The Bortle Scale
Explained

Before you can plan a Milky Way shoot intelligently, you need to understand one number: your Bortle class. It tells you more about whether you'll actually see the Milky Way than any other single factor except the moon.

What the Bortle scale measures

The Bortle Dark-Sky Scale is a nine-level numeric scale that measures the darkness of the night sky at any given location. It was developed by amateur astronomer John Bortle and published in Sky and Telescope magazine in 2001. The scale runs from 1 (the darkest skies on Earth) to 9 (the most severely light-polluted skies, typically found in the center of large cities).

The scale doesn't measure cloud cover, moon phase, or any other weather variable. It is purely a measurement of the ambient light pollution at a location, which is created by the cumulative glow of artificial lights on the ground reflecting off the atmosphere.

The gold-highlighted rows (2, 3, 4) represent the range where serious Milky Way photography becomes productive. Bortle 1 exists but is extremely rare. Most dedicated dark sky parks fall in the 2 to 3 range.

What it means for photography

Bortle class affects your images in two ways: it determines how bright the Milky Way appears relative to the background sky, and it affects how much detail you can pull from the galactic core in post-processing.

Bortle 1 and 2: Exceptional

At Bortle 1 and 2, the zodiacal light is visible, airglow bands are noticeable, and the Milky Way casts a visible shadow on a white surface. The galactic core appears with dramatic color contrast, and structures like dust lanes are visible to the naked eye. These sites are rare and usually require significant travel. Natural Bridges National Monument in Utah and a handful of sites in the Atacama Desert in Chile are well-documented examples.

Bortle 3 and 4: Very good

Most of the best accessible dark sky photography happens in the Bortle 3 to 4 range. The Milky Way is bright, colorful, and well-defined. M33 (Triangulum Galaxy) is faintly visible to the naked eye at Bortle 3. The galactic core at Bortle 4 still produces stunning images with most camera and lens combinations, and much of the American West falls in this range within a reasonable drive from major cities.

Bortle 5 and 6: Workable with effort

At Bortle 5 and 6, the Milky Way is visible but competes meaningfully with the sky glow on the horizon. You can still make good images, but you're working harder in post to achieve the same result. Wide-angle lenses at large apertures become more important, and the horizon glow becomes part of your compositional challenge rather than an avoidable problem.

Bortle 7 and above: Generally not viable

From Bortle 7 upward, the galactic core is difficult to see with the naked eye and blends into a generally bright background in images. You can still shoot star fields and bright targets, but what most people picture when they think "Milky Way photo" is not achievable at Bortle 7 or above.

How Milky Way Tonight uses Bortle data

Milky Way Tonight pulls your Bortle class live from the VIIRS (Visible Infrared Imaging Radiometer Suite) satellite data processed by lightpollutionmap.info. This gives you a measurement for your specific GPS coordinates, not a rough estimate based on your nearest city or zip code.

The measurement is based on the artificial brightness of the sky at your location, converted to a Sky Quality Meter (SQM) reading and then mapped to the Bortle scale using the thresholds established by the Falchi 2016 light pollution atlas.

The most common misconception

The most common mistake people make with Bortle is treating it as a fixed property of a general area rather than a specific coordinate. A ridge 3 miles outside a small town might be Bortle 4. The valley floor of that same town might be Bortle 6. The difference matters enormously, and it's exactly the reason the app measures at your coordinates rather than estimating from a city name.

The second most common mistake is conflating Bortle class with weather. Bortle measures light pollution, not cloud cover. A Bortle 2 site under 80% cloud cover is useless. A Bortle 4 site on a perfectly clear moonless night is excellent. Bortle sets your ceiling. Weather and moon phase determine whether you hit it on any given night.