Light Pollution Map UK: The Layer Most People Read Wrong

Open any light pollution map UK observers recommend and you will be looking at a wash of red, orange, yellow and green over a familiar coastline. The instinct is to find the nearest green patch and drive to it. That instinct is half right, and the half that is wrong sends people to sites that look dark on screen and disappoint under the sky. The problem is that the two main maps measure completely different things, and the one most people land on by default is measuring the wrong end of the light.

This page explains what each map actually shows, which layer to trust when you are planning a night out, and how to turn a colour on a screen into a realistic expectation of what you will see.

The two maps you will actually use

CPRE’s Night Blight map covers England and is built from satellite data broken into nine brightness bands. Its strength is administrative detail: you can search by postcode and pull statistics for your county, district, National Park or National Landscape, including what percentage of that area sits in each band and how it ranks nationally. If you want to know whether your own patch of England is getting brighter or darker, or you are arguing with a council about a planning application, this is the map.

lightpollutionmap.info covers the whole world and carries several data layers. It is the one astronomers use for planning, and it is the one where the layer choice matters enormously.

The British Astronomical Association also lists several older UK-specific maps, including the Avex map and a Philip’s folding paper map, which are still useful for a broad regional sense of where the dark ground is.

The layer trap: VIIRS shows the ground, not the sky

This is the single most useful thing to understand, and most pages skip it.

The VIIRS layer is satellite radiance data. It shows how bright the ground looks from space, which is to say where the lamps are. It is excellent at showing you a town, a motorway junction, a lit industrial estate or a greenhouse complex. It is not a measurement of how bright the sky above that point will look to you.

The World Atlas 2015 layer models the opposite direction: how much artificial light is scattered into the sky above a given point, seen by an observer on the ground looking up. That is the same quantity a Sky Quality Meter measures, and it is the number that determines whether you will see the Milky Way.

The two diverge in ways that matter. A spot in the lee of a hill can look mildly bright on VIIRS because there are lamps a mile away, yet have a genuinely dark sky overhead. A valley floor with almost no local lighting can sit under a bright dome pushed sideways from a city twenty miles off. If you want Bortle and SQM figures for a site, switch the map to World Atlas 2015 first. Reading Bortle off a VIIRS layer is a category error, and it is the reason people report “green zone” sites that felt like suburbs.

Turning a colour into a number, and a number into an expectation

The map gives you sky brightness in magnitudes per square arcsecond. Higher is darker, and the scale is compressed: 21.8 is properly dark, 20.5 is a decent rural site, 18.5 is a suburb. The Bortle scale bins that continuous number into nine classes, from Class 1 (excellent dark site) to Class 9 (inner city).

Roughly what each band buys you in Britain:

• Bortle 1 to 2 barely exists here outside the far north and west of Scotland, mid Wales and a few upland pockets. The Milky Way is structured and casts a faint shadow.
• Bortle 3 is the realistic target for a good UK dark-sky trip. The Milky Way shows clear detail overhead and the Andromeda Galaxy is easy naked eye.
• Bortle 4 covers a lot of usable rural England. Milky Way visible but washed at the horizon.
• Bortle 5 is the classic village or edge-of-town sky. Milky Way faint or absent, but planets, double stars and brighter clusters are unaffected.
• Bortle 6 to 9 is suburban to urban. Plenty still works here, which is the point of our guide to stargazing near London.

Our Bortle scale calculator converts between the SQM figure the map gives you and what you can expect to see, and the limiting magnitude calculator does the same job for faintest visible star.

What the map cannot tell you

Three things will ruin a night in a Bortle 3 site, and none of them appear on any light pollution map.

The Moon. A full Moon lifts a Bortle 3 sky to something like Bortle 6 for deep sky purposes. Driving two hours to a dark site three nights before full is wasted effort. Check the phase before you check the map.

Transparency and haze. Humid or dusty air scatters town light far more efficiently. The same site can read Bortle 4 on a crisp night after a cold front and Bortle 6 in a summer murk, with no change to the lamps.

Horizon and elevation. A map point is a single coordinate. What you need is a site where the direction you want to observe is shielded. A ridge between you and the nearest town is worth more than a nominally darker spot with a clear line of sight to it.

The practical order is: pick the darkest reasonable site once, then choose the night by Moon phase and forecast. Site selection is a one-off job; night selection is the one you repeat.

A method for finding your own dark site

1. Open lightpollutionmap.info and switch the layer to World Atlas 2015.
2. Draw a rough circle at whatever driving time you will genuinely accept. Ninety minutes is the honest limit for most people on a work night.
3. Find the darkest band inside that circle, not the darkest band in the country.
4. Cross-check the same spot on VIIRS. You are looking for local lamps the sky model smooths over: a floodlit yard, a service station, a security light.
5. Check access on a normal map. Somewhere you can park legally, safely and without a gate that locks at dusk. Laybys on fast A-roads are a poor idea.
6. Note which horizon is compromised, and plan your targets to sit on the clean side.
7. Go once on a clear night near new Moon and record what you can actually see. That is your baseline.

If you would rather start from a known good site, our roundups of the best stargazing spots in England, Scotland and Wales cover places with parking and public access, and the UK dark sky sites guide covers the formally accredited reserves and parks.

How bad is it, actually

CPRE’s Star Count asks the public to count the stars visible inside the rectangle of Orion, which is a crude but consistent naked-eye measure. Fewer than ten means severe light pollution; more than thirty means a truly dark sky.

In the 2023 count, the most recent set of national figures published, 51% of the 3,937 participants counted ten or fewer stars, and just 5% counted more than thirty. That is the practical situation for British stargazers: about half of us cannot see a meaningful night sky from home, and one in twenty has a genuinely dark one. It is also why the drive is usually worth it, and why learning what is visible with the naked eye from a mediocre sky is not a consolation prize.

Frequently asked questions

Which light pollution map is best for the UK? Use lightpollutionmap.info with the World Atlas 2015 layer for planning observing sessions, because it models sky brightness overhead and gives usable Bortle and SQM values. Use CPRE’s Night Blight map when you want England-specific detail by postcode, district or National Park, or evidence for a planning objection.

Why does the light pollution map disagree with what I see? Almost always because you are reading the VIIRS layer, which shows ground brightness from space rather than sky brightness from the ground. Terrain also matters: a hill between you and a town blocks light the model spreads evenly. Moon phase and haze then move the real sky by a Bortle class or two on the night.

What Bortle class can I realistically reach in England? Bortle 3 is achievable in parts of Northumberland, the North Pennines, Exmoor, Dartmoor, the Brecon fringe and mid Wales. Most rural England is Bortle 4 to 5. Bortle 1 to 2 effectively requires the Scottish Highlands and Islands.

Does a dark sky matter for planets? Very little. The Moon, Jupiter, Saturn and Mars are bright enough to punch through suburban skies, and their views depend on atmospheric steadiness rather than darkness. Dark skies matter for galaxies, nebulae and the Milky Way, which is why a city observer is better served by planetary targets.

Are British skies getting brighter or darker? Both, in different places. LED conversion has cut upward spill in some towns and raised blue-rich glare in others, and CPRE’s counts have moved up and down year to year with energy prices and lighting policy. The Night Blight map is the tool for checking your own area rather than relying on a national trend.

How dark does it need to be to see the Milky Way? Around Bortle 4 or darker, meaning roughly 21.0 mag/arcsec² or better, with the Moon down and clear air. At Bortle 5 you may catch the brightest section near Cygnus overhead on a good night; below that it is generally gone.

Sources

• CPRE: England’s light pollution and dark skies map (Night Blight)
• CPRE: Star Count 2023 results
• British Astronomical Association: Light pollution maps
• Light pollution map (lightpollutionmap.info)