Bortle Scale

Bortle Scale

The Bortle Scale is a scale consisting of nine levels that measure the brightness of the night sky from a particular location. John E. Bortle designed this scale and published it in the February 2001 issue of Sky & Telescope to help amateur astronomers assess the darkness of an observatory, and to compare darkness between observable areas.

The Bortle Scale is based on astronomer's observations of the ability to detect the number of stars and galaxies in the sky that are affected by the rate of darkness due to light pollution. For example, the Bortle Class 1 sky means that one can see the M33 (Triangular Galaxy) using the naked eye directly.

In the Bortle scale, the lower the value of the scale, the darker the area.

SQM value is compared to the Bortle Scale. The lower the Bortle scale, the higher the SQM value.
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