Bortle scale

Last updated
Representation of the Bortle scale How light pollution affects the dark night skies (dark-skies) (flipped left-right).jpg
Representation of the Bortle scale

The Bortle dark-sky scale (usually referred to as simply the Bortle scale) is a nine-level numeric scale that measures the night sky's brightness of a particular location. It quantifies the astronomical observability of celestial objects and the interference caused by light pollution. John E. Bortle created the scale and published it in the February 2001 edition of Sky & Telescope magazine to help amateur astronomers evaluate the darkness of an observing site, and secondarily, to compare the darkness of observing sites.

Contents

The scale ranges from Class 1, the darkest skies available on Earth, through to Class 9, inner-city skies. It gives several criteria for each level beyond naked-eye limiting magnitude (NELM). [1] The accuracy and utility of the scale have been questioned in 2014 research. [2] The table summarizes Bortle's descriptions of the classes. For some classes, there can be drastic differences from one class to the next, e.g, Bortle 4 to 5.

Table of dark-sky classifications

ClassTitleNELMApprox.
SQM [3]
mag/arcsec2
Description
1Excellent
dark-sky site
7.6–8.021.76 - 22.0
2Typical truly
dark site
7.1–7.521.6–21.75
  • the zodiacal light is distinctly yellowish and bright enough to cast shadows at dusk and dawn
  • airglow may be weakly visible near horizon
  • the gegenschein is visible
  • clouds are only visible as dark holes against the sky
  • surroundings are barely visible silhouetted against the sky
  • the summer Milky Way is highly structured
  • many Messier objects and globular clusters are naked-eye objects
  • M33 is easily seen with naked eye
  • limiting magnitude with 12.5" reflector is 16.5
3Rural sky6.6–7.021.3–21.6
  • the zodiacal light is striking in spring and autumn, and color is still visible
  • some light pollution evident at the horizon
  • clouds are illuminated near the horizon, dark overhead
  • nearer surroundings are vaguely visible
  • the summer Milky Way still appears complex
  • M15, M4, M5, and M22 are naked-eye objects
  • M33 is easily visible with averted vision
  • limiting magnitude with 12.5" reflector is 16
4Brighter rural6.3–6.520.8–21.3
  • the zodiacal light is still visible, but does not extend halfway to the zenith at dusk or dawn
  • light pollution domes visible in several directions
  • clouds are illuminated in the directions of the light sources, dark overhead
  • surroundings are clearly visible, even at a distance
  • the Milky Way well above the horizon is still impressive, but lacks detail
  • M33 is a difficult averted vision object, only visible when high in the sky
  • limiting magnitude with 12.5" reflector is 15.5
4.5Semi-Suburban/Transition sky6.1–6.320.3–20.8
  • clouds have a grayish glow at zenith and appear bright in the direction of one or more prominent city light domes
  • the Milky Way is only vaguely visible – 10–15 degrees above the horizon. However the Great Rift, when overhead and with good transparency, is still obvious.
  • Although the views of bright globular clusters through 10" aperture and larger are striking, the outer regions of galaxies are difficult or impossible to see.
  • limiting magnitude with 12.5" reflector is 15.2
5Suburban sky5.6–6.019.25–20.3
  • only hints of zodiacal light are seen on the best nights in autumn and spring
  • light pollution is visible in most, if not all, directions
  • clouds are noticeably brighter than the sky
  • the Milky Way is invisible near the horizon, and looks washed out overhead. The winter Milky Way, even directly overhead, is fairly subtle.
  • when it is half moon (first/last quarter) in a dark location the sky appears like this, but with the difference that the sky appears dark blue
  • limiting magnitude with 12.5" reflector is 15
6Bright
suburban sky
5.1–5.518.5–19.25
  • the zodiacal light is invisible
  • light pollution makes the sky within 35° of the horizon glow grayish white
  • clouds anywhere in the sky appear fairly bright
  • even high clouds (cirrus) appear brighter than the sky background
  • surroundings are easily visible
  • the Milky Way is only visible near the zenith
  • M33 is not visible, M31 is modestly apparent
  • limiting magnitude with 12.5" reflector is 14.5
7Suburban/urban
transition
4.6–5.018.00–18.5
  • light pollution makes the entire sky light gray
  • strong light sources are evident in all directions
  • clouds are brightly lit
  • the Milky Way is nearly or totally invisible
  • M31 and M44 may be glimpsed, but with no detail
  • through a telescope, the brightest Messier objects are pale ghosts of their true selves
  • when it is full moon in a dark location the sky appears like this, but with the difference that the sky appears blue
  • limiting magnitude with 12.5" reflector is 14
8City sky4.1–4.5<18.00
  • the sky is light gray or orange – one can easily read
  • stars forming familiar constellation patterns may be weak or invisible
  • M31 and M44 are barely glimpsed by an experienced observer on good nights
  • even with a telescope, only bright Messier objects can be detected
  • limiting magnitude with 12.5" reflector is 13
9Inner-city sky4.0
  • The sky is brilliantly lit
  • many stars forming constellations are invisible and many fainter constellations are invisible
  • aside from the Pleiades, no Messier object is visible to the naked eye
  • the only objects to observe are the Moon, the planets, bright satellites, and a few of the brightest star clusters

The band Days N' Daze referenced the scale in the title and lyrics of their song Nine on the Bortle. [4]

See also

In this 10-second exposure photo, facing south toward Sagittarius, light pollution obscures the stars and faintly visible Milky Way in the suburban night sky over Southern California. Suburban night sky.jpg
In this 10-second exposure photo, facing south toward Sagittarius, light pollution obscures the stars and faintly visible Milky Way in the suburban night sky over Southern California.

Related Research Articles

<span class="mw-page-title-main">Amateur astronomy</span> Hobby of watching the sky and stars

Amateur astronomy is a hobby where participants enjoy observing or imaging celestial objects in the sky using the unaided eye, binoculars, or telescopes. Even though scientific research may not be their primary goal, some amateur astronomers make contributions in doing citizen science, such as by monitoring variable stars, double stars, sunspots, or occultations of stars by the Moon or asteroids, or by discovering transient astronomical events, such as comets, galactic novae or supernovae in other galaxies.

<span class="mw-page-title-main">Libra (constellation)</span> Zodiac constellation in the southern celestial hemisphere

Libra is a constellation of the zodiac and is located in the Southern celestial hemisphere. Its name is Latin for weighing scales. Its old astronomical symbol is (♎︎). It is fairly faint, with no first magnitude stars, and lies between Virgo to the west and Scorpius to the east. Beta Librae, also known as Zubeneschamali, is the brightest star in the constellation. Three star systems are known to have planets.

<span class="mw-page-title-main">Lupus (constellation)</span> Constellation in the southern celestial hemisphere

Lupus is a constellation of the mid-Southern Sky. Its name is Latin for wolf. Lupus was one of the 48 constellations listed by the 2nd-century astronomer Ptolemy, and it remains one of the 88 modern constellations but was long an asterism associated with the just westerly, larger constellation Centaurus.

<span class="mw-page-title-main">Astrophotography</span> Imaging of astronomical objects

Astrophotography, also known as astronomical imaging, is the photography or imaging of astronomical objects, celestial events, or areas of the night sky. The first photograph of an astronomical object was taken in 1840, but it was not until the late 19th century that advances in technology allowed for detailed stellar photography. Besides being able to record the details of extended objects such as the Moon, Sun, and planets, modern astrophotography has the ability to image objects outside of the visible spectrum of the human eye such as dim stars, nebulae, and galaxies. This is accomplished through long time exposure as both film and digital cameras can accumulate and sum photons over long periods of time or using specialized optical filters which limit the photons to a certain wavelength.

Naked eye, also called bare eye or unaided eye, is the practice of engaging in visual perception unaided by a magnifying, light-collecting optical instrument, such as a telescope or microscope, or eye protection.

<span class="mw-page-title-main">Star party</span>

A star party is a gathering of amateur astronomers for the purpose of observing objects and events in the sky. Local star parties may be one-night affairs, but larger events can last a week or longer and attract hundreds or even thousands of participants. Many astronomy clubs have monthly star parties during the warmer months. Large regional star parties are held annually and are an important part of the hobby of amateur astronomy. A naturally dark site away from light pollution is typical.

<span class="mw-page-title-main">Skyglow</span> Diffuse luminance of the night sky

Skyglow is the diffuse luminance of the night sky, apart from discrete light sources such as the Moon and visible individual stars. It is a commonly noticed aspect of light pollution. While usually referring to luminance arising from artificial lighting, skyglow may also involve any scattered light seen at night, including natural ones like starlight, zodiacal light, and airglow.

<span class="mw-page-title-main">Deep-sky object</span> Any astronomical object that is not an individual star

A deep-sky object (DSO) is any astronomical object that is not an individual star or Solar System object. The classification is used for the most part by amateur astronomers to denote visually observed faint naked eye and telescopic objects such as star clusters, nebulae and galaxies. This distinction is practical and technical, implying a variety of instruments and techniques appropriate to observation, and does not distinguish the nature of the object itself.

<span class="mw-page-title-main">Limiting magnitude</span> Faintest item observable by an instrument

In astronomy, limiting magnitude is the faintest apparent magnitude of a celestial body that is detectable or detected by a given instrument.

<span class="mw-page-title-main">Night sky</span> Appearance of the sky in a clear night

The night sky is the nighttime appearance of celestial objects like stars, planets, and the Moon, which are visible in a clear sky between sunset and sunrise, when the Sun is below the horizon.

The dark-sky movement is a campaign to reduce light pollution. The advantages of reducing light pollution include an increased number of stars visible at night, reducing the effects of electric lighting on the environment, improving the well-being, health and safety of people and wildlife, and cutting down on energy usage. Earth Hour and National Dark-Sky Week are two examples of such efforts.

In astronomy, surface brightness (SB) quantifies the apparent brightness or flux density per unit angular area of a spatially extended object such as a galaxy or nebula, or of the night sky background. An object's surface brightness depends on its surface luminosity density, i.e., its luminosity emitted per unit surface area. In visible and infrared astronomy, surface brightness is often quoted on a magnitude scale, in magnitudes per square arcsecond (MPSAS) in a particular filter band or photometric system.

The Indian Astronomical Observatory (IAO) is a high-altitude astronomy station located in Hanle, India and operated by the Indian Institute of Astrophysics. Situated in the Western Himalayas at an elevation of 4,500 meters (14,764 ft), the IAO is one of the world's highest located sites for optical, infrared and gamma-ray telescopes. It is currently the tenth highest optical telescope in the world. It is India's first dark-sky preserve.

<span class="mw-page-title-main">International Dark Sky Week</span>

International Dark Sky Week is held during the week of the new moon in April, when people worldwide may turn off their lights to observe the beauty of the night sky without light pollution. This event was founded in 2003 by high school student Jennifer Barlow of Midlothian, Virginia. It has been endorsed by the International Dark-Sky Association, the American Astronomical Society, the Astronomical League, and Sky & Telescope.

<span class="mw-page-title-main">Visible-light astronomy</span>

Visible-light astronomy encompasses a wide variety of astronomical observation via telescopes that are sensitive in the range of visible light. Visible-light astronomy is part of optical astronomy, and differs from astronomies based on invisible types of light in the electromagnetic radiation spectrum, such as radio waves, infrared waves, ultraviolet waves, X-ray waves and gamma-ray waves. Visible light ranges from 380 to 750 nanometers in wavelength.

<span class="mw-page-title-main">International Year of Astronomy</span> 2009 UN theme year

The International Year of Astronomy (IYA2009) was a year-long celebration of astronomy that took place in 2009 to coincide with the 400th anniversary of the first recorded astronomical observations with a telescope by Galileo Galilei and the publication of Johannes Kepler's Astronomia nova in the 17th century. The Year was declared by the 62nd General Assembly of the United Nations. A global scheme, laid out by the International Astronomical Union (IAU), was also endorsed by UNESCO, the UN body responsible for educational, scientific, and cultural matters.

<span class="mw-page-title-main">Globe at Night</span>

Globe at Night is an international scientific research program that crowdsources measurements of light pollution in the night sky. At set time periods within each year, the project asks people to count the number of stars that they can see from their location and report it to the project's website. The coordinating researchers compile this information to produce a public, freely available map of global light pollution. By September 2011, almost 70,000 measurements had been made. The use of data collected by the public makes the program an example of citizen science. Globe at Night began as a NASA educational program in the US organized by the NOAO, and was expanded internationally during the 2009 International Year of Astronomy; it is an offshoot of the GLOBE Program, which focuses on school-based science education.

<span class="mw-page-title-main">Astronomical Society of Victoria</span> Amateur astronomy club in the state of Victoria, Australia

The Astronomical Society of Victoria (ASV) is an amateur astronomy club in the state of Victoria, Australia. It was founded in 1922, making it one of the oldest such clubs in the country, and with some 1500 members it claims to be one of the largest amateur astronomy organisations in the southern hemisphere. Membership is open to all with an interest in astronomy, and the society caters for people with a wide range of ages, backgrounds, abilities and interests.

John E. Bortle is an American amateur astronomer. He is best known for creating the Bortle scale to quantify the darkness of the night sky.

<i>The End of Night</i> (book) Non-fiction book by Paul Bogard

The End of Night: Searching for Natural Darkness in an Age of Artificial Light is a 2013 non-fiction book by Paul Bogard on the gradual disappearance, due to light pollution, of true darkness from the night skies of most people on the planet. Bogard examines the effects of this loss on human physical and mental health, society, and ecosystems, and how it might be mitigated.

References

  1. Bortle, John E. (February 2001). "Gauging Light Pollution: The Bortle Dark-Sky Scale". Sky & Telescope . Sky Publishing Corporation. Retrieved 2020-05-29.
  2. Crumey, Andrew (2014). "Human Contrast Threshold and Astronomical Visibility". Monthly Notices of the Royal Astronomical Society. 442 (3): 2600–2619. arXiv: 1405.4209 . Bibcode:2014MNRAS.442.2600C. doi:10.1093/mnras/stu992.
  3. "Dark Skies Awareness". Archived from the original on 2020-11-12. Retrieved 2016-02-18.
  4. "Days N' Daze – Nine on the Bortle Lyrics". Genius Lyrics.