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 author, 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 fuller 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 are visible, the structure of the Milky Way, the zodiacal light and the gegenschein, two very faint natural glows, and the appearance of clouds. Over a town, clouds reflect the light from street lighting. Under a very dark sky, by contrast, they show up 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 degrades diffuse objects, comets, nebulae and galaxies, far more than it does stars [1].

The nine classes, from dark sky to city centre

Class 1 is the darkest sky on the scale, class 9 the sky of a city centre. The limiting magnitudes in the table are the benchmarks Bortle published: they guarantee nothing to a given observer on a given night [1].

M33, the Triangulum Galaxy, serves as a benchmark because it is diffuse: it soon disappears against a bright background. How visible it is also depends on the season and on its height in the sky. In class 4, Bortle gives it as visible with averted vision only when it is 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 favourable 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 the light given off at ground level at night. The Earth Observation Group’s VIIRS data, for example, are filtered to remove, among others, pixels lit by the Sun or the Moon and cloudy pixels [3].

These data do not photograph the sky as someone out walking sees it: they measure the artificial light sent upwards. An atmospheric model then estimates how much of that light is scattered back and brightens the sky. The world atlas consulted on 31 October 2026 uses satellite data from 2025, and its author states that it is not based on direct measurements [4].

Enlarge diagram

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

The atlas’s author tested this against data from the US National Park Service, whose team measured the sky and noted its Bortle class on 397 nights: the measured brightness and the Bortle classes do not match 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 backwards. Its field is wide: its sensitivity falls to half its peak across a cone about 42° wide. The National Park Service states that it no longer measures reliably 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 artificial part [6]. This shows that a single reading at the zenith does not 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 does not replace checking 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 large towns, at a regional scale. Don’t just pick the darkest pixel: access, nearby villages and the glow from surrounding towns matter too.
3. Look at the horizon, direction by direction. A site that is dark at the zenith can have a light dome on the side facing a town: is that exactly where the thing you want to observe will be [5]?
4. Check the place itself: somewhere you are allowed to be, with a clear horizon and no direct lighting. A row of trees or a building hides part of the sky without removing the light pollution.
5. Check the conditions on the night: an annual map says nothing about the Moon, cloud, haze or temporary lighting, all of which greatly change what you see [3].

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

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

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

For a simple estimate, the citizen science programme Globe at Night suggests comparing a constellation chosen for the date and place with charts showing more or fewer stars. The Association française d’astronomie 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 a British summer, the wait can be much longer: in June, the Royal Observatory Greenwich points out, the Sun does not sink far enough below the horizon from London for truly dark skies, and astronomical twilight lasts most of the night [9].

This observation does not 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, note 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 Britain, a designated dark sky place makes a sensible shortlist, not a guarantee: the South Downs National Park, an International Dark Sky Reserve, was mapped from more than 25,000 individual measurements, and its authority reports that the Milky Way is visible on clear, dark nights over 66% of the park [10]. The right site combines a dark enough sky, a clear horizon in the right direction and kind weather. A map points the way; only the sky, once you are there, tells you what it is prepared 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.

The video is hosted by YouTube (Google): nothing loads until you play it, and Google may then set cookies.

Erik Vadas-Gauden

The one who started Outfarer

Outfarer started with Erik, who launched it with a few passionate friends. A nature lover above all, he writes about the subjects closest to his heart and reviews a good part of the magazine, including every survival and bushcraft article before it is published.

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