1.9 usable nights
1–3 in the middle half of years · 0.3 typically near a dark moon.
357 astronomical-dark hours in an average month
20-year trip-planning baseline
Best time to stargaze at Butte
July: 16.8 usable nights on average. Use this seasonal climate baseline to choose a month, then use the live forecast to choose a night.
Site darkness · modelled, not measured
Brightened skyBortle classes describe observed sky conditions; this brightness model does not establish a measured Bortle class.20.5–21.0 mag/arcsec²
The short answer
July has been strongest.
It averages 16.8 usable nights, versus 1.6 in December. A typical July lands between 15 and 19 usable nights.
Record2005–2024 · 20 complete years
Usable nightAt least 3 astronomical-dark hours and NASA POWER nighttime cloud ≤30%
Dark-moon nightUsable night with ≤25% illumination at local solar midnight
ResolutionCERES SYN1deg 1° cloud grid; regional climate, not a campsite guarantee
Seasonal fingerprint
Usable nights by month
Month by month
1.9 usable nights
0–3 in the middle half of years · 0.6 typically near a dark moon.
292 astronomical-dark hours in an average month
2.7 usable nights
1–3 in the middle half of years · 1.1 typically near a dark moon.
267 astronomical-dark hours in an average month
3.4 usable nights
2–5 in the middle half of years · 1.3 typically near a dark moon.
202 astronomical-dark hours in an average month
4.7 usable nights
2–7 in the middle half of years · 1.4 typically near a dark moon.
141 astronomical-dark hours in an average month
2.5 usable nights
1–3 in the middle half of years · 0.8 typically near a dark moon.
82 astronomical-dark hours in an average month
16.8 usable nights
15–19 in the middle half of years · 5.8 typically near a dark moon.
120 astronomical-dark hours in an average month
13.2 usable nights
12–14 in the middle half of years · 4.5 typically near a dark moon.
184 astronomical-dark hours in an average month
10.4 usable nights
8–12 in the middle half of years · 3.8 typically near a dark moon.
244 astronomical-dark hours in an average month
5.5 usable nights
4–7 in the middle half of years · 1.4 typically near a dark moon.
303 astronomical-dark hours in an average month
2.3 usable nights
1–4 in the middle half of years · 0.5 typically near a dark moon.
334 astronomical-dark hours in an average month
1.6 usable nights
1–2 in the middle half of years · 0.8 typically near a dark moon.
372 astronomical-dark hours in an average month
Use the baseline honestly
Climate chooses the month. Forecasts choose the night.
The cloud product is a roughly 1° regional grid. It can reveal seasonality, but it cannot know a mountain ridge, valley fog pocket, locked gate, smoke event or the exact sky you will get.
Source: NASA POWER daily data, parameter CLOUD_AMT_NIGHT from the CERES SYN1deg cloud product. GoodSeeing calculates astronomical darkness and lunar illumination independently for each place.