Bortle scale

A man named John Bortle made a scale.

This scale measures how bright the night sky is. It has nine levels. Level one is the darkest sky. You can see many stars there.

Level nine is for big cities. City lights make the sky very bright. This light hides the stars from view. This scale helps people find dark places. It is a great tool for star watchers.

How bright is your night sky? A man named John Bortle created a way to measure this. He called it the Bortle scale. It has nine levels.

Level 1 is the darkest sky on Earth. In these places, the sky is full of stars. You can see bright parts of the Milky Way. You can even see the Triangulum Galaxy with just your eyes.

As the levels go up, the sky gets brighter. This happens because of light pollution. Light pollution is extra light from cities. It makes the sky glow.

In a suburban area, like Level 6, the sky is not as dark. You might only see the brightest parts of the Milky Way. In big cities, which are Level 8 or 9, the sky is very bright. It might look gray or orange. Many stars become invisible. You might only see the Moon and bright planets.

This scale helps star watchers find the best dark spots.

The Bortle scale is a special tool used to measure how bright the night sky is. It helps people understand how much light pollution affects the view of space. Light pollution is the extra light created by cities and roads. This extra light can make the sky glow and hide the stars. Astronomers use this scale to find the best places to look at the universe.

The scale works by using nine different levels. Level 1 is the darkest sky you can find on Earth. In these places, the sky is filled with many stars. You can see colorful light called zodiacal light and even see the Milky Way clearly. As the numbers go up, the sky gets brighter and harder to see through. At Level 9, which is an inner-city sky, the sky is brilliantly lit.

An amateur astronomer named John E. Bortle created this scale. He wanted to help people compare different places for stargazing. He published his ideas in a magazine called Sky & Telescope in February 2001. He made the scale because more cities were making dark skies harder to find. This is a big problem for people who love to study the stars.

There are many specific details for each level of the scale. In a Level 2 site, the Milky Way is highly structured. In a Level 6 suburban sky, the Milky Way is only visible near the top of the sky. At Level 8, the sky might look light gray or orange. You might even be able to read a book by the light of the sky. In Level 9, you can only see the Moon, planets, and a few bright star clusters.

This scale connects to how we use light in our daily lives. Many places are switching to LED lights to save energy. However, the high light temperature of some LEDs can increase light pollution. Groups like DarkSky International want us to use warm-colored lights instead. They believe we can have bright cities and dark skies at the same time. This helps everyone enjoy the beauty of the night.

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.