Reference

Bortle Scale

Judge how dark your night sky really is, from a Class 1 wilderness sky to a Class 9 city core, using John Bortle's nine-level scale.

Explore the nine classes

Pick a class to see how the same patch of northern sky, the Big Dipper and its surrounding star field, degrades as light pollution grows. The seven Dipper stars survive everywhere, but the field stars thin out class by class, and Alcor, the faint mag 4.0 companion riding beside Mizar, fades in city skies and vanishes at Class 9.

The class explorer needs JavaScript to draw its sky. The reference table and full class descriptions below cover the same ground without it.

Classify a reading

If you know your sky's naked-eye limiting magnitude, the faintest star you can just detect near the zenith, or have a sky-quality meter reading in magnitudes per square arcsecond, enter either one and this converts it to a Bortle class. Enter both to cross-check them against each other.

The reading classifier needs JavaScript. The table below maps the same limiting-magnitude and SQM ranges to each class.

What can you see?

Answer from memory of a clear, moonless night at your usual observing spot, after your eyes have adapted for twenty minutes or so. Skip anything you have never checked. The result is indicative only: seeing and transparency change from night to night, and no short questionnaire replaces a careful reading of the full class descriptions below.

Question 1How does the Milky Way look on the best nights?

So bright that its Sagittarius region casts faint shadows on the ground Brilliant, with rich braided structure across the whole band Clearly visible with some structure and dark patches Visible overhead but washed out, with little fine detail Only a faint trace near the zenith Never visible at all Not sure, skip this question

Question 2Have you seen the zodiacal light, the tall cone of glow along the ecliptic after dusk or before dawn?

Yes, striking, with an obvious color or even casting weak shadows Yes, clearly, in spring or autumn Only a faint hint on the very best nights Never, despite looking for it Never looked for it, skip this question

Question 3Can you find the Triangulum Galaxy, M33, without optical aid?

Yes, easily, with direct vision Yes, but only with averted vision Barely, with averted vision and only when it rides high No, it needs binoculars or a telescope here Never tried, skip this question

Question 4How do clouds look at night?

Like dark holes, blacker than the sky around them Faintly lit near the horizon, dark overhead Noticeably brighter than the sky Brightly lit anywhere in the sky Not sure, skip this question

Question 5How well can you see your surroundings by sky light alone?

Not at all, my telescope and companions are basically invisible Only as silhouettes against the sky Nearby things are vaguely visible Clearly visible, even at a distance Easily visible everywhere Bright enough to read headline print

Question 6How much artificial sky glow is there?

None that I can detect in any direction A dome or two of light low on the horizon Glow from light sources in most directions The sky glows grayish white well up from the horizon The entire sky has a grayish white cast The whole sky is brightly lit, even at the zenith

The questionnaire needs JavaScript to tally a suggestion. The full nine-class reference table and descriptions below work without it.

The nine classes at a glance

Naked-eye limiting magnitude, NELM, is the faintest star an experienced observer can just detect near the zenith. SQM figures are sky-brightness meter readings in magnitudes per square arcsecond, where higher numbers mean darker sky. Select a class name to jump to its full description.

Bortle dark-sky scale, after Bortle (2001); SQM values are later approximations
Class	Name	Naked-eye limit	Approx. SQM	Sky at a glance
1	Excellent dark-sky site	7.6 to 8.0	about 21.8 to 22.0	Zodiacal light, gegenschein, and zodiacal band all visible; the Milky Way casts shadows; M33 obvious to the naked eye.
2	Typical truly dark site	7.1 to 7.5	about 21.6 to 21.8	Summer Milky Way highly structured; clouds appear as dark holes; M33 fairly easy with direct vision.
3	Rural sky	6.6 to 7.0	about 21.3 to 21.6	Some light pollution on the horizon; Milky Way still complex; M33 easy with averted vision.
4	Rural and suburban transition	6.1 to 6.5	about 20.4 to 21.3	Light domes in several directions; Milky Way impressive but lacking fine detail; M33 difficult.
5	Suburban sky	5.6 to 6.0	about 19.1 to 20.4	Milky Way washed out overhead, gone near the horizon; only hints of zodiacal light on the best nights.
6	Bright suburban sky	5.1 to 5.5	about 18.5 to 19.1	Milky Way only near the zenith; the lower sky glows grayish white; M33 out of naked-eye reach.
7	Suburban and urban transition	4.6 to 5.0	about 18.0 to 18.5	Whole sky grayish white; Milky Way invisible or nearly so; M31 and M44 faint and indistinct.
8	City sky	4.1 to 4.5	below about 18.0	Sky glows whitish gray or orange; familiar constellation patterns weak or incomplete.
9	Inner-city sky	4.0 at best	below about 18.0	Sky brilliantly lit even at the zenith; only the Moon, planets, and brightest clusters reward a look.

Class by class

Class 1

Excellent dark-sky site

Naked-eye limit 7.6 to 8.0, SQM about 21.8 to 22.0

The darkest skies on Earth. The zodiacal light, gegenschein, and the complete zodiacal band spanning the sky are all visible, and the zodiacal light shows striking color. M33 is an obvious direct-vision object, airglow is readily apparent low on the horizon, and the Sagittarius and Scorpius region of the Milky Way is bright enough to cast diffuse shadows on the ground. Jupiter or Venus can be bright enough to spoil dark adaptation, and your telescope and companions are essentially invisible except as blots against the stars.

Class 2

Typical truly dark site

Naked-eye limit 7.1 to 7.5, SQM about 21.6 to 21.8

Airglow may be weakly visible along the horizon. The summer Milky Way is highly structured to the unaided eye, and many Messier globular clusters are naked-eye objects. Clouds show only as dark holes against a starry background, the zodiacal light is distinctly yellowish and bright enough to cast weak shadows around dusk and dawn, M33 is fairly easy to see directly, and your surroundings appear only in silhouette against the sky.

Class 3

Rural sky

Naked-eye limit 6.6 to 7.0, SQM about 21.3 to 21.6

Some sign of light pollution sits on the horizon, and clouds are faintly lit there while remaining dark overhead. The Milky Way still shows complex structure, bright globular clusters such as M4, M5, M15, and M22 remain distinct naked-eye objects, and M33 is easy with averted vision. The zodiacal light is striking in spring and autumn with a hint of color, and nearer surroundings are vaguely visible.

Class 4

Rural and suburban transition

Naked-eye limit 6.1 to 6.5, SQM about 20.4 to 21.3

Light-pollution domes are plainly visible over settlements in several directions. The zodiacal light is still seen but no longer stretches even halfway to the zenith. Well above the horizon the Milky Way remains impressive yet lacks all but its coarser detail, and M33 becomes a difficult averted-vision object, seen only when high in the sky. Clouds are lit up in the direction of the light sources but stay dark overhead, and your surroundings are clearly visible even at a distance.

Class 5

Suburban sky

Naked-eye limit 5.6 to 6.0, SQM about 19.1 to 20.4

Only hints of the zodiacal light survive, and then only on the finest spring and autumn nights. The Milky Way is very weak or invisible near the horizon and looks washed out overhead. Sources of light pollution are evident in most, if not all, directions, and clouds are noticeably brighter than the sky itself.

Class 6

Bright suburban sky

Naked-eye limit 5.1 to 5.5, SQM about 18.5 to 19.1

The zodiacal light is gone. The Milky Way is apparent only toward the zenith, and the sky within about 35 degrees of the horizon glows grayish white. Clouds anywhere in the sky look fairly bright, surroundings are easily visible, and M33 cannot be seen without at least binoculars while M31, the Andromeda Galaxy, is only modestly apparent to the unaided eye.

Class 7

Suburban and urban transition

Naked-eye limit 4.6 to 5.0, SQM about 18.0 to 18.5

The entire sky background takes on a grayish white hue, with strong light sources evident in all directions. The Milky Way is totally invisible or nearly so. M31 and the Beehive Cluster, M44, may still be glimpsed with the naked eye but are very indistinct, clouds are brilliantly lit, and even in a moderate-sized telescope the brightest Messier objects are pale ghosts of their true selves.

Class 8

City sky

Naked-eye limit 4.1 to 4.5, SQM below about 18.0

The sky glows whitish gray or orange, bright enough that you can read newspaper headlines without difficulty. M31 and M44 are barely glimpsed by an experienced observer on good nights, only bright Messier objects can be detected with a modest telescope, and some familiar constellation figures are weak or missing stars entirely.

Class 9

Inner-city sky

Naked-eye limit 4.0 at best, SQM below about 18.0

The whole sky is brightly lit, even at the zenith. Many constellations are weakened or invisible, and aside from the Pleiades no Messier object can be seen with the naked eye. The only celestial sights that reward a casual look are the Moon, the planets, and a few of the brightest star clusters.

About this scale

The scale comes from John E. Bortle, who published it in the February 2001 issue of Sky and Telescope so that amateur astronomers could compare observing sites with a common yardstick. The class names, limiting magnitudes, and observational criteria above follow his original descriptions.

Treat the numbers as approximate. Bortle's article defined the classes by what an experienced observer can see, not by meter readings, so the SQM correspondences quoted here are later estimates that vary from source to source, and the boundaries between neighboring classes are soft. Naked-eye limiting magnitude also depends on your eyesight and experience as much as on the sky, and some observers find Bortle's naked-eye criteria for the darkest classes, such as M33 with direct vision, optimistic. A given site can shift by a class or more from night to night with haze, humidity, airglow, and the amount of lighting in use. Nothing you select on this page is stored or transmitted.