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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 is an American amateur astronomer. He is best known for creating the Bortle scale to quantify the darkness of the night sky. Bortle has made a special study of comets. He has recorded thousands of observations relating to more than 300 comets. From 1977 until 1994 he authored the monthly '"Comet Digest" in Sky and Telescope magazine.
A truly dark sky has a surface brightness of 2 × 10 −4 cd m −2 or 21.8 mag arcsec −2. [9] [clarification needed] The peak surface brightness of the central region of the Orion Nebula is about 17 Mag/arcsec 2 (about 14 milli nits) and the outer bluish glow has a peak surface brightness of 21.3 Mag/arcsec 2 (about 0.27 millinits). [10]
The limiting magnitude will depend on the observer, and will increase with the eye's dark adaptation. On a relatively clear sky, the limiting visibility will be about 6th magnitude. [2] However, the limiting visibility is 7th magnitude for faint stars visible from dark rural areas located 200 km (120 mi) from major cities. [3] (See the Bortle ...
The modern scale was mathematically defined to closely match this historical system by Norman Pogson in 1856. The scale is reverse logarithmic: the brighter an object is, the lower its magnitude number. A difference of 1.0 in magnitude corresponds to the brightness ratio of , or about 2.512. For example, a magnitude 2.0 star is 2.512 times as ...
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A large number of the instances of "Bortle Dark-Sky Scale" in books are references to Bortle's paper title "Introducing the Bortle Dark-Sky Scale", which is usually but not always capitalized. Based on the diversity of things it's called, and lowercasing, there's little evidence that there's a proper name here.
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