About Light Pollution
The Bortle Scale
The Bortle Scale is a visual metric on how bright a night sky is, ranging from 1 to 9. It is determined by what objects can be seen with the naked eye. For example, in a Bortle 1 dark sky site, the Milky Way core casts an obvious shadow and the Triangulum Galaxy is easily seen with the naked eye. In a Bortle 9, almost no stars are visible.
The Sky Quality Meter (SQM)
The Sky Quality Meter is another, more scientific measurment of light pollution. In units of magnitude per arcsecond squared, it ranges from 16 to 22 (sometimes 17 to 23 as seen in image). In a SQM of 16, barely any stars can be seen. In a SQM of 22, it is perfectly dark.
Bortle Scale - SQM Equivalency
The Bortle Scale and SQM can be approximated to each other. However, consensus varries based on the source. This image, from Astrobackyard, gives a rough estimate of what each Bortle is using SQM measurments. Personally, I use the app "Good to Stargaze" for my measurements and the website "www.lightpollutionmap.info" for planning dark sky trips.
Importance of Reducing Light Pollution
Urbanization and dense populations have created a world where most people live under light polluted skies. 80% of the world and 99% of the US population live under these skies, and for some the closest dark sky site is hours away. For example, a New York City resident would need to drive upwards of 5 hours to reach a Bortle 2 or less sky. 33% of Americans go their whole life without ever seeing the Milky Way with their own eyes. To learn more about preventing and reducing light pollution, visit www.darksky.org.Â