This is a way of measuring the quality (brightness) of the night sky for a given location and it was created and published in 2001 by John E. Bortle to help amateur astronomers evaluate the darkness of an observing site. This links to skyglow and light pollution, both of which have an impact on the class a certain place would fit in.
There are 9 levels, with class 9 being the most extreme amount of light pollution:
Class 1: Excellent dark-sky site
Zodiacal light, Gegenschein and Zodiacal Band are visible.
M33 (the Triangulum Galaxy) is visible to the naked eye.
The Scorpius and Sagittarius regions of the Milky Way cast diffuse shadows on the ground.
Viewing Jupiter or Venus degrades the dark adaptation of your eyes.
Class 2: Typical truly dark site
The summer Milky Way is highly structured to the unaided eye.
The brightest parts of the Milky Way look like "veined marble" when viewed with binoculars.
Zodiacal light is bright enough to cast weak shadows.
Clouds in the sky are visible only as dark holes in a starry background.
Several Messier globular clusters appear as naked-eye objects.
Class 3: Rural sky
There are slight signs of light pollution along the horizon.
Clouds appear faintly illuminated in the brightest parts of the sky.
The Milky Way still appears complex.
Globular clusters M4, M5, M15 and M22 are all distinct naked-eye objects.
M33 is easy to see with averted vision (the use of peripheral vision by
not looking directly at the object, but looking a little off to the side, while continuing to concentrate on the object).
The Zodiacal Light is striking in spring and autumn.
Class 4: Rural/suburban transition
Fairly obvious light-pollution domes apparent over population centres.
Zodiacal Light is clearly evident but doesn't even extend halfway to the zenith.
The Milky Way well above the horizon is impressive but lacks all but the most obvious structure.
M33 is a difficult averted-vision object.
Clouds in the direction of light-pollution sources are illuminated but only slightly.
Class 5: Suburban sky
Hints of the Zodiacal Light are seen on the best spring and autumn nights.
The Milky Way is very weak or invisible near the horizon and looks rather washed out over head.
Light sources are evident in most if not all directions.
Clouds are quite noticeably brighter than the sky itself.
Class 6: Bright suburban sky
No trace of the Zodiacal Light can be seen.
The Milky Way is apparent only toward the zenith.
The sky within 35° of the horizon glows greyish white.
Clouds anywhere in the sky appear fairly bright.
M33 is impossible to see without binoculars and M31 is only modestly apparent to the unaided eye.
Class 7: Suburban / urban transition
The entire sky background has a vague greyish-white hue. Strong light sources are evident in all directions.
The Milky Way is either nearly or totally invisible.
M44 or M31 may be glimpsed with the unaided eye but are very indistinct.
Clouds are brilliantly lit.
Even in moderate-sized telescopes, the brightest Messier objects are pale ghosts of their true selves.
Class 8: City sky
The sky glows whitish grey or orange.
M31 and M44 may be barely glimpsed by an observer on good nights.
Only the brightest Messier objects are detectable with a modest-sized telescope.
Some of the stars making up the familiar constellations are difficult to see.
Class 9: Inner-city sky
The entire sky is brightly lit, even at the zenith.
Stars making up familiar constellations are invisible.
Dim constellations eg. Cancer and Pisces are not seen at all.
Aside from The Pleiades, no Messier objects are visible to the unaided eye.
Bortle Class description
Level/class | Title | NELM* | SQM*² / mag/arcsec2 |
1 | Excellent dark sky | 7.6 - 8.0 | 21.99-22.0 |
2 | Typical truly dark sky | 7.1 - 7.5 | 21.89-21.99 |
3 | Rural sky | 6.6 - 7.0 | 21.69-21.89 |
4 | Rural/suburban transition sky | 6.1 - 6.5 | 20.49-21.69 |
5 | Suburban sky | 5.6 - 6.0 | 19.50-20.49 |
6 | Bright suburban sky | 5.1 - 5.5 | 18.94-19.50 |
7 | suburban/urban transition sky | 4.6 - 5.0 | 18.38-18.94 |
8 | City sky | 4.1 - 4.5 | <18.38 |
9 | Inner-city sky | 4.0 ≧ | <18.38 |
*Naked Eye Limiting Magnitude
*² Sky Quality meter
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