The Bortle Scale: Finding a Dark Sky and Reading Light Pollution Maps

What does the Bortle scale actually measure?

The Bortle scale describes the quality of a night sky in nine classes. Its creator, the American observer John E. Bortle, published it in the February 2001 issue of Sky & Telescope, drawing on nearly fifty years of observing [1]. He wanted a more complete method than simply spotting the faintest star visible to the naked eye.

His scale relies on several clues that complement one another: how many stars you can see, the structure of the Milky Way, the zodiacal light and the gegenschein, two very faint natural glows, and the way clouds look. Over a city, clouds reflect the glow of streetlights and other lighting. Under a very dark sky, they show up instead as dark holes in the star field [1].

The naked-eye limiting magnitudethe magnitude of the faintest star you can make out without an instrument; the higher the number, the fainter the star, often shortened to NELM, is one of those clues. But it varies with the observer’s eyesight, how well their eyes have adapted to the dark and the state of the atmosphere [1] [2]. Bortle also notes that modest light pollution hurts diffuse objects, comets, nebulae and galaxies, far more than it hurts stars [1].

The nine classes, from dark sky to downtown

Class 1 is the darkest sky on the scale, class 9 the sky over a city center. The limiting magnitudes in the table are the benchmarks Bortle published: they don’t guarantee anything to a given observer on a given night [1].

M33, the Triangulum Galaxy, works as a benchmark because it’s diffuse: it quickly vanishes against a bright background. How visible it is also depends on the season and how high it sits in the sky. In class 4, Bortle describes it as visible with averted vision only when it’s more than 50° above the horizon [1].

Enlarge diagram

In class 4, the Milky Way is still visible, but its detail is fading; beyond that, it pales until it disappears. Classes 1 to 3 offer increasingly favorable conditions for faint structures. Even so, no universal threshold guarantees any particular observation.

Why doesn’t a light pollution map give you the Bortle class?

Global maps rely on satellite images that detect light given off on the ground at night. The Earth Observation Group’s VIIRS data, for example, are filtered to remove, among other things, pixels lit by the Sun or the Moon and cloudy pixels [3].

These data don’t photograph the sky the way someone standing outside sees it: they measure the artificial light sent upward. An atmospheric model then estimates how much of that light gets scattered back and brightens the sky. The world atlas, consulted on October 31, 2026, uses satellite data from 2025, and its author states that it isn’t based on direct measurements [4].

Enlarge diagram

This is where the two approaches part ways. A zenith brightness map describes the sky directly overhead. Bortle judges the whole dome of the sky, from the zenith down to the horizon, using clues seen with the naked eye. So a map can show a dark zenith while a strong glow from a city is still visible in one direction.

The atlas’s author showed this using National Park Service data gathered over 397 nights: he compares the measured sky brightness with the Bortle class that the agency’s teams noted on site, and the two don’t line up in any fixed way [5].

Bortle, SQM and limiting magnitude: three different yardsticks

The SQMSky Quality Meter, a small device that measures the brightness of the sky in magnitudes per square arcsecond is pointed at the zenith. The higher the reading, the darker the sky, because the magnitude scale runs backward. Its field of view is wide: about 42° across, measured where its sensitivity drops to half its maximum. The National Park Service notes that it no longer reliably measures a sky darker than 21.5 magnitudes per square arcsecond [2].

To describe a site in detail, the National Park Service photographs the entire celestial hemisphere with calibrated cameras, in high-resolution mosaics. That way it measures brightness and glare across the whole sky, then separates the natural part from the human-caused part [6]. It’s a good reminder that a single reading at the zenith doesn’t describe the whole sky.

How do you use a map to choose a place to observe from?

A map cuts down the number of sites worth looking into; it doesn’t replace checking out the place itself.

1. Start with a map whose method is published: the date of the data, the sensor, what the value shown actually is. An annual atlas picks out dark areas, not the conditions on the night ahead [3] [4].
2. Look for areas well away from big cities, at a regional scale. Don’t just pick the darkest pixel: access, nearby towns and the glow around them matter too.
3. Look at the horizon, one direction at a time. A site that’s dark at the zenith can have a light dome on the side facing a city: is that exactly where the thing you want to observe will be [5]?
4. Check the place itself: somewhere you’re allowed to be, with a clear horizon and no direct lighting. A line of trees or a building blocks part of the sky without getting rid of the light pollution.
5. Check conditions on the night: an annual map says nothing about the Moon, clouds, haze or temporary lighting, all of which make a big difference to what you see [3].

How do you check the quality of the sky on site?

Get away from streetlights, headlights and lit buildings, and skip moonlit nights if you want to judge faint stars. The National Park Service only takes its reference measurements on moonless nights, under a sky that’s at least 95% clear. That’s its measurement protocol, not a rule for every outing [6].

Then give your eyes time to adjust to the dark. According to the British Astronomical Association, that takes 15 to 20 minutes or more, and a bright white light undoes the effort; a dim red flashlight is a better choice for reading a chart [7].

For a simple estimate, the citizen science program Globe at Night suggests comparing a constellation chosen for your date and location with charts showing more or fewer stars. The Association française d’astronomie, France’s national astronomy association, describes the method: wait for full darkness, about an hour and a half after sunset, avoid the Moon and direct lights, let your eyes adjust, then pick the chart that looks most like the sky you see [8]. In summer, when nights are short, full darkness comes much later.

This observation doesn’t automatically make a place a class 3 or class 4 site: it tests the map’s estimate against reality. For the comparison to be useful, write down the date, the time, the Moon, the state of the sky, the direction of any glows and the objects you actually saw, then try again on another night. Binoculars help you judge diffuse objects such as M33.

In the United States, a certified dark sky place makes a sensible shortlist, not a guarantee. The National Park Service, which has worked with the organization DarkSky for nearly two decades, says dozens of its park units hold Dark Sky Place certification; in 2007, Natural Bridges National Monument became the world’s first certified International Dark Sky Park [9]. The right site combines a dark enough sky, a clear horizon in the right direction and cooperative weather. A map points the way; only the sky, once you’re there, tells you what it’s willing to show.

Worth watching

An astrophotographer compares light pollution maps with the Bortle scale, then measures the sky with a Sky Quality Meter: the same pitfalls as above, put to the test.

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Erik Vadas-Gauden

The one who started Outfarer

Outfarer started with Erik, who launched it with a few passionate friends. A nature lover first and foremost, he writes about the topics closest to his heart and reviews a good chunk of the magazine, including every survival and bushcraft article before it’s published.

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