The sky-quality scale
Sky darkness is measured in magnitudes per square arcsecond (mag/arcsec²) — the higher the number, the darker the sky. It maps onto the Bortle scale, from 1 (pristine) to 9 (inner city). The Kerry reserve's core sits around 21.9, a Gold-Tier dark sky.
The Milky Way casts shadows; zodiacal light and airglow are obvious. Among the darkest skies on Earth.
The Milky Way is richly structured; M33 visible to the naked eye. True dark-sky conditions.
The Milky Way is clearly visible with detail; only a faint glow near the horizon.
The Milky Way is visible overhead; some light domes near towns.
Only the brightest part of the Milky Way shows; suburban glow is noticeable.
The Milky Way is washed out; the sky glows grey. Bright suburban conditions.
No Milky Way; a bright, orange city sky. Only the Moon, planets and brightest stars show.
Why it matters
Dark skies are a rare and fragile natural resource. They matter for astronomy, for nocturnal wildlife and human health, and increasingly for tourism — the Kerry reserve draws visitors specifically to see stars they can't see at home. Continuous measurement lets the reserve prove its darkness for annual certification, spot light-pollution creep early, and guide visitors to the best skies on any given night.