THE ORIGIN
John Bortle and the 2001 Scale
John E. Bortle, a veteran American comet and variable star observer, published his nine-class scale in the February 2001 issue of Sky & Telescope magazine. The scale addressed a practical problem: astronomers needed a common vocabulary to compare observing site quality, and existing measures (sky background brightness in magnitudes per square arcsecond) were not intuitive for the average observer. Bortle's descriptions are based on what an experienced observer with dark-adapted eyes can see under each class of sky — concrete, observational, and immediately applicable.
The scale runs from Class 1 (the darkest skies on Earth, accessible only in remote wilderness) to Class 9 (inner-city skies so light-polluted that only the Moon, planets, and the brightest stars are visible). Each class has a specific set of observable features that distinguish it from adjacent classes. The scale is not perfectly linear — the jump from Class 4 to Class 5 is perceptually larger than the jump from Class 8 to Class 9 — but it provides a practical, universally understood shorthand for observing conditions.
1 · DARKEST
3 · RURAL
5 · SUBURBAN
7 · CITY EDGE
9 · CITY CENTRE
ALL NINE CLASSES
The Complete Scale
1
EXCELLENT DARK-SKY SITE
The zodiacal light is so bright that it casts visible shadows. The Milky Way projects complex structure visible to the naked eye. The Milky Way's central region casts a faint shadow on the ground when it is overhead. Air glow — the natural luminescence of the upper atmosphere — is visible. Limiting naked-eye magnitude reaches approximately 7.6–8.0. M33 (the Triangulum Galaxy) is visible with direct vision. Fewer than 1% of the global population lives within reach of Class 1 skies.
VISIBLE UNAIDED: Zodiacal light, gegenschein, zodiacal band, Milky Way structure in full detail, M33 direct, faint airglow
2
TRULY DARK SITE
Airglow is weakly visible near the horizon. The zodiacal light is very obvious and slightly yellowish. The Milky Way shows considerable structure with dark nebulae visible against it. M33 is clearly visible with direct vision. Limiting naked-eye magnitude approximately 7.1–7.5. Some light pollution domes may be visible on the horizon toward distant cities, but the overhead sky is very dark. Most remote dark-sky preserves fall in this class.
VISIBLE UNAIDED: Zodiacal light prominent, Milky Way dark nebulae, M33 direct, faint gegenschein
3
RURAL SKY
Some light pollution evident near horizon but overhead sky very dark. Zodiacal light still striking, reaching to the zenith in spring and autumn. Milky Way appears complex and rich. M15, M4, M5, and M22 appear partially resolved. Limiting magnitude approximately 6.6–7.0. This is the class accessible to rural observers in low-population-density regions far from major cities — central Australia, much of inland North America, and parts of southern Africa.
VISIBLE UNAIDED: Zodiacal light to zenith, Milky Way complex, faint globulars obvious, M33 with averted vision
4
RURAL / SUBURBAN TRANSITION
Light pollution domes obvious over one or two directions. Zodiacal light still evident but does not reach the zenith. The Milky Way still impressive but lacks the finest detail of Class 3. Limiting naked-eye magnitude approximately 6.1–6.5. Many good amateur observing sites in rural areas with some nearby towns fall in this class. The Sagittarius star clouds and the dust lanes of the Milky Way are still clearly visible.
VISIBLE UNAIDED: Milky Way still showing dust lanes, zodiacal light in spring/autumn, M33 hard with direct vision
5
SUBURBAN SKY
Light pollution is obvious over most of the horizon. The Milky Way is washed out at the zenith and completely lost near the horizon. Zodiacal light is detectable only in the best conditions in spring and autumn. Limiting naked-eye magnitude approximately 5.6–6.0. Most suburban observers live under Class 5 or worse skies. The Orion Nebula (M42) is obvious, but fainter nebulae require optical aid. This is where most of humanity observes the night sky.
VISIBLE UNAIDED: Milky Way faint at zenith only, bright globulars, M42 obvious, Andromeda galaxy visible as a smear
6
BRIGHT SUBURBAN SKY
Light pollution dominates most of the sky. The Milky Way is only glimpsed at the zenith, if at all. Zodiacal light is invisible. Limiting magnitude approximately 5.1–5.5. Naked-eye sky shows obvious brightening toward city centres. The Pleiades and Double Cluster are obvious, but fainter star clusters are difficult to see without optical aid. Typical of small city edges and outer suburbs.
VISIBLE UNAIDED: Milky Way only barely at zenith, bright open clusters, major double stars, obvious constellations
7
SUBURBAN / URBAN TRANSITION
The Milky Way is invisible. The entire sky background has a grey-white appearance even overhead. Limiting magnitude approximately 4.6–5.0. Bright star clusters are visible in binoculars but the sky background inside the telescope eyepiece is noticeably light grey. Many satellite passes are visible during twilight. This is typical of outer suburbs and small city edges. Only bright constellations, planets, and stars brighter than about magnitude 5 are easily seen.
VISIBLE UNAIDED: Milky Way absent, constellations obvious, planets, Moon, brightest stars
8
CITY SKY
The sky glows whitish or orange-grey. Stars are visible down to about magnitude 4.0–4.5. Only the most obvious constellations are recognisable. Only a handful of Messier objects are visible with a telescope. The sky background through a telescope is pale and washed out. M42 (Orion Nebula) and the brightest globular clusters are still detectable in telescopes, but require effort. Light-adapted eyes see the sky as quite bright. Typical of city residential areas.
VISIBLE UNAIDED: Major constellations only, bright planets, Moon, stars to ~mag 4.5
9
INNER-CITY SKY
The entire sky is glaringly bright. Stars are visible to only about magnitude 3 or 4. The Moon, planets, and the brightest stars are obvious but deep-sky objects are essentially invisible. Even at high magnification in a telescope, the sky background is uncomfortably bright. Familiar constellation figures are difficult to trace because many of their fainter component stars are washed out. Typical of central business districts and urban cores worldwide.
VISIBLE UNAIDED: Moon, Venus, Jupiter, brightest stars, a few constellation patterns
SPACE MIRRORS AND THE BORTLE SCALE
How Orbital Mirrors Interact with Dark Skies
Space mirrors do not change a site's Bortle class in a permanent sense — they do not add to the background sky brightness that sets the Bortle rating. What they do is produce brief, transient additions to the sky brightness during their orbital passes, typically lasting 3–5 minutes. The effect is qualitatively similar to a bright satellite flare or a moderately bright meteor — a temporary brightening that disrupts observations in progress but does not shift the underlying sky quality permanently.
The relevant comparison is to the full Moon. A full Moon at the zenith raises effective sky background brightness enough to push a Class 3 rural site to Class 5 or 6 conditions for extended periods. A space mirror at magnitude −2 to −3 passing overhead is similarly bright to a near-full Moon in the sky at the moment of closest approach — but only for minutes, not hours. After the pass, the underlying sky class reasserts itself.
The concern for professional astronomy is different: not the transient brightness of a single pass, but the cumulative probability that a sensitive long-duration exposure will be disrupted by a mirror pass at some point. A single mirror over a 3-month observing campaign represents a calculable disruption probability. A constellation of 57 mirrors running concurrently represents a much higher disruption probability — analogous to the documented impact of Starlink satellite trails on astronomical imaging. See our Astronomy Impact page for the scientific community's specific objections, and Light Pollution for the broader sky brightness analysis.
FREQUENTLY ASKED
Bortle Scale Questions
Who invented the Bortle scale?+
John E. Bortle, a veteran American amateur astronomer specialising in comets and variable stars. The scale was first published in the February 2001 issue of Sky & Telescope magazine. It was quickly adopted by the amateur astronomy community worldwide and is now the standard vocabulary for describing dark-sky site quality.
What Bortle class is a typical suburban backyard?+
Most suburban backyards fall in Bortle Class 5–7. A Class 5 suburban sky allows the Milky Way to be glimpsed faintly at the zenith under the best conditions. Class 6 is typical of outer suburbs where the Milky Way is barely detectable. Class 7–8 covers most city residential areas where the Milky Way is invisible. Class 9 is reserved for central urban cores.
Does a space mirror pass change a site's Bortle class?+
Not permanently. The Bortle class reflects the site's underlying sky background brightness, which is set by terrestrial light pollution. A space mirror pass produces a brief transient brightening during the 3–5 minute pass — comparable to a bright satellite flare or a near-full Moon in that part of the sky. After the pass, the underlying class reasserts itself. The concern for astronomers is not class change but disruption of long-exposure observations and the cumulative probability of interference from a large constellation.
What is limiting magnitude and how does it relate to Bortle class?+
Limiting magnitude is the faintest star visible to the unaided dark-adapted eye under a given sky. It correlates closely with Bortle class: Class 1 allows limiting magnitudes around 7.6–8.0; Class 5 allows about 5.6–6.0; Class 9 allows about 3–4. Each class typically represents a limiting magnitude step of roughly 0.5–0.6 magnitudes. Since each magnitude step represents a brightness factor of ~2.5, the sky brightness difference between Class 1 and Class 9 is enormous — roughly 100× darker.
Are there places on Earth still at Bortle Class 1?+
Yes, but they are increasingly rare and mostly inaccessible. True Class 1 skies are found in the driest, most remote desert regions — parts of the Atacama Desert in Chile, remote areas of the Namib Desert, Antarctica away from research stations, and very high-altitude sites in Central Asia. Light pollution maps show that true Class 1 is disappearing even in traditionally dark areas as cities grow. The Milky Way is now invisible to approximately one third of humanity, and to more than 60% of Europeans and Americans.