SKY DARKNESS / QUICK DIRECTORY

Colorado stargazing by Bortle class

Start with how dark you want the sky. Jump to a Bortle class, scan the matching active Colorado stops, then open the place that fits your night.^[1][2]

QUICK NAV / 01–09

Lower number. Darker sky.

These are the current guide ratings for active practical observing or program locations. Bortle itself is observational. Some stop ratings have direct field or source support; provisional estimates may use VIIRS/Black Marble nighttime-radiance and light-pollution-map context until a better observation is available.^[1][2][3][4]

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 CLASS1