The Bortle Scale

The Bortle scale is a nine-level numeric scale that measures the night sky’s brightness of a particular location. It quantifies the astronomical observability of celestial objects and the interference caused by light pollution. John E. Bortle created the scale and published it in the February 2001 edition of Sky & Telescope magazine to help amateur astronomers evaluate the darkness of an observing site, and secondarily, to compare the darkness of observing sites. The scale ranges from Class 1, the darkest skies available on Earth, through Class 9, inner-city skies.

(Source: Wikipedia)


40 Percent of Americans Never Known True Dark

Those who us who turn to the stars as a hobby have a scale to rank the darkness of the night, the Bortle Scale where 9 is the brightest skies and 1 is the darkest. In America the majority of people spend their lives in areas of 5 through 8 and rarely see anything different. This huge light pollution is impacting us in ways we don’t yet fully understand; lately studies have been linking light-overuse and such things as sleep disorders diabetes, obesity and cancer. It is important to remember that we’re not the only important organisms on the world and that this light is having serious consequences on the circadian cycles of many animals, and effect which is having a knock-on effect through ecosystems worldwide. However, we must not think of the purely measurable effects but also about what is being lost to us as a species in less tangible ways. The dark of night gave so much to our ancestors, thousands upon thousands of stars inspiring their imagination and influencing their art, their mythologies, their religious beliefs and their place in the universe. But more than that it offered an often intangible solitude and a perfect quiet. It is such a shame that so many people know a sky with no more than 10 stars when in reality it is so much more. I hope that we can start to reverse this process, this wasteful use of light to bring back the beauty of the night.

What can you do about it?
Around the world are a series of Dark Sky Preserves, Reserves and Parks. These areas all have their own programs to reduce light pollution, be it encouraging cities to use more efficient lights, to reduce traffic late at night, to encourage citizens to pull their curtains at night, ever little bit helps. You can get involved by hitting up Google to find your local dark sky reserve or petition to start a dark sky reserve and get involved by spreading awareness and doing your bit! Together we can bring the beauty of the night sky back to the citizens of America and the world. The night sky inspires us in so many ways and I hope you get the chance to see the sky in a beautiful 1 or 2 ranked sky!

There’s a statistic that I quote, which is that eight of every ten kids born in the United States today will never experience a sky dark enough to see the Milky Way. The Milky Way becomes visible at 3 or 4 on the Bortle scale. That’s not even down to a 1. One is pretty stringent. I’ve been in some really dark places that might not have qualified as a 1, just because there was a glow of a city way off in the distance, on the horizon. You can’t have any signs of artificial light to qualify as a Bortle Class 1.

A Bortle Class 1 is so dark that it’s bright. That’s the great thing-the darker it gets, if it’s clear, the brighter the night is. That’s something we never see either, because it’s so artificially bright in all the places we live. We never see the natural light of the night sky.


The Bortle scale was originally published in Sky & Telescope magazine in 2001. It classifies the darkness of skies from point of view of an astronomer, ranging from 1 (“an observer’s Nirvana!”) to 9, in which “the only celestial objects that really provide pleasing telescopic views are the Moon, the planets, and a few of the brightest star clusters.” This illustration of the scale comes via Stellarium.

Light Pollution Simulation / Bortle Scale

Medical research on the effects of excessive light on the human body suggests that a variety of adverse health effects may be caused by light pollution or excessive light exposure, and some lighting design textbooks use human health as an explicit criterion for proper interior lighting. Health effects of over-illumination or improper spectral composition of light may include: increased headache incidence, worker fatigue, medically defined stress, decrease in sexual function and increase in anxiety.Light pollution poses a serious threat in particular to nocturnal wildlife, having negative impacts on plant and animal physiology. It can confuse animal navigation, alter competitive interactions, change predator-prey relations, and cause physiological harm. The rhythm of life is orchestrated by the natural diurnal patterns of light and dark, so disruption to these patterns impacts the ecological dynamics.Astronomy, both amateur and professional, is very sensitive to light pollution. The night sky viewed from a city bears no resemblance to what can be seen from dark skies. Skyglow (the scattering of light in the atmosphere) reduces the contrast between stars and galaxies and the sky itself, making it much harder to see fainter objects. (wiki)