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
https://www.cleardarksky.com/csk/faq/2.html
Introduction To Light Pollution
http://www.bigskyastroclub.org/lp_bortle.html
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