Bortl scale

Bortle scale ( English Bortle scale ) - is a nine-level scale that measures the brightness of the night sky in a specific place. It quantitatively determines the astronomical observability of celestial objects and interference caused by light pollution . John E. Bortle created the scale and published it in the journal Sky & Telescope in February 2001, in order, first, to help amateur astronomers to estimate the darkness of the observation site, and secondly, to compare the darkness of the observation places. The scale is built from Class 1 , the darkest sky on Earth , to Class 9 - city skies. It provides several criteria for each level of the limiting magnitude seen by the naked eye ( born naked-eye limiting magnitude (NELM) ) ^[1] . The accuracy and usefulness of the scale has been called into question in recent studies ^[2] . However, it should be understood that the scale was created by an amateur for lovers and it is impossible to demand much from it.

The following table describes the classes of the Bortl scale:

Class	Short description	NELM	SQM	Description
one	Perfectly dark-sky site	7.6 ^m -8.0 ^m	21.7-22.0	The clouds of the Milky Way in Scorpio and Sagittarius cast shadows on Earth . The zodiacal light , anti - radiance , the zodiacal belt are visible in their entirety. In a 32 cm telescope, you can see stars of 17.5 ^m , while a 50 cm instrument with will show stars of 19 ^m . The presence of Venus or Jupiter in the sky worsens the dark adaptation of the eyes .
2	Typically truly dark site	7.1 ^m -7.5 ^m	21.5-21.7	The zodiacal light is distinctly yellow and bright enough to cast shadows at dusk and at dawn . The intrinsic glow of the atmosphere may be poorly visible near the horizon . Many Messier objects and globular clusters are visible to the naked eye . The clouds ^{[ what?} ^] are visible only as dark holes in the sky. The Milky Way Summer is very structured. M33 is easily visible with the naked eye . You can see stars 16 ^m −17 ^m in a 30 cm telescope .
3	Rural sky	6.6 ^m -7.0 ^m	21.3-21.5	Zodiacal light is visible in spring and autumn (when it extends 60 ° from the horizon ), its color differs with difficulty. Some light pollution on the horizon . The clouds may be dimly lit near the horizon - where the sky is brighter - but above the head they are dark. The Milky Way still looks complicated. Globular clusters , for example, M4 , M5 , M15 , M22 , are clearly visible to the naked eye . M33 is visible with lateral vision. In a 32 cm telescope you can see stars 16 ^m .
four	Rural / suburban transition	6.1 ^m -6.5 ^m	20.4-21.3	The zodiacal light is visible quite well, but does not extend even to half of the firmament at the beginning of the morning and late evening dusk . Illumination cones are clearly visible over settlements in several directions. The clouds above the light source are lit, but above the head are still dark. The milky way high above the horizon looks impressive, but it does not differ much except for the most noticeable structures. M33 is barely visible with lateral vision and is detected only at a height of more than 50 °. You can see stars 15.5 ^m in the 32 cm telescope .
five	Suburban sky	5.6 ^m- 6.0 ^m	19.1-20.4	Even on the best spring and autumn nights, only a hint of the zodiacal light is visible. Light sources are visible in almost all directions. In most or even the entire sky, clouds are noticeably brighter than the background. The milky way is very weak, invisible near the horizon and hardly distinguishes above the head. In a 32 cm telescope, you can see stars of 14.5 ^m -15.5 ^m .
6	Illuminated suburban sky	5.1 ^m -5.5 ^m	18.0-19.1	Even on the best nights, no signs of the zodiacal light are visible. The Milky Way is visible only at its zenith . The sky to a height of 35 ° above the horizon is whitish. The clouds across the sky are quite bright. M33 cannot be found without binoculars , M31 is hardly visible to the naked eye . In the 32 cm telescope you can see the stars 14.0 ^m -14.5 ^m .
7	Suburban / urban transition	4.6 ^m -5.0 ^m	18.0-19.1	The sky background has an indefinite grayish-white hue. Bright light sources are visible in all directions. The milky way is practically invisible. The clouds are brightly lit. M44 and M31 are hardly visible to the naked eye . Even in medium - sized telescopes, Messier objects are only pale ghosts of their real form. In a 32 cm telescope, stars of 14.0 ^m -14.5 ^m can hardly be seen.
eight	City sky	4.1 ^m -4.5 ^m	<18.0	The sky is whitish or reddish, newspaper headlines are read easily. Even on good nights, an experienced observer barely sees the M31 and M44 . In small telescopes, you can find only the brightest objects of the Messier catalog . Some stars, which should form the familiar contours of the constellations, are barely visible or absent at all. In a 32 cm telescope, 13.0 ^m stars can hardly be seen.
9	Inner-city sky	4.0 ^m	<18.0	The whole sky is brightly lit, even at its zenith . Many of the stars that make up the familiar constellations are not visible, and faint constellations such as Cancer and Pisces are not visible at all. Except, perhaps, the Pleiades , no objects of Monsieur are visible to the naked eye . The only celestial objects that are pleasant to look at with a telescope are the Moon , planets and several star clusters (if you can find them).

Notes

↑ Bortle, John E. The Bortle Dark-Sky Scale (neopr.) . Sky & Telescope . Sky Publishing Corporation (February 2001).
↑ Crumey, Andrew. Human Contrast Threshold and Astronomical Visibility // English Monthly Notices of the Royal Astronomical Society : journal. - Oxford University Press , 2014 .-- Vol. 442 , no. 3 . - P. 2600-2619 . - DOI : 10.1093 / mnras / stu992 . - . - arXiv : 1405.4209 . Archived March 5, 2016.

Links

(English) Bortle, John E. Gauging Light Pollution: The Bortle Dark-Sky Scale (neopr.) . Sky & Telescope . Sky Publishing Corporation (July 18, 2006).
(English) International Dark-Sky Association
(Russian) Estimation of the darkness of the sky

[1] Bortle, John E. The Bortle Dark-Sky Scale (neopr.) . Sky & Telescope . Sky Publishing Corporation (February 2001).

[2] Crumey, Andrew. Human Contrast Threshold and Astronomical Visibility // English Monthly Notices of the Royal Astronomical Society : journal. - Oxford University Press , 2014 .-- Vol. 442 , no. 3 . - P. 2600-2619 . - DOI : 10.1093 / mnras / stu992 . - . - arXiv : 1405.4209 . Archived March 5, 2016.

Bortl scale

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