Think of the number as a quick observational sketch, not a permanent score stamped onto an entire park or town. Sky brightness can vary across the same area, and conditions can change what an observer sees. Where a representative field observation is not available, this guide may use a provisional best-fit integer estimate informed by nighttime-light and sky-brightness map context at the practical observing or program location. NASA Black Marble / VIIRS is satellite radiance data, not a Bortle measurement, so those modeled guide values remain estimates until they are field checked.[1][2][3][4]
Want the fast version? Open the complete darkness directory and jump straight from a Bortle number to the matching locations.
Bortle icon key / darker sky → brighter sky
VISIBLE SKY / 01–09
What each Bortle class lets you see
A practical comparison adapted from Handprint’s Bortle indicator table. Lower numbers reveal fainter stars, galaxies and natural sky structure; brighter classes progressively trade those targets for the Moon, planets and brighter stars.[5]
| Class | Naked-eye limit | 320 mm limit | M33 | M31 | Milky Way | Zodiacal light | Artificial glow / ground |
|---|---|---|---|---|---|---|---|
| B1Excellent dark-sky site | Magnitude 7.5 or fainter | Fainter than magnitude 17 | Obvious naked-eye | Easy, bright naked-eye target | Extremely structured; brightest regions can cast diffuse shadows | Striking | Airglow can be apparent; ground objects mostly silhouettes |
| B2Typical truly dark site | Magnitude 7.0–7.49 | About magnitude 16.5 | Easy with direct vision | Easy naked-eye target | Highly structured | Bright; can show a faint yellow cast | Very little artificial glow; large nearby objects are only vaguely visible |
| B3Rural sky | Magnitude 6.5–6.99 | About magnitude 16.0 | Easy with averted vision | Easy naked-eye target | Complex structure remains obvious | Obvious | Light pollution mainly on the horizon; overhead remains dark |
| B4Rural / suburban transition | Magnitude 6.0–6.49 | About magnitude 15.5 | Difficult with averted vision | Obvious naked-eye | Visible; mainly the larger structures remain | Can reach about halfway to the zenith | Low light pollution domes; distant clouds and large ground objects become distinct |
| B5Suburban sky | Magnitude 5.5–5.99 | About magnitude 14.5–15.0 | Not expected as a naked-eye target | Easy with direct vision | Washed out | Faint | Artificial glow surrounds the horizon; clouds can look brighter than the sky |
| B6Bright suburban sky | Magnitude 5.0–5.49 | About magnitude 14.0–14.5 | Not expected as a naked-eye target | Easy with averted vision | Visible mainly near the zenith | Generally not visible | Light pollution reaches roughly 35° high; clouds are fairly bright |
| B7Suburban / urban transition | Magnitude 4.5–4.99 | About magnitude 14.0 | Not expected as a naked-eye target | Difficult with averted vision | Generally invisible | Not visible | Light pollution can reach the zenith; clouds may be brilliantly lit |
| B8City sky | Magnitude 4.0–4.49 | About magnitude 13 | Not visible naked-eye | Not a reliable naked-eye target | Invisible | Not visible | Strong low-sky glow; ground can be bright enough to read large print |
| B9Inner-city sky | Magnitude 4.0 or brighter | Not specified in the source table | Not visible naked-eye | Not a reliable naked-eye target | Invisible | Not visible | Artificial skyglow remains bright even at the zenith |
M33 and M31 refer to naked-eye visibility. “Averted vision” means looking slightly away from a faint target so the more light-sensitive part of the retina can detect it.
Bortle is only one stop-file label. The Practical Guide also explains access, camping, roads, facilities, cell service, telescope setup and guided versus self-guided use.
John E. Bortle introduced the nine-class scale in 2001. It combines several things an observer can see in the sky rather than reducing darkness to one satellite value, so modeled values in this guide are explicitly provisional estimates rather than certifications.[1][2][3][4]
Sources
- [1]
John E. Bortle — Introducing the Bortle Dark-Sky Scale, Sky & Telescope
- [2]
DarkSky International — How to conduct a night sky quality survey
- [3]
NASA Earthdata — Worldview guide: Black Marble nighttime at-sensor radiance from VIIRS
- [4]
Light Pollution Map — nighttime-light and sky-brightness map used as guide context
- [5]
Handprint — Bortle Dark Sky Scale observational indicator table