1.8 usable nights
0–2 in the middle half of years · 0.6 typically near a dark moon.
364 astronomical-dark hours in an average month
20-year trip-planning baseline
When to plan an observing trip to Galway
April: 3.5 usable nights on average. A regional planning baseline, not a forecast for your trip.
Site darkness · modelled, not measured
Brightened skyBortle classes describe observed sky conditions; this brightness model does not establish a measured Bortle class.20.4–20.9 mag/arcsec²
The short answer
April has been strongest.
It averages 3.5 usable nights, versus 0.0 in June. A typical April lands between 1 and 5 usable nights.
No astronomical darkness in June. Cloud cannot rescue a night when the sun never reaches 18° below the horizon.
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
1.3 usable nights
0–2 in the middle half of years · 0.4 typically near a dark moon.
291 astronomical-dark hours in an average month
3.4 usable nights
2–4 in the middle half of years · 1.4 typically near a dark moon.
254 astronomical-dark hours in an average month
3.5 usable nights
1–5 in the middle half of years · 1.1 typically near a dark moon.
158 astronomical-dark hours in an average month
0.5 usable nights
0–0 in the middle half of years · 0.1 typically near a dark moon.
33 astronomical-dark hours in an average month
0.0 usable nights
0–0 in the middle half of years · 0.0 typically near a dark moon.
0 astronomical-dark hours in an average month
0.0 usable nights
0–0 in the middle half of years · 0.0 typically near a dark moon.
3 astronomical-dark hours in an average month
1.3 usable nights
0–2 in the middle half of years · 0.3 typically near a dark moon.
125 astronomical-dark hours in an average month
1.9 usable nights
1–3 in the middle half of years · 0.5 typically near a dark moon.
215 astronomical-dark hours in an average month
1.8 usable nights
1–2 in the middle half of years · 0.6 typically near a dark moon.
295 astronomical-dark hours in an average month
1.6 usable nights
1–2 in the middle half of years · 0.3 typically near a dark moon.
341 astronomical-dark hours in an average month
1.1 usable nights
0–1 in the middle half of years · 0.3 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.