What is a Sky Quality Meter?
A Sky Quality Meter (SQM) is an electronic instrument designed to measure the brightness of the night sky. It reports the result in magnitudes per square arcsecond, commonly written as mag/arcsec².
In simple terms, an SQM gives us a numerical indication of how bright or dark the sky appears to the instrument. A higher SQM reading means a darker sky, while a lower reading means a brighter sky.
This makes SQM measurements particularly useful when comparing different locations. Artificial light is an important contributor to sky brightness, but readings can also be influenced by natural conditions such as airglow, moonlight and the atmosphere.
How SQM readings work
SQM readings can seem confusing at first because higher numbers represent darker skies. This is because the measurement uses the astronomical magnitude scale, where a brighter sky produces a lower numerical value.
As a general rule, a reading around 18 or 19 indicates a relatively bright night sky, while readings above 21 indicate much darker conditions. Readings around 21.5 and above can indicate exceptionally dark skies, although the exact result depends on the instrument, conditions, direction and time of measurement.
Brighter skies
Below 20
Dark rural skies
21+
Very dark
21.5+
These figures are best treated as practical reference points rather than strict boundaries. SQM readings can change from night to night, so one measurement should not be treated as a permanent score for an entire location.
SQM vs the Bortle Scale
The SQM and Bortle Scale are both useful ways of describing night sky conditions, but they are not the same measurement.
The Bortle Scale is a nine-class visual classification based on what an observer can see in the night sky, including the Milky Way, zodiacal light and the visibility of fainter stars. SQM provides a numerical measurement of sky brightness in magnitudes per square arcsecond.
Because they measure darkness in different ways, an SQM value should not be treated as an exact Bortle class. At Starnook, we use numerical measurements alongside the wider characteristics of a location to build a more useful picture of the stargazing experience.
Why SQM matters for stargazing stays
Dark skies reveal far more than the brightest stars. As sky brightness decreases, the Milky Way becomes more prominent and increasingly faint stars and deep sky objects become visible.
For a stargazing stay, this can make a significant difference. A darker sky can turn an ordinary clear night into an opportunity to see the Milky Way, meteor showers, star clusters and other objects that are difficult or impossible to see from brighter locations.
SQM is therefore useful when assessing the darkness of a location, but it is only part of the picture. Horizon views, nearby lights, shelter, access to the outdoors and the position of the accommodation can all affect the experience.
How Starnook uses SQM data
We use recognised dark sky measurements as one part of how we assess stargazing locations. Where reliable SQM information is available, it helps us understand and compare the underlying darkness of the sky.
We also look beyond the number. Nearby lighting, horizon visibility, privacy, surrounding landscape and how easily guests can step outside and experience the sky all contribute to what makes a stay genuinely good for stargazing.
That matters because the darkest number does not automatically mean the best place to spend a night beneath the stars. The aim is to find stays where genuinely dark skies and a great guest experience come together.