Bortle scale

It quantifies the astronomical observability of celestial objects and the interference caused by light pollution.

Amateur astronomer John E. Bortle created the scale and published it in the February 2001 edition of Sky & Telescope magazine to help skywatchers evaluate the darkness of an observing site, and secondarily, to compare the darkness of observing sites.

It gives several criteria for each level beyond naked-eye limiting magnitude (NELM).

[1] The accuracy and utility of the scale have been questioned in 2014 research.[2][why?]

The table summarizes Bortle's descriptions of the classes.

Representation of the Bortle scale
In this 10-second exposure photo, facing south toward Sagittarius , light pollution obscures the stars and faintly visible Milky Way in the suburban night sky over Southern California .
In this 4-minute exposure photo, facing up the sky pointing directly up on Kappa Ophiuchi in the constellation of Ophiuchus . With other constellations such as Scutum , Hercules , Corona Borealis , Scorpius , Bootes , Libra , a hint of Lyra star ( Vega ) and a hint of Sagittarius star ( Polis ) and Messier objects such as M16 and M24 . Light pollution is small, doesn't obscure the stars and visible Milky Way (at the corner) in the exurban night sky over Kepala Batas, Kedah . The light pollution here is class 3 in the Bortle scale.