Cosmic Passport

Why cities have no stars

Twenty or thirty stars in a city centre, over two thousand in the countryside. The Bortle scale grades night skies from 1 to 9.

Light covers the sky

Streetlights and signs scatter off particles in the air, brightening the whole sky. Any star fainter than that background simply disappears.

The Bortle scale, 1 to 9

Class 1 is a perfect dark sky where the Milky Way casts shadows. A city centre is class 8 or 9, leaving twenty or thirty bright stars.

Two thousand in the country

Under a class 3 rural sky you can count over two thousand stars unaided, with the Milky Way clearly visible.

Twenty minutes to adapt

Step outside from a lit room and you see almost nothing. Full dark adaptation takes 20–30 minutes — and one glance at a phone screen resets it.

Check the sky grade where you live
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