What is light pollution?
Simply put, light pollution is the presence of artificial light in the night environment. Light pollution has an enormous impact on the night sky and consequently, the field of astronomy. It is not uncommon to hear amateur astronomers and astrophotographers talk about heading out to dark sky sites for their observing or imaging sessions.
Moving away from sources of light pollution is the easiest and most accessible way to bypass light pollution, but how do you know where to go? Light pollution maps, like this one, are great tools for finding dark skies and for determining just how polluted your backyard is. Most light pollution maps will measure how light polluted the skies are by recording “magnitudes per square arcsecond”, essentially, how bright is a square arcsecond of sky. These values can be hard to interpret, so most astronomers use a simple scale known as the Bortle Scale.
What is the Bortle Scale?
The Bortle scale is a multi-level scale that was devised by John E. Bortle to measure how bright the night sky is in a location. The Bortle Scale was designed to be used by anyone, regardless of equipment, and uses visible objects as a yardstick for brightness. The scale ranges from Class 1 skies, the darkest skies possible on the Earth, through to Class 9 skies, the skies of major population centres.
Let’s take a journey through Perth and out into the dark to get an idea about the impact light pollution has on our skies. Each Bortle Class is described and a Sky Quality Meter (SQM) reading in magnitudes per arcsecond squared is included below:
Bortle 8/9 (SQM reading: <18.38)
For the worst night sky in WA, you have to go no further than Perth city centre. With the naked eye, most stars and even constellations will be invisible. Only the brightest objects in the sky, like planets, the Moon and some bright stars, will be visible.
Bortle 7 (SQM reading: 18.38-18.94)
Just a few kilometres away from the city centre, the Bortle Scale is 7. The light pollution in these areas makes the entire sky appear light grey in colour, and the Milky Way is effectively invisible. Clouds are lit up brightly by terrestrial light sources can be viewed in all directions.
Bortle 6 (SQM reading: 18.94-19.50)
By the time some of the outer suburbs are reached, like Ballajura, Bibra Lake, Cannington and Joondalup are reached, the night sky becomes Bortle 6. Bortle 6 skies are categorised by grey/white skies below 35° above the horizon, and faintly lit clouds in the sky. The Milky Way can be faintly defined in the sky, along with the Magellanic Clouds.
Bortle 5 (SQM reading: 19.50-20.49)
For skies of Bortle Class 5, a trip to Whiteman Park or the Darling Scarp is necessary. These skies are dark enough to view the Milky Way overhead and the Magellanic Clouds. Light pollution will be visible in most, if not all directions, depending on where you are viewing from.
Bortle 4 (SQM reading: 20.49-21.69)
The Perth Observatory is situated under skies of Bortle Class 4. Light pollution can be seen as domes in several directions, Clouds can be seen illuminated in the direction of light sources, but overhead clouds remain dark. The zodiacal light (a triangular white glow visible in the night sky, caused from sunlight scattered by interplanetary dust) is pronounced and identifiable and the Milky Way is clearly visible above the horizon, as well as the Emu in the Sky.
Bortle 3 (SQM reading: 21.69-21.89)
Bortle Class 3 skies can be found as close to Perth as an hour’s drive north and east, or a few hour’s drive south. In Bortle Class 3 skies, there is a small amount of light pollution visible as domes on the horizon. The zodiacal light is visible up to about half-way to zenith (directly overhead). Defined structures and dark veils of gas and dust can be observed directly in the Milky Way.
Bortle 2 (SQM reading: 21.89-21.99)
Bortle 2 skies are approaching the darkest possible skies. The zodiacal light is bright enough to cast shadows at dusk and dawn, and appears yellowish in colour. The Milky Way can be seen in great detail with the naked eye as well as many deep sky objects too! The Magellanic Clouds are pronounced and easily identifiable. Most locations on WA’s Astrotourism Towns Map are in Bortle Class 2.
Bortle 1 (SQM reading:: 21.99-22)
Bortle 1 skies are as dark as they come. On a moonless night, shadows are cast by the Scorpius and Sagittarius regions of the Milky Way, zodiacal light is completely visible, and most constellations become unrecognisable due to the amount of stars. The sky appears black to the unaided eye.
How to measure your backyard
An easy and accessible way to measure light pollution levels of your area is to use an app like “Dark Sky Meter” or “Loss of Night”. The Dark Sky Meter App uses your device’s camera to record the brightness and return a value in magnitudes per arc second squared. These values can easily be converted to Bortle Scales using the list above.
A more accurate way to measure light pollution in your local area, is to use a Sky Quality Meter. Sky Quality Meters are designed to return accurate readings of the night sky in magnitudes per arc second squared. These values can then be submitted to the manufacturer’s database, or uploaded to the Globe at Night citizen science program.
Riley is an experienced astronomy guide who has been working within the astronomy community for several years. He is incredibly passionate about the night sky and holds a Bachelor’s Degree in Science, Math and Education.
Riley Johnston
Author
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