The Bortle Scale: Measuring Light Pollution

Bortle Scale International Dark Sky Association Buying Land Light Pollution COVID

The Bortle scale is a way of measuring the quality, otherwise known as brightness, of the night sky for a particular location. The scale is really helpful as it quantifies the astronomical observability of celestial objects and the interference caused by light pollution. The scale was created by John E. Bortle, who published his work 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.

There are nine levels to the Bortle scale with Class 9 being the most extreme amount of light pollution.

Bortle Class Color Type Of Sky
1 Black Excellent, truly dark-skies
2 Grey Typical, truly dark skies
3 Blue Rural Sky
4 Green Rural/suburban transition
5 Yellow Suburban sky
6 Orange Bright suburban sky
7 Red Suburban/urban transition
8 White City Sky
9 White Inner City Sky

Classes 1 and 2 are truly dark sky in which one experiences the best skies possible. The milky way is at its greatest visibility and the greatest array of night sky objects are available.

Classes 3 and 4 of rural skies are characterized by the milky way being visible and multiple objects being visible with the naked eye.

Classes 5 through 9 have a signature of little to no Milky Way visibility, limited magnitude of 15 or below using a telescope, and visible light pollution. There will be limited detail that can be seen at these level on various specific celestial items.

One can gather sky quality information regarding light pollution and the Bortle scale using a sky quality meter, which can be purchased and learned more about from the IDA. Using this information can help track sky quality over time and help accurately map your surrounding area!

References And Further Reading

Introduction To Light Pollution

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