What is the Bortle scale?
The Bortle scale rates how dark the night sky is, from 1, the darkest skies on Earth, to 9, the sky over a city center. John E. Bortle, an American amateur astronomer, published it in Sky & Telescope magazine in 2001 so observers could compare sites by what they could actually see.
Each class describes the view on a clear night with no moon: how much of the Milky Way shows, how many stars you can pick out, and how strongly towns glow on the horizon.
What can you see in each Bortle class?
| Class | What you see | Sky brightness | Faintest stars |
|---|
Sky brightness in mag/arcsec² is a later conversion used with Sky Quality Meters, and published tables differ; this site uses one common table. Faintest stars are the naked-eye limits Bortle gave, as a magnitude (higher means fainter). Bortle defined the classes by what you see, not by meter readings.
Why does a higher number mean a darker sky?
Sky brightness is measured in magnitudes per square arcsecond, and astronomers count magnitudes backwards: the bigger the number, the fainter the light. A natural sky with no artificial light reads about 22. A big city center reads about 17 or brighter: Las Vegas, the brightest place on this site, comes out at 16.6.
Every step of 1 makes the sky about 2.5 times brighter. Five steps make it 100 times brighter, so a city sky at 17 is roughly 100 times brighter than a natural sky at 22.
How Yozora Atlas works out a Bortle class
Nobody has measured the sky at every town and park. To give every place a class, Yozora Atlas estimates the brightness of the sky overhead from NASA satellite images of light at night, then converts it with the thresholds in the table above.
The model was checked against 487 hand-held sky meter readings from the Globe at Night project and against the US National Park Service's own sky glow model. 87 to 88% of the meter sites land in the right class or one class off. It can't tell class 8 from class 9, so city centers show as 8. How the readings are made.
Examples: Dinosaur Provincial Park (class 3) and Calgary (class 8).