Bortle scale

Lore & Background

The Bortle scale ranges from Class 1, the darkest skies available on Earth, through to Class 9, inner-city skies. The classes are described primarily in terms of the visibility of notable celestial objects and light sources in the sky, but correspond closely with naked-eye limiting magnitude (NELM) and sky quality meter (SQM) measurement of skyglow. At higher classes, light pollution above the horizon is obvious, diffuse light sources such as the Milky Way and Messier objects are invisible to the naked eye, and fewer point light sources such as stars and planets can be seen. At lower classes, light pollution domes are only present in the direction of cities or are absent altogether, the sky is filled with stars, and faint diffuse light sources such as the zodiacal light are contrastful and brilliant.

Bortle was motivated to publish the scale in 2001 by increasing light pollution making true dark-sky sites inaccessible to many amateur astronomers. Ongoing urbanization and the global transition to LED lighting have continued this trend. James Brodrick of the US Department of Energy wrote in 2018 that 'All manmade lighting is "unnatural" and thus, has potentially undesirable side effects. Some of these are unavoidable if we're to continue to enjoy the benefits.' The higher light temperature of LED lights has been a particular contributor to increasing light pollution, in ways sometimes not reflected in older studies of light pollution. However, The Washington Post wrote in 2023 that 'there is a world where more energy-efficient LED lights exist and don't significantly disrupt nightscapes or our health.' DarkSky International calls for responsible outdoor lighting that is carefully targeted, no brighter than necessary, and warm-colored.

Reader's Guide

The Bortle scale has attained widespread usage among amateur and professional astronomers. It quantifies the link between artificial lighting and reduced night sky quality in an intuitive manner, making it a common tool in scientific communication. A wide database of SQM measurements has been created, allowing the Bortle rating to be estimated for anywhere in the world. 2014 research suggests Bortle may have overestimated the visibility of dim objects for the typical observer, even in the darkest skies. Diffuse light sources such as M33 are more difficult to see than point light sources of the same visual magnitude; Bortle's description of M33 (V 5.72) as a naked-eye object is atypical. A naked-eye limiting magnitude (NELM) over 7.1, which Bortle suggests for Class 2, represents a 'substantial raising of achievement and expectation'. A NELM of 6.0 to 6.5 is a more commonly cited figure. Bortle was aware of NELM varying with the visual acuity of the observer, and he devised the scale as an alternative to raw NELM.

Did You Know?

• The Bortle scale was created by amateur astronomer John E. Bortle and published in the February 2001 edition of Sky & Telescope magazine.
• The scale ranges from Class 1 (darkest skies) to Class 9 (inner-city skies).

Origin and Motivation

Amateur astronomer John E. Bortle devised a nine-level numeric system to quantify how bright the night sky appears at a given location, ultimately publishing it in the February 2001 issue of Sky & Telescope magazine. His driving motivation was a growing frustration: escalating light pollution was rendering genuinely dark observing sites increasingly unreachable for the average skywatcher. Rather than relying on a single raw number like naked-eye limiting magnitude, Bortle wanted a descriptive framework that captured the full visual character of a site—how many stars pop, whether the Milky Way blazes overhead, and whether the zodiacal light stands out with striking contrast. The scale was designed to let observers at any location evaluate and compare the darkness of their skies against one another, factoring in the interference that artificial light introduces. By anchoring each class to recognizable celestial landmarks and the behavior of both point and diffuse light sources, Bortle gave the amateur community a shared vocabulary for discussing sky quality that went well beyond a simple brightness reading.

Structure and Class Descriptions

The Bortle scale spans nine discrete classes, with Class 1 representing the darkest skies still found on Earth and Class 9 depicting the heavily illuminated conditions of an inner city. Each class is defined primarily by what a naked-eye observer can actually see: the number of visible stars and planets, whether diffuse structures like the Milky Way or Messier galaxies are apparent, and how prominent the zodiacal light appears. At the lower-numbered classes, artificial light domes are either confined to the direction of distant cities or entirely absent, the sky is densely packed with stars, and faint extended sources glow with vivid contrast. As the class number climbs, light pollution becomes an obvious overhead presence, the Milky Way and Messier objects fade from naked-eye view, and the total count of discernible point sources drops sharply. Although the descriptions are qualitative, they correspond closely to quantitative metrics such as naked-eye limiting magnitude and sky quality meter readings, giving the scale a measurable backbone beneath its descriptive language.

Adoption and Scientific Utility

Since its debut, the Bortle scale has become a standard reference point for both amateur and professional astronomers worldwide. Its strength lies in translating the abstract relationship between artificial lighting and degraded night-sky quality into an intuitive, easily communicable framework, which has made it a go-to tool in scientific writing and public outreach. A broad database of sky quality meter measurements has since been assembled, enabling a Bortle rating to be estimated for virtually any location on the planet. The scale's influence extends beyond astronomy: in 2018, James Brodrick of the US Department of Energy acknowledged that all manmade lighting is inherently unnatural and carries potentially undesirable side effects, a sentiment that resonates with the Bortle framework's implicit warning. By providing a common numerical shorthand, the scale has helped bridge the gap between technical photometry and the lived experience of looking up at the night, making the conversation about light pollution more accessible and actionable for a wide audience.

Criticisms and the Evolving Lighting Landscape

Research published in 2014 raised questions about whether Bortle's descriptions overstate what a typical observer can actually perceive, even under the darkest skies. Diffuse extended objects such as the galaxy M33 (visual magnitude 5.72) are notably harder to detect than point sources of identical brightness, yet Bortle listed M33 as a naked-eye object—a characterization later analysts deemed atypical. Similarly, the Class 2 suggestion of a naked-eye limiting magnitude exceeding 7.1 was described as a substantial elevation of expectation, with 6.0 to 6.5 being the more commonly cited range. Bortle himself recognized that limiting magnitude varies with individual visual acuity, which is partly why he built the scale as a descriptive alternative to raw NELM. Meanwhile, the global shift to LED lighting and continued urbanization have intensified skyglow, with LEDs' higher color temperature posing challenges not fully captured in older studies. DarkSky International advocates for carefully targeted, warm-colored outdoor lighting that is no brighter than necessary, while a 2023 Washington Post piece noted that a world of energy-efficient LEDs that spare the nightscape remains possible.