The Milky Way over the UK: which towns can still see it, and how far it has retreated

How fast the night sky is fading

The atlas describes the sky in 2014, the year of its satellite data. Since then the sky has brightened. Between 2011 and 2022, volunteers around the world sent more than 51,000 observations, each choosing which of eight star charts best matched their sky. From their reports, Christopher Kyba and colleagues calculated that the night sky over Europe grew brighter by 6.5% a year, and by 9.6% a year on average across all the observers’ locations (Science, 2023).

Satellites did not see this rise: for the same European locations, light measured from space fell slightly, by 0.3% a year. Kyba believes the difference probably comes from light sent sideways, by advertising and façade lighting, which the satellite barely catches, and from the switch to white LEDs, rich in blue light, a colour its sensor barely sees. The map above most likely shows a darker sky than today’s.

The chart shows what the European rate would mean for London, starting from its 2014 value: the solid line covers the years of Kyba’s study, the dashed one extends the rate to 2044, when a child born in 2026 turns 18. It only shows what would happen if the European rate held; any one town may brighten faster or slower.

“If the development were to continue at that rate, a child born in a place where 250 stars are visible will only be able to see 100 stars there on his 18th birthday,” Christopher Kyba said when the study was published, describing the global average of 9.6% a year.

Where the Milky Way has gone in the UK, and where it remains

The Milky Way is our own galaxy seen from inside: billions of stars too distant to tell apart, merged into a pale band of light. The band is faint, about as bright as the natural glow of the night sky, so a little artificial light is enough to cover it.

In 2014 the Milky Way could no longer be seen from 972 of the 1,672 cities and towns, including London, Birmingham, Leeds, Glasgow, Sheffield, Manchester and Edinburgh. The atlas lists the London to Liverpool and Leeds region among the large areas on Earth where the Milky Way is lost. For 26% of people in the UK, the sky is so bright that the eye never adapts to the dark.

The darkest skies over a town of 5,000 people or more: Fishguard (Pembrokeshire), Great Torrington (Devon), Looe (Cornwall), Bude (Cornwall) and Windermere (Cumbria).

From central London, the nearest place where the summer Milky Way returns is 36 km to the south-east. An almost untouched sky begins 340 km away, in Gwynedd.

What a council or a neighbour can do

• Light pointed only downwards. A shielded fitting sends no light directly upwards; one that also shines sideways lights the sky above the town.
• Less light, and not all night. Towards morning, as many lights go off, the sky visibly darkens; dimming after midnight does the same.
• Warm, amber light. Blue light scatters more in the air, and the dark-adapted eye is most sensitive to blue-green.
• The effect is immediate. When a lamp goes out, its light leaves the sky at that moment.

How this was calculated

• Sky brightness comes from the World Atlas of artificial night sky brightness (Falchi et al., Science Advances, 2016): a map with pixels of 30 arcseconds (under a kilometre), computed from Suomi-NPP satellite images taken in 2014 and calibrated with more than 35,000 ground measurements. The value is the artificial light at the zenith, for 1 am.
• Each town is placed at its OpenStreetMap point; the list covers all 1,672 places tagged there as a city or town. Populations, used only to order the lists, come from OpenStreetMap or, where it has none, from GeoNames.
• The thresholds come from the atlas paper (its colour table and Figure 9): from 0.223 mcd/m² the winter Milky Way can no longer be seen, from 0.445 the summer one is only a trace, from 0.688 it is gone, and from 3 the eye no longer adapts to the dark. Below 0.014 (8% of the natural glow), the sky overhead is almost untouched.
• Distances are in a straight line from the town’s point to the nearest pixel inside the UK below the threshold (a closer place may exist abroad); for untouched sky, an area of at least 3 × 3 pixels. The road is longer, and hills and woods can hide part of the sky.
• Stars come from the HYG catalogue, down to magnitude 6.6. The faintest visible star follows from the sky brightness through Schaefer’s formula in the form Unihedron uses, with 0.25 magnitudes of dimming per thickness of air the light crosses. The count under the answer is the average over a whole night (a full turn of the sky), the one in the picture is for the hour shown; both assume dark-adapted eyes, away from street lamps. Above 3 mcd/m² the eye can no longer adapt and the number is only theoretical. In a lit street far fewer are seen.
• The sky picture puts each star where it really is for the town, year and hour. The Milky Way band is the diffuse light from ESO’s photograph of the whole sky, with point stars removed; its visibility depends on its contrast with the sky, set so that it disappears at the atlas thresholds. The rise in brightness towards the horizon is a simple curve, and the glow of a neighbouring city in one direction is not drawn.
• The rate of 6.5% a year is the European average measured by Kyba et al. (2023) between 2011 and 2022, applied equally to every town.
• Check. From the same map, the Milky Way is gone across 13.4% of the UK and 1.3% of Romania, as in the atlas table (13.5% and 1.3%).

Sources

• Falchi, F., Cinzano, P., Duriscoe, D. et al. (2016). The new world atlas of artificial night sky brightness. Science Advances 2(6), e1600377. Map data: GFZ Data Services, CC BY-NC 4.0.
• Kyba, C. C. M., Altıntaş, Y. Ö., Walker, C. E., Newhouse, M. (2023). Citizen scientists report global rapid reductions in the visibility of stars from 2011 to 2022. Science 379, 265–268. European rate and quote: GFZ press release.
• Whole-sky photograph: ESO/S. Brunier, The Milky Way panorama (CC BY 4.0).
• Star catalogue: HYG v4.1 (Hipparcos, Yale Bright Star, Gliese), CC BY-SA 4.0.
• Towns: © OpenStreetMap contributors (ODbL); place names near the targets: GeoNames (CC BY 4.0); borders and areas: Natural Earth.