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 created the scale and published it in the February 2001 edition of
Sky & Telescope magazine to help
amateur astronomers evaluate the darkness of an observing site, and secondarily, to compare the darkness of observing sites. The scale ranges from Class 1, the darkest skies available on Earth, through Class 9, inner-city skies. It gives several criteria for each level beyond
naked-eye limiting magnitude (NELM). The accuracy and utility of the scale have been questioned in recent research.