We have lost the night.
Roughly 80% of people on Earth no longer see the Milky Way from where they live. The same satellites that gave us the iconic "Earth at night" image now let us classify any spot on the globe by sky darkness. Four panels walk from the language of dark-sky observers, to the map, to what it costs you in exposure time.
Nine steps from black to yellow.
John E. Bortle proposed a nine-level scale in 2001 to give astronomers a shared language for sky darkness. Step 1 is the genuine pristine sky, where the Milky Way casts shadows. Step 9 is inner-city sky, where only Moon, planets and a handful of stars remain.
Hover any class to read the description.
SQM is Sky Quality Meter, magnitudes per square arcsecond — a logarithmic darkness reading where each whole number is roughly 2.5× brighter or darker. Hover any class for the original Bortle description.
From orbit, the lights all blur together.
NASA's Suomi NPP satellite captured the Earth at night in the VIIRS Black Marble composite. Each pixel is classified into a Bortle class using both its own brightness and a 19 km area median around it, so a dark park surrounded by a city is correctly classified by its city, not by itself. Click anywhere to read the local sky.
Stars on the map mark verified dark-sky sites — click one for its Bortle and SQM. Pixels above 65° latitude are desaturated and hatched because VIIRS is noisy there (winter darkness gaps, auroral contamination). Below that, the map is honest. The whole map and its colour palette are lifted 1:1 from Astrophotal — the charts around it are in Pete-DNA.
What it costs to chase the same photon.
Sky brightness doubles, your signal-to-noise halves, and you have to expose for four times as long to compensate. Across the nine Bortle classes, broadband imaging gets exponentially more painful — but narrowband filters that pick out the H-alpha and OIII emission lines flatten the curve dramatically. Hover the chart to read each class.
- Broadband (LRGB)
- Narrowband 7 nm
- Narrowband 3 nm
t = 10(SQMref − SQMsite) / 2.5 · narrowband filters isolate emission lines and reject most artificial skyglow.
Reference is Bortle 2, a typical dark site. The curve uses linear SQM interpolation between integer Bortle values. Narrowband shifts are conservative: a 7 nm filter cuts roughly 60% of the light-pollution penalty, a 3 nm filter roughly 80%. Real filters and cameras vary.
Not every target loses the same.
Broadband, narrowband and planetary imaging react very differently to light pollution. Knowing which discipline you're after tells you whether a Bortle 7 backyard is impossible or perfectly fine.
Broadband (LRGB)
Most affected by light pollution. The background sky glow eats contrast on faint nebulosity and galaxy arms. Best results from Bortle 1-4 sites.
Narrowband (Hα, OIII, SII)
Filters that isolate specific emission lines reject most artificial light. Effective even from Bortle 7-8 suburban locations.
Planetary & Lunar
Bright targets that easily outshine any light pollution. Lucky imaging works well even from city centres.
The bars show approximate sensitivity to light pollution, not signal quality of the result. A Bortle 8 backyard with a 3 nm Ha filter can deliver a stunning Heart Nebula. The same backyard cannot do broadband Andromeda.
The map shows the world from above. The chart shows what that costs you when you point the camera up.
Map: NASA VIIRS Black Marble (GIBS Earthdata, 2016 composite) · Per-pixel 2D Bortle classification (local brightness + 19 km area median) · Basemap CARTO Dark, OSM contributors · Bortle scale: Bortle 2001 (Sky & Telescope). Lifted 1:1 from astrophotal.com/plan/light-pollution, charts in Pete-DNA · All charts multi-trigger on viewport re-entry