The Bortle Scale-Matt’s Space Pics

The Bortle Scale

The Bortle Scale: Understanding Light Pollution

The Bortle Scale, developed by astronomer John Bortle in 2001, is a way for astronomers and amateur stargazers to measure the amount of light pollution in the night sky. The scale ranges from 1 to 9, with 1 being the darkest sky and 9 being the most light-polluted. Understanding the Bortle Scale is an important step towards preserving the night sky and protecting our environment.

Light pollution is the term used to describe the excessive artificial light that is emitted into the night sky. This pollution can have negative effects on both humans and wildlife, as well as the environment as a whole. With the increasing use of artificial light, the sky is becoming brighter and darker skies are becoming rarer. Thus, it is important to understand the impact of light pollution and take steps to reduce it.


The Bortle Scale is divided into nine levels. At the lowest end of the scale, a Bortle Class 1 sky is the darkest possible sky. This sky is not found in many places anymore due to light pollution. A Class 2 sky is also very dark, but with some mild light pollution from nearby towns or cities. Class 3 skies are typical of suburban areas, with moderate light pollution. Class 4 skies are city skies, with high light pollution. Class 5 skies are suburban/urban transition skies, and are typically seen near large cities. Class 6 skies are typical of small towns, with some light pollution from streetlights and houses. Class 7 skies are typical of rural areas with few streetlights or buildings, but still some light pollution from nearby cities. Class 8 skies are very remote rural areas, with very little artificial light and few human inhabitants. Finally, Class 9 skies are found in the heart of large deserts or oceans, with no artificial light visible.

The Bortle Scale serves not only as a way to measure and compare the levels of light pollution, but also as a reminder of the beauty of dark skies. Observing the night sky with no artificial light interference allows people to see the Milky Way, constellations, and even fainter celestial objects such as nebulas and galaxies. These objects are not visible in a light-polluted sky, and thus people are missing out on the wonders of the universe.

Moreover, light pollution has serious environmental implications, as it can disrupt the natural behavior of wildlife, affect biodiversity and disturb ecosystems. Many species of animals, such as birds and sea turtles, use the darkness of the night sky to navigate, hunt, or rest. Light pollution can disorient these animals and impair their ability to survive, which can have major consequences on the entire ecosystem.

Furthermore, excessive artificial light also has negative effects on human health. Ongoing exposure to artificial light at night can disrupt circadian rhythm and suppress the production of melatonin, which can lead to sleep deprivation and various health problems such as obesity, depression, and diabetes. Therefore, preserving the night sky and reducing light pollution is crucial not only for the environment, but also for human well-being.

In conclusion, the Bortle Scale is an important tool for measuring and comparing the levels of light pollution in the night sky. By understanding the impact of light pollution on the environment, wildlife, and human health, we can work to reduce it. Additionally, by preserving the darkness of the night sky, we can appreciate the beauty and wonder of the universe that surrounds us. As John Bortle himself wrote, “Starlight is one of the purest and most fundamental gifts that nature can bestow, and it is a gift that belongs to all of us.”

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