HOW TO READ IT
One number is a sketch of the sky, not the whole night.
A Bortle class is an observational estimate based on visible sky conditions, not a permanent property of an entire park or town. DarkSky recommends repeating the assessment when brightness varies across an area.[1][2]
Use the number to compare darkness quickly, then read the stop page for the horizon, headlights, access and everything else that decides whether the place actually works.
WHAT YOU CAN SEE / BORTLE 4
What to expect under a Bortle 4 sky
A strong practical stargazing sky, but no longer pristine. The Milky Way is plainly visible with only its larger structures standing out. M31 is obvious to the naked eye, while M33 becomes difficult even with averted vision. Zodiacal light can extend about halfway to the zenith and low light domes are noticeable.[3]
- Naked-eye star limit
- Magnitude 6.0–6.49
- 320 mm telescope limit
- About magnitude 15.5
- Triangulum Galaxy / M33
- Difficult with averted vision
- Andromeda Galaxy / M31
- Obvious naked-eye
- Milky Way
- Visible; mainly the larger structures remain
- Zodiacal light
- Can reach about halfway to the zenith
- Artificial glow + ground
- Low light pollution domes; distant clouds and large ground objects become distinct
These are observational expectations for a clear, moonless, well-dark-adapted night, not a guarantee for every observer or every direction from a site.
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.[3]
| 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.