The Bortle scale — how dark is your sky?

In 2001, an amateur astronomer named John Bortle published a short article in Sky & Telescope describing how dark a night sky really is. He proposed a nine-point scale, with one being the darkest skies that exist anywhere on the planet and nine being central Manhattan. Twenty-five years later, every serious stargazer knows their Bortle class, and the scale has become the standard way to talk about light pollution.

It’s also a useful diagnostic. If somebody complains they can’t see the Milky Way from their garden, the answer is almost always that their garden is Bortle 6 or 7, and the Milky Way only shows up reliably at Bortle 4 or darker. The scale doesn’t change your eyes. It changes your expectations.

The classes, briefly

Bortle 1 is pristine wilderness — the high Atacama, central Mongolia, the Australian outback, parts of Namibia. The Milky Way is bright enough to cast shadows. You can see roughly six and a half thousand stars with the naked eye, on a moonless night, after thirty minutes of dark adaptation. Most humans alive in 2026 will never experience a Bortle 1 sky in their lifetime.

Bortle 2 is truly dark countryside, far from any town, with no light dome in any direction. The Milky Way still casts shadows in some directions but not all. About six thousand stars visible.

Bortle 3 is rural, perhaps thirty kilometres from the nearest town. The Milky Way is detailed but no longer overwhelming. Some light domes visible on the horizon. Around five thousand stars.

Bortle 4 is the edge of suburbia, or the deeper countryside in a populous region. The Milky Way is visible but pale, and you can see it best by looking slightly off-center (a technique called averted vision). Three or four thousand stars.

Bortle 5 is suburban. The Milky Way is faint, broken in places, and only visible to dark-adapted eyes. Light domes are clearly visible. Two thousand stars at most.

Bortle 6 is bright suburban or small-city. The Milky Way is barely there, even at the zenith. The whole sky has a noticeable glow. Maybe a thousand stars.

Bortle 7 is the transition between suburban and urban. The sky has a permanent gray-white wash. No Milky Way at all. A few hundred stars.

Bortle 8 is city center proper. Only the brightest constellations are recognizable; the rest of the sky is a featureless gray. Perhaps a hundred stars.

Bortle 9 is inner city — the cores of Tokyo, London, New York. You can see the Moon, the planets, and maybe two dozen of the brightest stars. The aurora can’t reach you. The Milky Way left a long time ago.

Finding your own class

The fastest way is to visit lightpollutionmap.info. Enter your city and look at the color of the spot where you live. The colors translate roughly: white or red is Bortle 8 or 9, orange is 7, yellow is 6, green is 5, blue is 4, gray or black is 3 or darker. The data comes from VIIRS DNB satellite measurements that get updated every year.

Most of Europe and the eastern United States is Bortle 5 to 8 within an hour of any major city. Truly dark skies — Bortle 3 or better — require driving sixty kilometres or more in the right direction, or visiting a designated dark sky preserve.

What changes between classes

The interesting thing is how fast the gain accelerates as you move toward darker. Going from Bortle 8 to Bortle 4 — say, driving an hour out from a city — doubles or triples the number of stars you can see. Going from Bortle 5 to Bortle 2 — another hour, or visiting a national park — multiplies it by something like ten. There is no upper bound. People who travel to a Bortle 1 site for the first time routinely describe it as one of the most memorable nights of their life. Photos can show you what to expect, sort of, but the experience of standing under it is different in ways that are hard to capture.

Your eyes also need time. Twenty to thirty minutes of full darkness for complete adaptation. Glance at a phone screen and you reset to about five minutes. If you’ve made the effort to get to a dark site, the discipline around your phone makes more difference than people expect.

How to use Bortle on a given night

For planets and the Moon, your Bortle class barely matters. Those objects are bright enough to cut through most light pollution. Even from Bortle 9, Jupiter and Venus are fine.

For deep-sky observing — galaxies, nebulae, the Milky Way — Bortle is everything. Aim for Bortle 4 or darker. The Moon also matters, because a bright moon effectively raises your Bortle class by one or two for the duration. A Bortle 5 sky with no moon shows more than a Bortle 3 sky during a full moon.