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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.
As a fourth-magnitude star, η Carinae is comfortably visible to the naked eye in all but the most light-polluted skies in inner-city areas according to the Bortle scale. [46] Its brightness has varied over a wide range, from the second-brightest star in the sky for a few days in the 19th century, to well below naked-eye visibility.
On clear nights around new moon, the sky darkness of Joshua Tree is rated a class 2–4 on the Bortle scale. [51] [52] This ranges from an "average dark sky" (class 2) in the easternmost region of the park to a sky quality typical of a rural/suburban transition (class 4) in the western regions near Palm Springs.
The limiting magnitude for naked eye visibility refers to the faintest stars that can be seen with the unaided eye near the zenith on clear moonless nights. The quantity is most often used as an overall indicator of sky brightness, in that light polluted and humid areas generally have brighter limiting magnitudes than remote desert or high altitude areas.
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The darkness of the night sky is classified on the Bortle scale from 1 ("excellent", i.e., extremely dark) to 9 ("inner-city sky", i.e., partially dark). Hanle is categorised as an excellent dark astronomical site with Bortle colour key "Black". [9] The dark sky is important for the conservation of nocturnal animals and ecology.
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. Measurement of the surface brightnesses of celestial objects is called surface photometry.