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Light Pollution: The Bortle Dark-Sky Scale

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The Bortle Dark-Sky Scale is a nine-level rating system that measures sky quality by providing observable standards. John E. Bortle, a retired fire chief and amateur astronomer, devised the rating system in 2001 as a means to help amateur astronomers evaluate the darkness of an observation site and compare sites. The scale ranges from one, an excellent dark-sky site, to nine, an inner-city sky, and specifies observable criteria for each class. Learn more about light pollution: http://www.delmarfans.com/educate/basics/lighting-pollution/

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Light Pollution: The Bortle Dark-Sky Scale

  1. 1. -; '_ _, ‘ L. j‘. . _. . TH E B0 RTLE ’- . jIIoj. Iig-h‘tp'9II'ufion is visible». A‘ al ba'nd, _ ge‘gen‘schein, .airglow and M33 galaxy are visible. ’ ' A " _ 1' Sc"o'__rpiu's and Sya_gittafriu's_ regionjdf the Milky"Way cast shadows. - V ‘ ’ TYPICAL TRULY DARK SITE . , , , . . ‘ ' Milky Way are visible. Clouds appear as dark holes in-the sky. - '_S| iI. I'I"OUI1vd1Ing'0bjeC"_B are barely visible except when silhouetted against the sky. “*3” | .R U RAL 5 KY Q. ‘ S pollution is evident along the horizon. Clouds appear illuminated near - ': the: horizon but are’-dark overhead. The Millq Way and global clusters like ' M4," M5,. M15 and M22 are visible. Zodiacal light and M33 are weak -and/ or difficult to see and objects 20-30 feet away are vaguely visible. I RU RAL/ S UBURBAN TRANSITION _ _Slq glow is ‘visible ‘in several directions. Clouds are illuminated near light pollution sources but are still dark overhead. Zodiacal light, Mi| ky’Way, and M33 ‘are still visible but with some limitations. - ' . _ ‘Objects are clearly visible ata distance. AIII SKY Light sources ‘are visible in all directions. .. 4 Clouds‘ appearbrighter than the sky itself. Zodiacal light and the Milky Way are barely visible. BRIG HTS U U RBAN 5 KY Skies within 35 degrees of the horizon glow a grayish white and the clouds are fairly bright Zodiacal light is no longer visible and the Milky Way is only visible near the zenith. M33 is not visible without binoculars and M31 is faintiy visible to the naked eye. S U BU RBAN/ U TRANS lTl0 N Strong light sources are visible in all directions. Sky background appears grayish white hue and clouds are brilliantiy | it. The Milky Way is nearly or totally invisible and M44 or M31 are still visible to the naked eye but lack detail. Bright Messier objects are faintly visible with moderate telescopes. “§“| C ITY S KY The sky glows whitish gray or orange. M31 and M44 are barely visible on good nights. Only bright Messier objects are visible with a modest telescope. Stars forming familiar constellations are difficultto see or invisible. °“3“| INNER-CITY SKY The entire sky is brightiy lit. Many stars making up constellations are invisible and constellations such as Cancer and Pisces are invisible. Pleiadas is the only Messier object visible to the naked eye. Moon, planets and a few bright star clusters are the only observable celestial objects. www_de, ma, fan5_mm

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