SKY · FIELD GUIDE
Light Pollution & the Bortle Scale — How Dark Is Your Sky?
Most people have never seen more than a few dozen stars — not because the sky is empty, but because the glow of artificial light hides almost all of it. Here's how sky darkness is measured, and how to get under a real dark sky.
You can do everything right — a clear, Moonless night, the galactic core riding high — and still see almost nothing, if you're standing under a bright sky. Light pollution is the quiet reason most people have never seen the Milky Way, and unlike cloud or the Moon, it's a property of where you are, not of the night.
What light pollution actually does
Artificial light from streetlights, buildings and signage scatters off molecules and particles in the air, brightening the whole sky. Faint things — and most of the night sky is faint — simply can't compete with that glow. The result is a sky that fades from thousands of stars under truly dark conditions to a few dozen in a city centre. The stars are still there; you just can't see them through the haze of our own lighting.
The Bortle scale: a darkness rating you can use
In 2001 the amateur astronomer John Bortle published a 9-point scale to describe sky darkness in plain, observational terms. It runs from:
- Class 1–2 — truly dark: the Milky Way is bright and structured, casts faint shadows at its best, and faint objects are easy. The darkest skies on Earth.
- Class 3–4 — rural: the Milky Way is clearly visible with some detail; light domes sit on the horizon. Excellent for most stargazing.
- Class 5–6 — suburban: the Milky Way is washed out or only visible overhead. Brighter objects still show.
- Class 7–9 — urban: a greyish or brightly-lit sky where the Milky Way is gone and only the brightest stars, the planets and the Moon survive.
The centre of almost every major city lands at class 8 or 9. That's why the city pages here give an honest Bortle estimate for each location — and, more usefully, point you toward darker skies.
Finding a real dark sky
The encouraging part: the improvement comes fast. Because a city's light dome fades with distance, even an hour or two out can take you several Bortle classes darker. The most reliable destinations are Dark Sky Parks and Reserves — areas recognised (usually by DarkSky International) for exceptional darkness and responsible lighting. They're scattered across every continent, and there's very often one within a half-day's drive of a major city.
Our per-city pages list real, named dark-sky destinations within reach of each location — national parks, certified reserves and well-known dark regions — so "where do I actually go?" has a concrete answer rather than a vague "somewhere rural."
A note on our data
Sky-brightness has been mapped globally from satellite measurements (the work behind the World Atlas of Night Sky Brightness), and a high-resolution version of that map is the natural next step for this layer. For now, the per-city Bortle figures here are honest estimates — the kind an experienced observer would assign to a major metro — not measured readings, and they're labelled as such. The dark-sky destinations, by contrast, are real, named places you can look up and visit.
Frequently asked questions
What is light pollution?
It's the brightening of the night sky by artificial light — streetlights, buildings, signage and floodlights scattering off the air. It washes out faint stars and the Milky Way, so from a bright city you might see only a few dozen of the brightest stars, while from a truly dark site you'd see thousands plus the glowing band of the galaxy. It's also one of the few forms of pollution you can largely escape just by driving away from it.
What is the Bortle scale?
A 9-point scale, introduced by John Bortle in 2001, that rates how dark a night sky is — from class 1 (a pristine wilderness sky where the Milky Way casts shadows) to class 9 (a brightly-lit inner city where only the Moon, planets and a handful of stars show). It's a practical, naked-eye way to describe what you can actually expect to see at a given location.
What Bortle class is a typical city?
The centre of almost any large city sits at Bortle 8 or 9 — the Milky Way is completely invisible and only the brightest stars cut through the glow. Suburbs are usually class 6–7, smaller towns 4–5, and you generally need to reach rural areas well away from population for class 3 or darker, where the Milky Way becomes genuinely impressive.
What is a Dark Sky Park or Reserve?
These are places formally recognised — most often by the International Dark-Sky Association (IDA, now DarkSky International) — for exceptionally dark skies and good lighting practices. A Dark Sky Park protects a specific area like a national park; a Dark Sky Reserve covers a larger region with a dark core and a supportive surrounding community. They're reliable destinations when you want a guaranteed dark sky.
How far do I have to travel to escape light pollution?
It depends on how big the city is, but the improvement is fast at first: even an hour or two out can drop you several Bortle classes. Big metros throw a light dome that can reach 50–100 km, so the darkest skies often mean a longer trip or a known dark-sky site. Our city pages list real dark-sky destinations within range of each location.
Can I see the Milky Way from a suburb?
Rarely, and never well. The bright core might be faintly detectable overhead from a darker suburb (Bortle 5–6) on a Moonless night, but to actually see the Milky Way as the structured river of light it should be, you need Bortle 4 or darker — which almost always means leaving the suburbs behind.
SEE IT LIVE
Everything in this guide is on the live sky map.