Understanding the Bortle Scale: How Dark Is Your Night Sky?

Light pollution is an interesting concept. It sounds like something dangerous and invasive, in the same way pollution can be used to describe waste in rivers or woodlands, or the gases emitted from motor vehicles. Whilst invasive in its own way, light pollution carries a different definition — the artificial light created by modern lighting and machinery. This light dissipates across the atmosphere and illuminates the night sky above our heads, and its character has shifted over time too, from orange sodium-based street lighting to modern white, LED-based alternatives.

Deep in the countryside, you’ll be able to spot the core of the Milky Way, including its arm structure and dust clouds. You’ll be able to track all of the constellations, see a rich and dense field of stars, and even capture fuzzy patches for nebulae and our neighbouring galaxies (e.g. Andromeda). However, in the city, surrounded by artificial lights, a lot of this detail is stripped away. Whilst you can still see stars, and track constellations, the richer colour and detail from the Milky Way and deep sky objects will blend into the diffuse artificial light.

The Bortle scale was created by amateur astronomer John E. Bortle in 2001 to help fellow astronomers and stargazers distinguish and compare the night sky at different locations. It’s a simple nine-point scale, varying from class 1 — the darkest skies on Earth, often deep in deserts — to class 9 — the inner-city skies. Each class is determined by the visibility of some of the more prominent celestial objects, most notably the Milky Way’s structure and colour.

The Nine Classes at a Glance

Roughly what each class actually looks like, in practice:

• Class 1 — Excellent dark-sky site. The Milky Way casts a visible shadow, zodiacal light is obvious, and the naked eye can pick out detail deep into the sky. Vanishingly rare — the deepest deserts, remote islands, a handful of protected reserves.
• Class 2 — Typical truly dark site. The Milky Way is still highly structured, with visible dust lanes and a mottled texture; airglow may be noticeable near the horizon.
• Class 3 — Rural sky. Some light pollution is evident low on the horizon, but the Milky Way still shows plenty of structure overhead.
• Class 4 — Rural/suburban transition. The Milky Way is visible but starting to lose detail, and light domes from nearby towns show on the horizon.
• Class 5 — Suburban sky. The Milky Way is only visible near the zenith, if at all, washed out lower down by skyglow.
• Class 6 — Bright suburban sky. The Milky Way is essentially invisible; only the brighter deep-sky objects and star clusters hold up.
• Class 7 — Suburban/urban transition. The whole sky takes on a grey-white glow; only the brightest stars cut through it.
• Class 8 — City sky. The sky glows orange or white; only the Moon, planets, and the very brightest stars are visible.
• Class 9 — Inner-city sky. The sky is lit up brilliantly; only the Moon, planets, and perhaps a handful of the brightest stars remain visible.

Seeing the Difference

None of these are my own shots — although I have every intention of visiting these places to capture my own at some point — but the following shots show the difference far better than words alone can.

Photo credits: Milky Way over Paranal — ESO/P. Horálek (CC BY 4.0); suburban sky — Twinsday, Wikimedia Commons (CC BY-SA 3.0); city sky — Derek Petersen, Wikimedia Commons (CC BY 3.0).