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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.
The scale used to indicate magnitude originates in the Hellenistic practice of dividing stars visible to the naked eye into six magnitudes. The brightest stars in the night sky were said to be of first magnitude ( m = 1), whereas the faintest were of sixth magnitude ( m = 6), which is the limit of human visual perception (without the aid of a ...
The nine chapters of Bogard's book map to the nine levels of the Bortle scale, which attempts to quantify the subjective brightness and suitability for astronomy of the sky in different environments. Bogard has said of the scale, invented in 2001, "one of the reasons why identifying different depths of darkness is so important is that we don't ...
The Bortle scale is a nine-level measuring system used to track how much light pollution there is in the sky. A Bortle scale of four or less is required to see the Milky Way whilst one is "pristine", the darkest possible. [55]
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.
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As a fourth-magnitude star, Eta 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.