How dark is your sky?

Discover the Bortle scale, a simple way to judge night-sky quality wherever you are

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Your new telescope arrives and you plan to take it for a spin through the stars. But a smog of light sits low in the sky, washing away your views to distant galaxies - along with your astronomy dreams. Quickly you realise you need a quality dark-sky site. But how dark is dark? And how do we compare one location's sky to another? Enter the astronomer's friend: the Bortle scale.

Amateur astronomer John E Bortle set out to create a sky-brightness scale that would be easily accessible to stargazers and scientists alike. Launched in 2001, it was quickly adopted by anyone curious about the darkness of their skies, and how theirs stacked up against others.

Light pollution, even in small amounts, impacts our view of the cosmos. Sadly, 99 per cent of Americans and Europeans live under light-polluted skies. This means that the majority of us have to travel if we want to experience a truly dark sky that offers unobscured views of distant stars, galaxies and other phenomena that are otherwise lost to light.

The Bortle scale runs from an increasingly rare Class 1: excellent dark-sky site, to the smothered skies of Class 9: inner-city sky. Here's a breakdown of what each level means, so you know what to expect from any rated stargazing site.

Class 1: Excellent dark-sky site