The Bortle Dark-Sky Scale

Use the chart below with the Light Pollution maps to guage the quality of your night skies where you live.

The column labeled "Naked-eye Limiting Magnitude" indicates the dimmest stars visible under each class of light pollution. The larger the magnitude number is, the dimmer the star is. Each whole number represents a factor of 2.51 in brightness. In other words, a magnitude 5 star appears approximately two and one-half times brighter than a magnitude 6 star, while a magnitude 4 star appears approximately five times brighter than a magnitude 6 star.

Class Color Key Naked-eye Limiting Magnitude Sky Description Milky Way Astronomical Objects Zodiacal Light / Constellations Airglow and Clouds Night Time Scene
1   7.6 - 8.0 Excellent, truly dark-skies. MW shows great detail and light from the Scorpio / Sagittarius region casts obvious shadows on the ground. M33 (the Triangulum Galaxy) is an obvious object. Zodiacal light has an obvious color and can stretch across the entire sky. Bluish airglow is visible near the horizon and clouds appear as dark blobs againt the backdrop of the stars. The brightness of Jupiter and Venus is annoying to night vision.  Ground objects are barely lit and trees and hills are dark.
2   7.1 - 7.5 Typical, truly dark skies. Summer MW shouws great detail and has veined appearance. M33 is visible with direct vision, as are many globular clusters. Zodiacal light bright enough to cast weak shadows after dusk and has an apparent color. Airglow may be weakly apparent and clouds still appear as dark blobs. Ground is mostly dark, but objects projecting into the sky are discernible.
3   6.6 - 7.0 Rural sky. MW still appears complex, dark voids and bright patches and meandering outline are all visible. Brightest Globular Clusters are distinct, but M33 is only visible with averted vision.  M31 (the Andromeda Galaxy) is obviously visible. Zodical light is striking in Spring and Autumn, extending 60 degrees above the horizon. Airglow is not visible and clouds are faintly illuminated, except at the zenith. Some light pollution eveidnet along the horizon.  Ground objects are vaguely apparent.
4   6.1 - 6.5 Rural / suburban transition. Only well above the horizon does the MW reveal any structure. Fine details are lost. M33 is a difficult object, even with averted vision. M31 is still readily visible. Zodiacal light is clearly evident, but extends less than 45 degrees after dusk. Clouds are faintly illuminated except at the zenith. Light pollution domes are obviouse in several directions.  Sky is noticeably brighter than the terrain.
5   5.6 - 6.0 Suburban sky. MW appears washed out overhead and is lost completely near the horizon. The oval of M31 is detectable, as is the glow in the Orion Nebula. Only nints of zodiacal light in Spring and Autumn. Clouds are noticibly brighter than the sky, even at the zenith. Light pollution domes are obviouse to casual observers. Ground objects are partly lit.
6   5.1 - 5.5 Bright, suburban sky. MW only apparent overhead and appears broken as fianter parts are lost to sky glow. M31 is detectable only as a faint smudge; Orion Nebula is seldom glimpsed. Zodiacal light is not visible. Constellations are seen and not lost against a starry sky. Clouds anywhere in the sky appear fairly bright as they reflect back light. Sky from horizon to 35 degrees glows with grayish color. Ground is well lit.
7   4.6 - 5.0 Suburban / urban transition. MW is totally invisible or nearly so. M31 and the Beehive Cluster are rarely glimpsed. The brighter constellations are easily recognizable. Clouds are brilliantly lit. Entire sky background appears washed out, with a grayish or yellowish color.
8   4.1 - 4.5 City sky. Not visible at all. The Pleiades Cluster is visible, but very few other objects can be detected. Dimmer constellations lack key stars. Clouds are brilliantly lit. Entire sky background has an orangish glow and it is bright enough to read at night.
9   4.0 at best Inner city sky. Not visible at all. Only the Pleiades Cluster is visible to all but the most experienced observers. Only the brightest constellations are discernable and they are missing stars. Clouds are brilliantly lit. Entire sky background has a broight glow, even at the zenith.
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