How dark is the sky in Idaho?
Sky darkness for 27 towns and parks in Idaho, worked out from NASA satellite images of light at night (2025). 21 of them have skies of Bortle 4 or darker, where the Milky Way shows overhead. The darkest is Nez Perce National Forest (21.91, Bortle 2), the brightest Boise (17.88, 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 Idaho
- Nez Perce National ForestBortle 221.91
- Salmon-Challis National ForestBortle 221.91
- Salmon National ForestBortle 221.90
- Clearwater National ForestBortle 321.89
- Central IdahoBortle 321.86
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 Idaho
All 27 places in Idaho
Amber names are parks and certified dark-sky places. The figure is the Bortle class: 1 darkest, 9 city center.
Boise8Boise National Forest4Caldwell7Caribou-Targhee National Forest4Castle Rocks State Park3Central Idaho3City of Rocks National Reserve3Clearwater National Forest3Coeur d'Alene National Forest4Craters of the Moon National Monument and Preserve3Craters of the Moon National Preserve3Hagerman Fossil Beds National Monument4Idaho Falls7Kaniksu National Forest3Ketchum4Massacre Rocks State Park4Meridian8Morley Nelson Snake River Birds of Prey National Conservation Area4Nampa7Nez Perce National Forest2Payette National Forest4Pocatello7Salmon National Forest2Salmon-Challis National Forest2Sawtooth National Forest3Sawtooth National Recreation Area3St. Joe National Forest3