The Bortle Scale: How Dark Is Your Sky, Really?

The Bortle scale is the simplest way to answer a question every skywatcher eventually asks: how dark is my sky, really? It is a nine-step ruler for night-sky quality, running from a glowing city center all the way to a truly pristine wilderness sky — and once you know your number, the whole night sky starts to make sense.

What the Bortle scale measures

Created by amateur astronomer John Bortle, the scale rates how much light pollution washes out the stars from a given spot. Class 1 is the darkest sky on Earth, where the Milky Way casts faint shadows and thousands of stars crowd the view. Class 9 is an inner-city sky, where only the Moon, the planets, and a handful of the brightest stars break through the orange glow.

Most people live somewhere in the middle. A typical suburb sits around Class 5 or 6, where you can still trace bright constellations but the Milky Way has vanished. The difference between a Class 4 sky and a Class 7 sky is enormous — often the gap between seeing a few dozen stars and seeing a thousand.

How to find your Bortle class

You do not need any equipment to estimate it. Step outside on a clear, moonless night, let your eyes adjust for twenty minutes, and notice what you can see. The faintest stars and the visibility of the Milky Way are your best clues to where you land on the scale.

• If the Milky Way is bright, detailed, and impossible to miss, you are likely Class 1 to 3.
• If you can just barely glimpse the Milky Way overhead, you are around Class 4 to 5.
• If the sky glows and only bright stars show, you are probably Class 6 to 9.
• Drive 30 to 60 minutes from city lights and check again — the change is dramatic.
• Always look away from streetlights and shield your eyes from any nearby glare.

Knowing your Bortle class also tells you what to chase. If you are starting out, our guide to stargazing for beginners and your first night under the stars will help you make the most of any sky, and if dark skies are calling, here is how to find a dark sky near you and why it matters.

Curious which stars are reachable from your own backyard right now? Open the Starly sky map, set it to your location, and see exactly what your sky has to offer tonight.