Key takeaways
- The Bortle scale ranks the darkness of a site from class 1 (an excellent dark sky) to class 9 (an inner city sky), by what a visual observer can see.
- A higher number of the faintest visible star means a darker sky, but a higher Bortle class means a brighter one.
- A brighter sky fills your frames with light and noise sooner, so you shoot shorter frames and collect many more of them.
- The class is a coarse, subjective starting point. Let your own test frames have the last word.
The Bortle scale is a nine step ruler for the darkness of a night sky, from class 1, an excellent dark sky site, to class 9, the sky above an inner city. It was published by the amateur astronomer John Bortle in Sky & Telescope magazine in February 2001.
If you have read anything about places to photograph the night sky, you have probably met it. It is a very useful idea, provided you remember what kind of ruler it is.
Where the scale comes from
Bortle described each class in terms of what a visual observer can actually see: whether the Milky Way casts a shadow, how faint a star can be with the naked eye, how bright the zodiacal light looks and what the horizon is doing. It was written for people with telescopes and for people who like to sit under the sky, which is why it describes the sky rather than measuring it.
The nine classes at a glance
| Class | Description | Faintest star by eye | The Milky Way |
|---|---|---|---|
| 1 | Excellent dark sky site | 7.6 to 8.0 | Vivid, and bright enough to cast a faint shadow |
| 2 | Typical truly dark site | 7.1 to 7.5 | Highly structured |
| 3 | Rural sky | 6.6 to 7.0 | Complex, with some light dome at the horizon |
| 4 | Rural to suburban transition | 6.1 to 6.5 | Still impressive, with less detail |
| 5 | Suburban sky | 5.6 to 6.0 | Weak, and washed out near the horizon |
| 6 | Bright suburban sky | 5.1 to 5.5 | Visible only high overhead, if at all |
| 7 | Suburban to urban transition | 4.6 to 5.0 | Not visible |
| 8 | City sky | 4.1 to 4.5 | Not visible |
| 9 | Inner city sky | 4.0 at best | Not visible, and only a handful of stars show |
To get a feeling for what these numbers mean, move the slider in the picture below. It draws the same imaginary patch of sky for every class and leaves out the stars that would be too faint to see.
Move the slider to compare a dark site with a bright one.
This picture is a simplified illustration for explanation. The number of stars follows the real trend, but it is not a map of any particular sky.
What it means for your photographs
In practice, the class tells you how quickly the sky itself will fill your frames with light. Under a very dark sky the background stays dim for minutes at a time, and your exposures can be long. Under a bright suburban sky the background reaches the same brightness much sooner, so single frames must be shorter, and you make up for it by taking many more of them. The extra light from the sky adds noise as well as brightness, which is why a bright sky needs more total time to reach the same clean result. A light pollution filter or a narrowband filter can recover some of what the sky takes away, but it cannot restore light from the object that never reached the sensor.
| Classes | Typical approach | Targets that suit |
|---|---|---|
| 1 to 3 | Long exposures, broadband imaging, wide lenses for the Milky Way | Everything, including faint galaxies and dust |
| 4 to 5 | Moderate exposures, a light pollution filter can help, plan for more total time | Galaxies, bright nebulae, the Milky Way with care |
| 6 to 7 | Short exposures, narrowband filters for nebulae, a great many frames | Emission nebulae, clusters, the moon and planets |
| 8 to 9 | Narrowband, the moon, the planets and bright clusters | Bright, compact targets |
The reason that more frames help so much is the subject of why a hundred frames beat one perfect frame.
Beyond Bortle: other ways to measure the sky
The Bortle scale is only one of several tools, and the others are more objective.
| Method | What it tells you | Best used for |
|---|---|---|
| Bortle class | A visual judgement on a nine point scale | A quick first impression of a site |
| Sky quality meter reading | The sky brightness in magnitudes per square arcsecond, where a dark site is around 21.5 to 22 and a bright city is around 17 or lower | An objective number you can repeat and compare |
| Light pollution map | Satellite measurements of artificial light, shown as a colour map | Choosing a site before you travel |
| Naked eye limiting magnitude | The faintest star you can see on the night | A check that includes haze, moonlight and cloud |
| Your own test frames | How bright the background is in a standard exposure | The result that matters for photography |
Why the numbers matter is clear from a 2016 study in Science Advances, the New world atlas of artificial night sky brightness. It estimated that more than 80 per cent of the world's population lives under light-polluted skies, and that the Milky Way is hidden from more than a third of humanity.
The limits of a single number
The scale is subjective and coarse. Two sites of the same class can differ greatly in one direction because a town lies to the south, and a site can be dark at the zenith and bright at the horizon. Nights differ as well, since humidity, haze and snow cover all change how much light is scattered back down. It is sensible to treat the class as a starting point and to let your own frames have the last word.
A class is a starting point. Your own test frames have the last word.
How AstroNight estimates it
AstroNight does not measure the sky. It estimates a class for the place you choose by comparing your location with a built in list of around forty thousand cities and their populations, using a model in which the glow of a town fades with distance, and it shows the outcome as a single number that helps to tune its suggestions. It is a reasoned estimate rather than a measurement, and where you know from experience that your site is darker or brighter than the estimate suggests, your own knowledge deserves more weight.
Frequently asked questions
What Bortle class is best for astrophotography?
The lowest class you can reach, because a darker sky lets you expose for longer and collect a cleaner picture of faint galaxies and the Milky Way. Classes 1 to 3 are ideal. You can still do good work from classes 5 to 7 with narrowband filters and more total exposure time, and from classes 8 and 9 the moon, planets and bright clusters remain open to you.
Is a Bortle 4 sky good enough for the Milky Way?
Yes. At class 4 the Milky Way is still impressive to the eye, though with less detail, and a camera records far more than the eye sees. Expect a light dome on some horizons and plan the composition to point away from it.
How accurate is a Bortle class taken from a map?
Treat it as accurate to about one class. Maps estimate the light from satellite and population data rather than measuring your sky, so local factors such as a nearby streetlight, a lake or snow cover can move the real result either way.
Does a light pollution filter help in a bright sky?
A little. A broadband light pollution filter removes some colours of artificial light and can lift contrast on galaxies and stars. A narrowband filter does far more for emission nebulae, but neither one replaces a dark sky.
What is a sky quality meter?
A small handheld device that measures the brightness of the night sky in magnitudes per square arcsecond. A higher number means a darker sky, and because it gives a number rather than a description it is a more objective companion to the Bortle class.
Sources and further reading
AstroNight is now on the App Store. The free planner and coach for astrophotography works on iPhone and iPad with iOS 17 or later, offline, with no account. Get it on the App Store.
