The Bortle dark-sky scale (usually referred to as simply 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 to Class 9, inner-city skies. 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] The table summarizes Bortle's descriptions of the classes. For some classes, there can be drastic differences from one class to the next, e.g, Bortle 4 to 5.
Class | Title | NELM | Approx. SQM[3] mag/arcsec2 |
Description |
---|---|---|---|---|
1 | Excellent dark-sky site |
7.6–8.0 | 21.76 - 22.0 |
|
2 | Typical truly dark site |
7.1–7.5 | 21.6–21.75 |
|
3 | Rural sky | 6.6–7.0 | 21.3–21.6 |
|
4 | Brighter rural | 6.3–6.5 | 20.8–21.3 |
|
4.5 | Semi-Suburban/Transition sky | 6.1–6.3 | 20.3–20.8 |
|
5 | Suburban sky | 5.6–6.0 | 19.25–20.3 |
|
6 | Bright suburban sky |
5.1–5.5 | 18.5–19.25 |
|
7 | Suburban/urban transition |
4.6–5.0 | 18.00–18.5 |
|
8 | City sky | 4.1–4.5 | <18.00 |
|
9 | Inner-city sky | 4.0 |
|
The band Days N' Daze referenced the scale in the title and lyrics of their song Nine on the Bortle.[4]