The core clears 10 degrees during full darkness on most nights from April to August. In January, February, March, September, October, November and December it's out of reach, either too close to the Sun or too low in the dark.
Core score by month
| Month | Core score | Best week | Moon-free core hours a night | Average cloud |
|---|---|---|---|---|
| January | 0 | Core not visible | 0 | 70% |
| February | 0 | Core not visible | 0 | 67% |
| March | 0 | March 25 to 31 | 0.0 | 67% |
| April | 4 | April 6 to 12 | 0.6 | 69% |
| May | 8 | May 4 to 10 | 1.1 | 73% |
| June | 13 | June 1 to 7 | 1.8 | 70% |
| July | 12 | July 1 to 7 | 1.4 | 65% |
| August | 6 | August 1 to 7 | 0.7 | 64% |
| September | 0 | September 1 to 7 | 0.1 | 65% |
| October | 0 | Core not visible | 0 | 73% |
| November | 0 | Core not visible | 0 | 78% |
| December | 0 | Core not visible | 0 | 74% |
Times are for the best seven nights in each month. Moon-free means the moon is below the horizon. The core counts as visible above 10 degrees in full darkness, when the Sun is more than 18 degrees below the horizon.
The best nights in 2027
- May: May 4 is the best night. The core is above 10 degrees and the moon is down from 3:15 am to 4:15 am, with the moon 2% lit that night.
- June: June 4 is the best night. The core is above 10 degrees and the moon is down from 1:15 am to 2:55 am, with the moon 0% lit that night.
- July: July 1 is the best night. The core is above 10 degrees and the moon is down from 1:15 am to 2:35 am, with the moon 5% lit that night.
Clock times are Eastern time with daylight saving applied. The best nights fall around the new moon, so the dates shift by about 11 days each year.
About Isle Royale National Park
Isle Royale is a wilderness island in Lake Superior, and you get there by ferry or seaplane. There are no cars. Hikers walk the long ridge trail, paddlers explore the coves, and the wolves and moose have been studied here since 1958 in the longest predator and prey project in the world.
People have mined copper on the island for thousands of years, long before European miners arrived. The national park was established in 1940. The park closes completely each winter.
Isle Royale National Park is a national park in Michigan. The National Park Service places its center at 48.01 degrees north and 88.83 degrees west, and the sky calculations on this page use that point. Visitor information, fees and alerts are on the park's own page. National Park Service (opens in a new tab).
How Isle Royale National Park compares
Its best monthly core score of 13 ranks 52 of 54 national park units that can see the core. The highest is Lassen Volcanic National Park at 86, and the lowest is Voyageurs National Park at 13. Cloud at Isle Royale National Park runs from 64% in August to 78% in November.
The score ranks months and parks. It isn't a forecast for any night, and it leaves out local weather, smoke and light from nearby towns. How the core score works.
How dark is the sky?
The Park Service models how much artificial light brightens the sky across the lower 48. At the point used for Isle Royale National Park, it puts the added glow at about 9% of the natural brightness of a moonless sky (a ratio of 0.089). That makes it 12th darkest of the 47 parks it covers. Lower is darker. This is a model, not a reading taken at the park, and it describes one point, so the edges of a large park can differ.
Questions people ask
When is the best time to see the Milky Way at Isle Royale National Park?
May, June and July are the best months. The best week of 2027 is June 1 to 7.
Can you see the Milky Way at Isle Royale National Park all year?
The core clears 10 degrees during full darkness on most nights from April to August. In January, February, March, September, October, November and December it's out of reach, either too close to the Sun or too low in the dark.
How dark is the sky at Isle Royale National Park?
The Park Service models how much artificial light brightens the sky across the lower 48. At the point used for Isle Royale National Park, it puts the added glow at about 9% of the natural brightness of a moonless sky (a ratio of 0.089). That makes it 12th darkest of the 47 parks it covers. Lower is darker. This is a model, not a reading taken at the park, and it describes one point, so the edges of a large park can differ.
Does the moon matter at Isle Royale National Park?
Yes. A bright moon hides the Milky Way. The best night in June is June 4, when the moon is 0% lit and the core is above 10 degrees from 1:15 am to 2:55 am.
For a far-northern comparison, see Alaska.
