FIELD KEY / SKY DARKNESS
Bortle 1 is dark. Bortle 9 is bright.
The Bortle scale is a nine-step way to describe what a night sky actually looks like. Lower numbers mean more natural darkness and more faint structure overhead.
Think of the number as a quick sketch, not a permanent score stamped onto an entire park or town. One horizon can glow while another is dark. Smoke, snow, weather and nearby lights can change the view. Where a representative field measurement is not available, this guide uses a best-fit integer estimate from modeled sky-brightness and light-pollution data at the practical observing or program location, including NASA Black Marble / VIIRS data represented by Light Pollution Map and World Atlas context. Because Bortle is an observational scale, treat modeled estimates as roughly plus or minus one class until they are field checked.
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
Excellent dark-sky site
The dark end of the scale. The Milky Way shows structure and the sky itself starts to feel like part of the landscape.
SEE CLASS 1 ↗Typical truly dark site
Very dark rural sky with strong naked-eye Milky Way structure and only modest light domes.
SEE CLASS 2 ↗Rural sky
A dark rural observing sky. Distant light is visible, but deep-sky observing remains strong.
SEE CLASS 3 ↗Rural / suburban transition
Still a useful stargazing sky, but light domes are obvious and faint diffuse objects lose contrast.
SEE CLASS 4 ↗Suburban sky
Skyglow is obvious and the Milky Way is weaker. Bright clusters, nebulae, planets and the Moon still have plenty to offer.
SEE CLASS 5 ↗Bright suburban sky
Skyglow dominates much of the view. The Milky Way is difficult or absent and faint galaxies fade hard into the background.
SEE CLASS 6 ↗Suburban / urban transition
A bright sky where the Milky Way is generally gone and the easy wins are brighter stars, planets and the Moon.
SEE CLASS 7 ↗City sky
Strong urban glow leaves mainly the brighter stars, planets and Moon as easy naked-eye targets.
SEE CLASS 8 ↗Inner-city sky
The bright end of the scale. Only the brightest stars, planets and Moon stand out easily against the sky.
SEE CLASS 9 ↗John E. Bortle introduced the scale in 2001. It uses several things you can actually see in the sky rather than a single satellite value or brightness reading, so modeled values on this guide are estimates rather than certifications. Light Pollution Map — NASA Black Marble / VIIRS sky brightness ↗