A scale built for a human eye, not a sensor

Astronomer John Bortle published his 9-point scale in 2001 to describe what a dark-adapted human eye sees looking straight up, from Class 1 (a site dark enough to cast a shadow from the Milky Way alone) to Class 9 (an inner-city sky where only the Moon and a handful of the brightest objects cut through). It was designed from direct observation, not from a sensor reading.

Where satellite radiance agrees with it

Upward light radiating from a city and what a person sees looking up from that same city are closely related: more artificial light almost always means a worse view. That's why this site's brightness bands are a genuinely useful stand-in for a Bortle class most of the time, and why Oklahoma City, OK's Inner-city reading and a Bortle 8 or 9 rating would very likely agree.

Where they can disagree

A satellite reading can't see cloud cover, moon phase, humidity, or the direction a light dome falls relative to where someone is standing, all of which move what a human eye actually reports on a given night. Two cities with near-identical radiance can report different real-world Bortle classes if one sits near a bright neighbour whose light dome spills toward it and the other doesn't. Treat this site's band as a strong starting estimate, not a substitute for a night spent actually looking up.

If you want a real Bortle rating

A certified or community-reported Bortle class for a specific site usually comes from a local astronomy club's own logs or a dark-sky park's rangers, people who have actually stood there after dark. This site's job is narrower and different: a same-day estimate for any of 3,206 US cities, not a substitute for that first-hand report.

Where to go next

See the range this plays out across: Truckee, CA, Roswell, NM, Oklahoma City, OK.

Your Brightness Band Isn't the Whole Answer: When to Actually Look Up · What This Dataset Does and Doesn't Cover