How dark is the sky in Portugal?
Sky darkness for 48 towns and parks in Portugal, worked out from NASA satellite images of light at night (2025). 10 of them have skies of Bortle 4 or darker, where the Milky Way shows overhead. The darkest is Guadiana Valley Natural Park (21.59, Bortle 4), the brightest Lisbon (17.92, 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 Portugal
- Guadiana Valley Natural ParkBeja · Bortle 421.59
- International Douro Natural ParkBragança · Bortle 421.41
- Southwest Alentejo and Vicentine Coast Natural ParkBeja · Bortle 421.40
- Montesinho Natural ParkBragança · Bortle 421.34
- Serra de São Mamede Natural ParkPortalegre · Bortle 421.27
Among the places on this site. Plenty of land between them is just as dark, with no town or park to name it by.
Darkest place in each district of Portugal
| District | Places | Dark places Bortle 1 to 4 | Darkest place |
|---|---|---|---|
| Aveiro | 4 places | 0 dark | Arouca20.28 · Bortle 5 |
| Beja | 3 places | 2 dark | Guadiana Valley Natural Park21.59 · Bortle 4 |
| Braga | 3 places | 0 dark | Esposende19.78 · Bortle 5 |
| Bragança | 3 places | 2 dark | International Douro Natural Park21.41 · Bortle 4 |
| Castelo Branco | 1 place | 0 dark | Castelo Branco18.93 · Bortle 7 |
| Coimbra | 2 places | 0 dark | Figueira da Foz19.91 · Bortle 5 |
| Faro | 6 places | 1 dark | Ria Formosa Nature Park20.57 · Bortle 4 |
| Guarda | 2 places | 1 dark | Serra da Estrela Natural Park20.88 · Bortle 4 |
| Leiria | 4 places | 1 dark | Serras de Aire e Candeeiros Natural Park20.64 · Bortle 4 |
| Lisbon | 3 places | 0 dark | Azambuja20.06 · Bortle 5 |
| Portalegre | 2 places | 1 dark | Serra de São Mamede Natural Park21.27 · Bortle 4 |
| Porto | 5 places | 0 dark | Baião20.37 · Bortle 5 |
| Santarém | 1 place | 0 dark | Santarém19.50 · Bortle 6 |
| Setúbal | 2 places | 0 dark | Sesimbra19.70 · Bortle 5 |
| Viana do Castelo | 4 places | 1 dark | Peneda-Gerês National Park21.05 · Bortle 4 |
| Vila Real | 1 place | 1 dark | Alvão Natural Park20.90 · Bortle 4 |
| Viseu | 1 place | 0 dark | Viseu18.97 · Bortle 6 |
| Évora | 1 place | 0 dark | Évora19.68 · Bortle 5 |