How dark is the sky in Illinois?
Sky darkness for 36 towns and parks in Illinois, worked out from NASA satellite images of light at night (2025). 11 of them have skies of Bortle 4 or darker, where the Milky Way shows overhead. The darkest is Shawnee National Forest (21.10, Bortle 4), the brightest Chicago (16.77, 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 Illinois
- Shawnee National ForestBortle 421.10
- Apple River Canyon State ParkBortle 421.05
- Cache River State Natural AreaBortle 420.99
- Mississippi Palisades State ParkBortle 420.96
- Cave-in-Rock State ParkBortle 420.84
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 Illinois
All 36 places in Illinois
Amber names are parks and certified dark-sky places. The figure is the Bortle class: 1 darkest, 9 city center.
Apple River Canyon State Park4Aurora8Bloomington8Bolingbrook8Cache River State Natural Area4Cave-in-Rock State Park4Champaign8Chicago8Cicero8Decatur8Des Plaines8Elgin8Evanston8Fort Kaskaskia State Historic Site4Giant City State Park4Homer Glen8Joliet8Kankakee National Wildlife Refuge and Conservation Area5Lincoln Log Cabin State Historic Site4Middle Fork River Forest Preserve4Mississippi Palisades State Park4Naperville8North Peoria8Oak Lawn8Orland Park8Palatine8Peoria8Rockford8Schaumburg8Shawnee National Forest4Springfield8Tinley Park8Trail of Tears State Forest4Waukegan8Wheaton8White Pines Forest State Park4