The Bortle scale: how dark is your sky, really

«You can see plenty of stars from our place.» Anyone with a house outside town says it, and almost everyone overrates their sky. To settle the matter, in 2001 John Bortle, a comet observer with decades of nights behind him, published in Sky & Telescope a nine-class scale: from class 1, a remote site under a sky most people have never seen in their lives, to class 9, the heart of a city. The idea was practical: give observers a common language for comparing sites. Twenty-five years on it is still the standard amateurs use to describe their skies, and for phone photographers it is the first number worth knowing: the class you live in decides what makes sense from the balcony and what deserves a road trip.

Nine classes, three worlds

The scale reads best in blocks. Classes 1 to 3 are true darkness: deserts, high mountains, deep countryside. Under a class 1 sky the zodiacal light cuts across the heavens after sunset and the summer Milky Way is bright enough to cast shadows on the ground; the naked eye reaches stars beyond magnitude 7.5. In class 3 a few light domes sit low on the horizon, but overhead the Milky Way keeps its structure, its star clouds and dark lanes of dust.

Classes 4 to 6 are the suburbs and small-town country. The zenith holds up, the horizon does not. Class 4 is countryside bordering on villages, with the Milky Way still respectable high in the sky. Class 5 is the typical sky of anyone living half an hour from a mid-sized city: the Milky Way can be made out at the zenith on the best nights and fades on the way down. Class 6 wipes it out almost entirely.

Classes 7 to 9 are the city. In class 7 the sky background is greyish in every direction. Class 8 is the city proper, class 9 its core, where the Moon, the planets and a few dozen stars survive: from magnitude 7.5 at a remote site you drop to barely 4. It is the sky most readers live under, and that is no accident: according to the world atlas of light pollution, about a third of humanity cannot see the Milky Way from home. Among Europeans, six out of ten.

Reading your class with the naked eye

You need a clear night, no Moon, and three checks. No instruments.

The first is the Milky Way. Distinct from horizon to horizon, with detail: class 3 or better. Visible but washed out, only high in the sky: somewhere around 4 or 5. An uncertain paleness at the zenith puts you in class 6. No trace at all, even on a clear August night: 7 or worse.

The second is M31, the Andromeda galaxy, two and a half million light-years out. If you find it with the naked eye without effort, an elongated smudge not far from the Great Square of Pegasus, your sky is class 5 or darker. If it only shows fleetingly, when you look slightly to one side, you are in 6 or 7. From a city centre, no chance.

The third check is the horizon. Count the amber light domes over towns and cities: how high they climb tells you how close and how big they are. And there is the cloud test, which never misses: under a dark sky clouds drift past as black silhouettes, over a city they glow, lit from below.

What changes with a phone, band by band

Live stacking (it is what AstroStackerPro does for a living) adds up dozens of short exposures, and every class of darkness gained changes what emerges from the sum.

Class 8 does not end the game. The Moon could not care less about light pollution, and shooting it with a phone from a city balcony pays off immediately. So do the planets. Bright clusters like the Pleiades or the Beehive come out after a few minutes of stacking, and in winter the core of the Orion nebula punches through even an urban sky background. The great absentees are faint nebulae and wide fields: the orange glow drowns them.

In class 5 the field opens up. The Orion nebula shows colour and extent, the clusters number in the dozens, longer integration starts to pay. On dry nights the Milky Way at the zenith leaves a trace in the stacked frames before the eye catches it: the sensor, on this point, is more patient than you are.

In class 3 it becomes a different sport. The Milky Way runs across the screen with its star clouds, wide fields become the main course and the limit goes back to being technique: we cover the settings in our guide to the Milky Way with a smartphone. If you have to choose between yet another accessory and a tank of fuel towards a class 3 sky, choose the fuel. Two classes gained are worth more than any lens.

Where the dark lives

The first tool is a light pollution map built on satellite data: several exist online, free, and they show at a glance the islands of darkness within an hour or two of driving. Then come the places that protect the night by statute: national parks, certified dark-sky reserves, high plateaus far from lit coastlines. Mountains help twice over, because they take you away from the lights and lift you above the haze that spreads them.

There is also a trick that costs no extra miles: direction. From the same spot, the sky on the far side from a city can be two classes better than the sky right above it. Pick the site to suit the target: if the core of the summer Milky Way sits in the south, it is the southern horizon that must be clean, and the city belongs at your back. Last variable, the Moon: when full it eats darkness better than any streetlight, and turns a class 3 into a mediocre small-town night. Check the lunar calendar before you fill the tank.

Darkness also has to be waited for

Once on site, resist the urge to shoot straight away. The eye takes twenty to thirty minutes to adapt: the pupil snaps open quickly, but the heavy lifting is done by the chemistry of the retina, unhurried. It is the small private show of every trip out. At first the sky looks bare, then the stars float up in groups, as if from a seabed, until the eye picks out the dust of faint stars between the constellations and the dark stops feeling empty. One glance at a screen on full brightness wipes it all and restarts the clock. So keep the screen at minimum, and use a red light for fiddling with tripod and bag: the rods of the retina tolerate it far better than white light.

Fair warning

Stacking in the city works, and the list of urban prey (Moon, planets, clusters, the core of the odd bright nebula) fills plenty of evenings. But the Milky Way is no urban prey: it wants a class 4 as the bare minimum, better a 3, and no algorithm will pull it out clean from an amber sky background. Knowing your Bortle class is good for exactly this, telling when the problem is technique and when, simply, it is the place.