What the Bortle Scale measures
The Bortle Scale is a 1-to-9 ranking of night-sky darkness published by amateur astronomer John Bortle in Sky & Telescope in February 2001. Class 1 is a pristine wilderness sky; Class 9 is the inside of a major city. Each class corresponds to a band of sky brightness, typically measured in magnitudes per square arc-second (mag/arcsec²), often abbreviated SQM after the Sky Quality Meter handheld photometer.
Counter-intuitively, higher SQM numbers mean darker skies. A pristine site reads 21.8–22.0 mag/arcsec²; an inner-city site can be under 17 mag/arcsec². The reason is logarithmic: each step of one magnitude is a factor of 2.5× in brightness, so the ~5 magnitudes between Bortle 1 and Bortle 9 is roughly 100× darker.
The 9 classes
Class 1 — Excellent dark-sky site
SQM 21.7–22.0. The Milky Way is structured enough to cast visible shadows on the ground. The zodiacal light is obvious; M33 (the Triangulum Galaxy) is a naked-eye target. Globular clusters resolve into stars in a 6-inch scope. Very few sites in the Lower 48 of the US reach Class 1; you need to be deep in a designated Dark Sky Sanctuary or wilderness area.
Class 2 — Typical truly dark site
SQM 21.5–21.7. Milky Way highly structured but no longer shadow- casting. The Gegenschein (the faint glow opposite the sun) is visible. Excellent for almost any observation; this is what most astronomy clubs are chasing when they organize "star parties."
Class 3 — Rural sky
SQM 21.3–21.5. Milky Way is complex and shows dark lanes. Some light domes are visible on the horizon from towns 20+ miles away. M31 is obvious naked-eye; you can see globular clusters as faint smudges without optical aid.
Class 4 — Rural / suburban transition
SQM 20.4–21.3. The Milky Way overhead still shows structure but washes out near the horizon. Several light domes from distant cities. Most Messier objects are findable but require some searching. This is the practical minimum for rewarding Milky Way photography and serious deep-sky observation.
Class 5 — Suburban sky
SQM 19.1–20.4. Milky Way is very weak; only visible overhead at all. Bright Messier objects like M13, M42, and M31 are still nice through a small scope but galaxies require careful averted vision. Typical of outer suburbs around medium-sized cities.
Class 6 — Bright suburban sky
SQM 18.4–19.1. Milky Way is invisible or barely seen near zenith. The sky has a perceptible grey-orange tint even directly overhead. Only the brightest 30 or so Messier objects are practical without going to deep exposures.
Class 7 — Suburban / urban transition
SQM 18.0–18.4. Milky Way invisible. Sky has a uniform greyish-orange glow. Bright planets and the moon are still excellent telescope targets, but deep-sky observation is mostly limited to globular clusters, M31 / M32, and open clusters. M13 looks like a faint smudge.
Class 8 — City sky
SQM about 17.0–18.0. Most stars in familiar constellation patterns are missing; you might see 20 stars in the whole sky. Only Moon, planets, very bright stars, and a few bright open clusters are useful targets. The Pleiades shows about 5–7 stars instead of the usual dozen.
Class 9 — Inner-city sky
SQM below 17.0. The sky is brightly lit, often orange or salmon-pink. Only Moon, planets, and the very brightest stars are visible (around 10–15 in the whole sky). The night sky offers almost nothing of observational interest other than the Moon and bright planets.
How CVAN measures your Bortle class
CVAN uses the NOAA NCEI VIIRS Day-Night Band satellite radiance for your exact pixel — the same data behind widely-used light-pollution maps. The raw radiance (in nanowatts per cm² per steradian) is converted to artificial sky brightness at zenith via the Falchi (2016) empirical fit. That artificial component is added to the dark-sky natural background (about 22.0 mag/arcsec²) and the result is reported as SQM and binned into a Bortle Class.
Three numbers appear in the app:
- SQM in mag/arcsec² — the total sky brightness at zenith.
- LPI, the Light Pollution Index — how many times brighter your sky is than the natural background. LPI of 1.0 is a pristine site; LPI of 30+ is heavily light-polluted.
- Zone — VIIRS-derived darkness zone (1 pristine through 8 city) plus the Bortle class.
Finding a dark-sky site
Open the CVAN map picker and look for the deepest blue colours on the light-pollution overlay. As a rule of thumb:
- Bortle 4 is usually 30–60 miles outside major US cities, depending on terrain and surrounding population.
- Bortle 2 typically requires 100+ miles from any city of 100,000 people, often in mountains or remote farmland.
- Bortle 1 sites are nearly all designated Dark Sky Sanctuaries — Cherry Springs PA, Big Bend TX, Death Valley CA, Natural Bridges UT, etc.
The International Dark-Sky Association maintains a global directory at darksky.org/places — most of those qualify as Class 2 or better.