How dark is the sky in Montana?
Sky darkness for 23 towns and parks in Montana, worked out from NASA satellite images of light at night (2025). 20 of them have skies of Bortle 4 or darker, where the Milky Way shows overhead. The darkest is Upper Missouri River Breaks National Monument (21.92, Bortle 2), the brightest Billings (17.98, 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 Montana
- Upper Missouri River Breaks National MonumentBortle 221.92
- Helena-Lewis and Clark National ForestBortle 321.87
- Glacier National ParkBortle 321.83
- Beaverhead-Deerlodge National ForestBortle 321.82
- Medicine Rocks State ParkBortle 321.82
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 Montana
All 23 places in Montana
Amber names are parks and certified dark-sky places. The figure is the Bortle class: 1 darkest, 9 city center.
Beaverhead-Deerlodge National Forest3Bighorn Canyon National Recreation Area3Billings8Bitterroot National Forest4Custer-Gallatin National Forest3First Peoples Buffalo Jump State Park4Glacier National Park3Great Falls8Greycliff Prairie Dog Town State Park3Helena National Forest4Helena-Lewis and Clark National Forest3Lewis and Clark National Forest3Little Bighorn Battlefield National Monument4Madison Buffalo Jump State Park4Makoshika State Park4Medicine Rocks State Park3Missoula7Missouri Headwaters State Park4Pompeys Pillar National Monument4Pryor Mountains National Forest4Rattlesnake National Recreation Area4Upper Missouri River Breaks National Monument2Waterton-Glacier International Peace Park3