How dark is the sky in Wales?
Sky darkness for 20 towns and parks in Wales, worked out from NASA satellite images of light at night (2025). 12 of them have skies of Bortle 4 or darker, where the Milky Way shows overhead. The darkest is Coed y Bont (21.55, Bortle 4), the brightest Cardiff (18.07, Bortle 8).
19
Bortle class, 1 pristine to 9 city center
park or dark-sky place
city
Numbers like 21.95 or 16.97 are the brightness of the sky overhead on a clear, moonless night, in mag/arcsec². The scale runs backwards: a lower number means a brighter sky, from about 22 for a natural sky to about 17 in a city center.
How the readings are made
Darkest skies in Wales
- Coed y BontBortle 421.55
- Elan ValleyBortle 421.50
- the Llŷn PeninsulaBortle 421.43
- the Cambrian MountainsBortle 421.42
- the Pembrokeshire CoastBortle 421.40
Among the places on this site. Plenty of land between them is just as dark, with no town or park to name it by.
Brightest city skies in Wales
All 20 places in Wales
Amber names are parks and certified dark-sky places. The figure is the Bortle class: 1 darkest, 9 city center.