The Bortle dark-sky scale (usually referred to as simply the Bortle scale) is a nine-level numeric scale that measures the

night sky 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. Natural light sources in a night sky include moonlig ...

brightness Brightness is an attribute of visual perception in which a source appears to be radiating/reflecting light. In other words, brightness is the perception dictated by the luminance of a visual target. The perception is not linear to luminance, and ...

of a particular location. It quantifies the

astronomical Astronomy is a natural science that studies celestial objects and the phenomena that occur in the cosmos. It uses mathematics, physics, and chemistry in order to explain their origin and their overall evolution. Objects of interest include ...

observability of celestial objects and the interference caused by

light pollution Light pollution is the presence of any unwanted, inappropriate, or excessive artificial Visible spectrum, lighting. In a descriptive sense, the term ''light pollution'' refers to the effects of any poorly implemented lighting sources, during the ...

Amateur astronomer 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 m ...

John E. Bortle created the scale and published it in the February 2001 edition of ''

Sky & Telescope ''Sky & Telescope'' (''S&T'') is a monthly magazine covering all aspects of amateur and professional astronomy, including what to see in the sky tonight and new findings in astronomy. Other topics covered include: *observing guides for planets, ...

'' magazine to help skywatchers evaluate the darkness of an observing site, and secondarily, to compare the darkness of observing sites. 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 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. In astronomy, the naked eye m ...

limiting magnitude (NELM). The accuracy and utility of the scale have been questioned in 2014 research. 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, because the scale's SQM's at lowest are typically the same as the highest for the next, however 4-5 is the only one that deviates from that pattern.

Table of dark-sky classifications

References

External links

* by ''

''
Interactive demo of the Bortle Scale International Dark-Sky AssociationLight Pollution Map with Bortle Scale data (accurate to 1 decimal place)
{{Portal bar, Society, Astronomy, Stars, Science Amateur astronomy Light pollution Scales 2001 introductions Eponyms

Table of dark-sky classifications

See also

References

External links