Bortle scale

The Bortle scale is a numeric system used to measure night sky brightness. It helps astronomers understand how much light pollution interferes with observing the universe. Light pollution occurs when artificial light from cities and roads scatters into the atmosphere. This creates a glow that makes it harder to see faint celestial objects. The scale consists of nine distinct levels. These levels range from the darkest possible skies to the brightest inner-city environments.

To use the scale, observers look at specific features in the sky. They notice how many stars are visible to the naked eye. They also check for diffuse light sources like the Milky Way. The scale relates closely to technical measurements. These include naked-eye limiting magnitude (NELM), which is the brightness of the faintest star a person can see. It also relates to sky quality meter (SQM) readings, which measure skyglow. By looking at these details, people can assign a specific class to a location.

The scale is divided into nine classes. Class 1 represents an excellent dark-sky site. In these rare places, the zodiacal light is colorful and visible. Even the Triangulum Galaxy, known as M33, can be seen with the naked eye. Class 2 is a typical truly dark site where the Milky Way shows high structure. As you move toward Class 4, which is a brighter rural sky, light pollution domes appear on the horizon. These domes are glowing areas caused by distant cities.

Middle levels of the scale represent transition zones. Class 4.5 is a semi-suburban or transition sky. In this zone, clouds may have a grayish glow. The Milky Way is only vaguely visible near the horizon. Class 5 is a standard suburban sky. Here, light pollution is visible in almost all directions. The Milky Way looks washed out when viewed overhead. Class 6 is a bright suburban sky. In these areas, the Milky Way is only visible near the zenith, which is the point directly above an observer.

The highest levels of the scale describe very bright environments. Class 7 is a suburban or urban transition sky. The entire sky appears light gray due to light pollution. The Milky Way might look like a faint, featureless cloud. Class 8 is a typical city sky. The sky often looks light gray or orange. In these locations, an observer might even be able to read a book by the skyglow. Finally, Class 9 is an inner-city sky. The sky is brilliantly lit, and many constellations become invisible.

Amateur astronomer John E. Bortle created this scale. He published it in the February 2001 edition of Sky & Telescope magazine. He wanted to help skywatchers evaluate and compare observing sites. He was motivated by the fact that increasing light pollution was making dark skies harder to find. Urbanization and the global shift to LED lighting have continued to affect sky darkness. Some research suggests Bortle may have overestimated how easily people see very dim objects. For example, M33 is described as a naked-eye object in Class 1, which some find atypical.

Understanding the Bortle scale is important for modern science and conservation. The transition to LED lighting has changed how we view the night. Many LEDs have a high light temperature, which can increase light pollution. This makes it even more important to use responsible outdoor lighting. Groups like DarkSky International advocate for warm-colored, targeted lights. They believe we can have efficient cities without losing our view of the stars. This helps protect the natural nightscape for everyone.