The Sky Quality Meter is sensitive to visual light and measures the brightness of the night sky in magnitudes per square arcsecond.
The higher the number, the darker the sky.
A Sky Quality Meter (SQM) reading of 20 or 21 indicates a dark sky where stargazing is at its best and the stars of the Milky Way are clearly visible. A reading of 16 or 17 indicates a light sky that is impacted by either artificial light or bright Moon light. The image below from Unihedron gives a rough indication of what the readings indicate.
The readings give you a good indication of just how good (or dark) the night sky actually is. It makes it easy to compare different locations and to see light pollution is making an adverse impact on the darkness of the night sky. Unihedron has further information on their website.
Magnitude per square arcsecond
Magnitudes per square arcsecond is a logarithmic measurement which means that a large change in sky brightness are equal to a relatively small numerical change.
There’s a lot more information about the Sky Quality Meter on the Unihedron website.
Recording Sky Quality Meter Readings
You may even like to purchase a device yourself and start taking sky quality meter readings of your own.
When you’ve taken an accurate reading, you can become a “Citizen Scientist” and submit your data to a world-wide database of night sky observations. It’s as easy as downloading an app! Readings can be submitted to:
Thinking about buying a Sky Quality Meter?
There are two models to choose from: SQM or SQM-L. Unihedron has some helpful information about which one might suit you. Make sure you read the Frequently Asked Questions. May your night skies be dark!
The Bortle Scale
If you’re keen to read more about another scale that measures light pollution, check out this great article by Riley Johnston at Stargazers Club WA – How Dark is Your Night Sky? It explains what the Bortle Scale is and gives you examples of places with different measurements.