What is the Bortle scale?

The Bortle scale describes how dark the night sky appears at a particular location. It has nine classes: Class 1 represents a very dark sky, while Class 9 describes a bright urban sky. It is not an instrument reading or a weather forecast, but a practical rating based on which objects are visible and how bright the skyglow is.

Choosing a site

In a city, the Moon, planets and brightest stars are usually visible, while faint stars and the Milky Way can be washed out by artificial light. The sky is often darker outside town, though a bright glow may remain along the horizon. Choose an open observing spot and, when possible, avoid looking toward a large city when observing a target.

The Bortle class does not account for tonight’s clouds or haze: even a dark site can give poor views in humid air. Compare a light-pollution map with the cloud forecast and assess the site in person. Beginners do not need to travel to a Class 1 location; a safe site and a darker horizon are a useful start.

Check clouds for your location on the NightSkyCast map