0.6 usable nights
0–1 in the middle half of years · 0.1 typically near a dark moon.
355 astronomical-dark hours in an average month
20-year trip-planning baseline
Best time to stargaze at Alliston
September: 7.8 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
Bright skyBortle classes describe observed sky conditions; this brightness model does not establish a measured Bortle class.19.7–20.3 mag/arcsec²
The short answer
September has been strongest.
It averages 7.8 usable nights, versus 0.2 in December. A typical September lands between 6 and 9 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
0.9 usable nights
0–1 in the middle half of years · 0.3 typically near a dark moon.
292 astronomical-dark hours in an average month
3.4 usable nights
2–5 in the middle half of years · 1.1 typically near a dark moon.
270 astronomical-dark hours in an average month
3.8 usable nights
2–5 in the middle half of years · 1.2 typically near a dark moon.
210 astronomical-dark hours in an average month
4.0 usable nights
2–5 in the middle half of years · 1.6 typically near a dark moon.
155 astronomical-dark hours in an average month
4.0 usable nights
2–6 in the middle half of years · 1.6 typically near a dark moon.
120 astronomical-dark hours in an average month
6.0 usable nights
5–7 in the middle half of years · 1.6 typically near a dark moon.
133 astronomical-dark hours in an average month
6.1 usable nights
4–8 in the middle half of years · 1.9 typically near a dark moon.
196 astronomical-dark hours in an average month
7.8 usable nights
6–9 in the middle half of years · 2.5 typically near a dark moon.
246 astronomical-dark hours in an average month
3.9 usable nights
2–5 in the middle half of years · 1.4 typically near a dark moon.
304 astronomical-dark hours in an average month
2.6 usable nights
1–4 in the middle half of years · 1.1 typically near a dark moon.
332 astronomical-dark hours in an average month
0.2 usable nights
0–0 in the middle half of years · 0.1 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.