Joensuu · September 2027
Northern lights in Joensuu in September 2027
Answer
September against the rest of the year
September cloud at Joensuu averages 72% in NASA POWER's 2001 to 2020 record, 4 points clearer than its yearly mean. August is Joensuu's clearest month at 70%. By civil night times clear sky, September scores 2.6 hours at Joensuu, 5th of 12. Kuopio, 111 km away, averages 71%, clearer by 1 points.
Civil night at Joensuu starts September at 7 h 48 min and reaches 10 h 53 min by September 30 as the nights lengthen by 3.1 hours. SWPC's 10 PM to 2 AM window is fully dark on every night. True astronomical darkness, the sun 18 degrees down, lasts 5.1 hours on the 15th.
September night by night
Full moon comes first, on Sep 16, then the new moon on Sep 30, in Europe/Helsinki dates. Joensuu gets 143 hours of civil night with the moon below the horizon across the month. The best of them for moon-free darkness inside 10 PM to 2 AM: Sep 1 (4 h), Sep 2 (4 h), Sep 3 (4 h).
- 1
2% lit
7.8 h
- 2
7% lit
7.9 h
- 3
13% lit
8.0 h
- 4
22% lit
8.1 h
- 5
31% lit
8.3 h
- 6
41% lit
8.4 h
- 7
51% lit
8.5 h
- 8
61% lit
8.6 h
- 9
70% lit
7.7 h
- 10
78% lit
6.0 h
- 11
85% lit
4.3 h
- 12
91% lit
2.8 h
- 13
96% lit
1.3 h
- 14
99% lit
0.0 h
- 15
100% lit
0.0 h
- 16
99% lit
0.0 h
- 17
96% lit
0.0 h
- 18
91% lit
0.0 h
- 19
85% lit
0.0 h
- 20
77% lit
0.0 h
- 21
67% lit
0.0 h
- 22
56% lit
0.0 h
- 23
45% lit
0.8 h
- 24
34% lit
2.7 h
- 25
23% lit
4.8 h
- 26
14% lit
6.8 h
- 27
7% lit
8.7 h
- 28
2% lit
10.5 h
- 29
0% lit
10.8 h
- 30
1% lit
10.9 h
Each cell is the night that starts on that date. Hours are civil-dark time with the moon below the horizon; the green rule scales with them. Green outline: new moon. Bone outline: full moon.
Activity in past Septembers
Joensuu needs Kp 4 for the average oval edge to sit overhead. 278 of the 900 September days from 1996 to 2025 (31%) reached it in the GFZ Potsdam record, one of the three most active months for that level; storm days with Kp 5 or more made up 14%. Joensuu sits at 59.4 degrees geomagnetic. By SWPC's rule the average oval edge is overhead at Kp 4, and aurora is usually visible on the northern horizon at G1 (Kp 5) or less.
September across Finland
Joensuu ranks 7th in Finland for civil night on September 15, behind Helsinki at 9.6 hours. Coming from August (5.8 hours at Joensuu), September adds darkness, and the nights keep growing into October (12.4 hours on the 15th). The September equinox falls on September 23, 08:56 Joensuu time, inside the weeks SWPC links with larger storms.
Darkness and moon, every night
| Night | Civil | Astronomical | Moon-free dark | Moon-free, 10 PM to 2 AM | Moon lit |
|---|---|---|---|---|---|
| Wed Sep 1 | 7 h 48 min | 1 h 51 min | 7 h 48 min | 4 h | 2% |
| Thu Sep 2 | 7 h 55 min | 2 h 14 min | 7 h 55 min | 4 h | 7% |
| Fri Sep 3 | 8 h 1 min | 2 h 34 min | 8 h 1 min | 4 h | 13% |
| Sat Sep 4 | 8 h 8 min | 2 h 51 min | 8 h 8 min | 4 h | 22% |
| Sun Sep 5 | 8 h 15 min | 3 h 7 min | 8 h 15 min | 4 h | 31% |
| Mon Sep 6 | 8 h 22 min | 3 h 22 min | 8 h 22 min | 4 h | 41% |
| Tue Sep 7 | 8 h 28 min | 3 h 35 min | 8 h 28 min | 4 h | 51% |
| Wed Sep 8 | 8 h 35 min | 3 h 48 min | 8 h 35 min | 4 h | 61% |
| Thu Sep 9 | 8 h 41 min | 4 h 1 min | 7 h 42 min | 4 h | 70% |
| Fri Sep 10 | 8 h 48 min | 4 h 13 min | 5 h 57 min | 2 h 34 min | 78% |
| Sat Sep 11 | 8 h 54 min | 4 h 24 min | 4 h 19 min | 54 min | 85% |
| Sun Sep 12 | 9 h 1 min | 4 h 35 min | 2 h 47 min | 0 min | 91% |
| Mon Sep 13 | 9 h 7 min | 4 h 46 min | 1 h 20 min | 0 min | 96% |
| Tue Sep 14 | 9 h 14 min | 4 h 56 min | 0 min | 0 min | 99% |
| Wed Sep 15 | 9 h 20 min | 5 h 6 min | 0 min | 0 min | 100% |
| Thu Sep 16 | 9 h 26 min | 5 h 15 min | 0 min | 0 min | 99% |
| Fri Sep 17 | 9 h 33 min | 5 h 25 min | 0 min | 0 min | 96% |
| Sat Sep 18 | 9 h 39 min | 5 h 34 min | 0 min | 0 min | 91% |
| Sun Sep 19 | 9 h 45 min | 5 h 43 min | 0 min | 0 min | 85% |
| Mon Sep 20 | 9 h 52 min | 5 h 52 min | 0 min | 0 min | 77% |
| Tue Sep 21 | 9 h 58 min | 6 h | 0 min | 0 min | 67% |
| Wed Sep 22 | 10 h 4 min | 6 h 9 min | 0 min | 0 min | 56% |
| Thu Sep 23 | 10 h 10 min | 6 h 17 min | 45 min | 0 min | 45% |
| Fri Sep 24 | 10 h 16 min | 6 h 25 min | 2 h 43 min | 29 min | 34% |
| Sat Sep 25 | 10 h 23 min | 6 h 33 min | 4 h 46 min | 2 h 28 min | 23% |
| Sun Sep 26 | 10 h 29 min | 6 h 41 min | 6 h 45 min | 4 h | 14% |
| Mon Sep 27 | 10 h 35 min | 6 h 49 min | 8 h 40 min | 4 h | 7% |
| Tue Sep 28 | 10 h 41 min | 6 h 56 min | 10 h 31 min | 4 h | 2% |
| Wed Sep 29 | 10 h 47 min | 7 h 4 min | 10 h 47 min | 4 h | 0% |
| Thu Sep 30 | 10 h 53 min | 7 h 11 min | 10 h 53 min | 4 h | 1% |
Questions
- Can you see the northern lights in Joensuu in September?
- On dark, clear nights with enough activity. 30 of 30 nights at Joensuu have civil darkness, and 31% of past September days reached Kp 4.
- How dark is Joensuu in September?
- Joensuu averages 9.4 hours of civil night a night in September 2027, from 7 h 48 min on the 1st to 10 h 53 min on the last night.
- How cloudy is Joensuu in September?
- NASA POWER's 2001 to 2020 climatology gives Joensuu's 1 degree cell 72% mean cloud in September, against 76% for the whole year. It is an all-hours average, not a forecast.
- What is the best night in September 2027 for aurora in Joensuu?
- By darkness and moon alone, Sep 1, with 4 h of moon-free darkness inside 10 PM to 2 AM at Joensuu. Activity on that date will only be known hours ahead.
- Is September or October better in Joensuu?
- By civil night times the clear share, September scores 2.6 hours at Joensuu and October 2.1. September comes out ahead.
Sources and verification
- SunCalchttps://github.com/mourner/suncalc
- NASA POWER climatologyhttps://power.larc.nasa.gov/docs/services/api/temporal/climatology/
- GFZ Potsdam Kp indexhttps://kp.gfz.de/en/data
- NOAA SWPC, Tips on viewing the aurorahttps://www.spaceweather.gov/content/tips-viewing-aurora
Quoted figures last checked against their sources on 2026-09-30; next review 2026-12-31. Computed figures come from SunCalc and the IGRF-14 dipole as described in the methodology. Every quote is listed in the sources ledger.