why?]
the zodiacal light is still visible, but does not extend halfway to the zenith at dusk or dawn
light pollution domes visible in several directions
clouds are illuminated in the directions of the light sources, dark overhead
surroundings are clearly visible, even at a distance
the Milky Way well above the horizon is still impressive, but lacks detail
M33 is a difficult averted vision object, only visible when high in the sky
limiting magnitude with 12.5" reflector is 15.5
4.5
Semi-Suburban/Transition sky
6.1–6.3
20.3–20.8
clouds have a grayish glow at zenith and appear bright in the direction of one or more prominent city light domes
the Milky Way is only vaguely visible – 10–15 degrees above the horizon. However the Great Rift, when overhead and with good transparency, is still obvious.
Although the views of bright globular clusters through 10" aperture and larger are striking, the outer regions of galaxies are difficult or impossible to see.
limiting magnitude with 12.5" reflector is 15.2
5
Suburban sky
5.6–6.0
19.25–20.3
only hints of zodiacal light are seen on the best nights in autumn and spring
light pollution is visible in most, if not all, directions
clouds are noticeably brighter than the sky
the Milky Way is invisible near the horizon, and looks washed out overhead. The winter Milky Way, even directly overhead, is fairly subtle.
when it is half moon (first/last quarter) in a dark location the sky appears like this, but with the difference that the sky appears dark blue
limiting magnitude with 12.5" reflector is 15
6
Bright suburban sky
5.1–5.5
18.5–19.25
the zodiacal light is invisible
light pollution makes the sky within 35° of the horizon glow grayish white
clouds anywhere in the sky appear fairly bright
even high clouds (cirrus) appear brighter than the sky background