Can you see the northern lights in Viljandi in September?

On dark, clear nights with enough activity. 30 of 30 nights at Viljandi have civil darkness, and 5% of past September days reached Kp 6.

How dark is Viljandi in September?

Viljandi averages 9.7 hours of civil night a night in September 2027, from 8 h 27 min on the 1st to 11 h on the last night.

How cloudy is Viljandi in September?

NASA POWER's 2001 to 2020 climatology gives Viljandi's 1 degree cell 68% mean cloud in September, against 71% for the whole year. It is an all-hours average, not a forecast.

What is the best night in September 2027 for aurora in Viljandi?

By darkness and moon alone, Sep 1, with 4 h of moon-free darkness inside 10 PM to 2 AM at Viljandi. Activity on that date will only be known hours ahead.

Is September or October better in Viljandi?

By civil night times the clear share, September scores 3.1 hours at Viljandi and October 3.1. September comes out ahead.

Viljandi · September 2027

Northern lights in Viljandi in September 2027

Answer

September gives Viljandi 9.7 hours of civil night on the 15th, the 7th darkest of its 12 months, under 68% mean cloud. 5% of September days since 1996 reached Kp 6, enough for the oval edge to reach Viljandi.

9.7 h

civil night, average night

SunCalc for 58.36, 25.59; NASA POWER; GFZ Kp

September against the rest of the year

2.4

2.9

3.5

3.1

2.2

1.0

1.5

2.6

3.1

2.3

2.2

September cloud at Viljandi averages 68% in NASA POWER's 2001 to 2020 record, 3 points clearer than its yearly mean. May is Viljandi's clearest month at 56%. By civil night times clear sky, September scores 3.1 hours at Viljandi, 3rd of 12. Tallinn, 129 km away, averages 70%, cloudier by 2 points.

Civil night at Viljandi starts September at 8 h 27 min and reaches 11 h by September 30 as the nights lengthen by 2.5 hours. SWPC's 10 PM to 2 AM window is fully dark on every night. True astronomical darkness, the sun 18 degrees down, lasts 6.3 hours on the 15th.

September night by night

Full moon comes first, on Sep 16, then the new moon on Sep 30, in Europe/Tallinn dates. Viljandi gets 149 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
8.4 h
2
7% lit
8.5 h
3
13% lit
8.6 h
4
22% lit
8.7 h
5
31% lit
8.8 h
6
41% lit
8.9 h
7
51% lit
8.5 h
8
61% lit
7.7 h
9
70% lit
6.6 h
10
78% lit
5.3 h
11
85% lit
3.9 h
12
91% lit
2.6 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
Full
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
0.1 h
22
56% lit
0.9 h
23
45% lit
2.1 h
24
34% lit
3.7 h
25
23% lit
5.4 h
26
14% lit
7.2 h
27
7% lit
8.9 h
28
2% lit
10.6 h
29
0% lit
10.9 h
30
1% lit
11.0 h
New

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

Viljandi needs Kp 6 for the average oval edge to sit overhead. 47 of the 900 September days from 1996 to 2025 (5%) 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%. Viljandi sits at 55.9 degrees geomagnetic. By SWPC's rule the average oval edge is overhead at Kp 6, and aurora is usually visible on the northern horizon at G1 (Kp 5) or less.

September across Estonia

No month-page town in Estonia has more civil night on September 15 than Viljandi; the national median is 9.7 hours. Coming from August (6.9 hours at Viljandi), September adds darkness, and the nights keep growing into October (12.3 hours on the 15th). The September equinox falls on September 23, 08:56 Viljandi time, inside the weeks SWPC links with larger storms.

Civil night on September 15 and September cloud, month-page towns in Estonia
Town	Civil night	Cloud
Viljandi	9.7 h	68%
Tallinn	9.7 h	70%

Darkness and moon, every night

Viljandi, September 2027: each night from local noon to the next noon
Night	Civil	Astronomical	Moon-free dark	Moon-free, 10 PM to 2 AM	Moon lit
Wed Sep 1	8 h 27 min	4 h 22 min	8 h 27 min	4 h	2%
Thu Sep 2	8 h 32 min	4 h 31 min	8 h 32 min	4 h	7%
Fri Sep 3	8 h 38 min	4 h 40 min	8 h 38 min	4 h	13%
Sat Sep 4	8 h 43 min	4 h 49 min	8 h 43 min	4 h	22%
Sun Sep 5	8 h 49 min	4 h 57 min	8 h 49 min	4 h	31%
Mon Sep 6	8 h 54 min	5 h 5 min	8 h 54 min	4 h	41%
Tue Sep 7	8 h 59 min	5 h 14 min	8 h 31 min	4 h	51%
Wed Sep 8	9 h 5 min	5 h 22 min	7 h 43 min	3 h 54 min	61%
Thu Sep 9	9 h 10 min	5 h 29 min	6 h 35 min	2 h 44 min	70%
Fri Sep 10	9 h 16 min	5 h 37 min	5 h 17 min	1 h 24 min	78%
Sat Sep 11	9 h 21 min	5 h 45 min	3 h 56 min	0 min	85%
Sun Sep 12	9 h 26 min	5 h 52 min	2 h 35 min	0 min	91%
Mon Sep 13	9 h 32 min	5 h 59 min	1 h 16 min	0 min	96%
Tue Sep 14	9 h 37 min	6 h 7 min	0 min	0 min	99%
Wed Sep 15	9 h 42 min	6 h 14 min	0 min	0 min	100%
Thu Sep 16	9 h 47 min	6 h 21 min	0 min	0 min	99%
Fri Sep 17	9 h 53 min	6 h 27 min	0 min	0 min	96%
Sat Sep 18	9 h 58 min	6 h 34 min	0 min	0 min	91%
Sun Sep 19	10 h 3 min	6 h 41 min	0 min	0 min	85%
Mon Sep 20	10 h 8 min	6 h 47 min	0 min	0 min	77%
Tue Sep 21	10 h 14 min	6 h 54 min	5 min	0 min	67%
Wed Sep 22	10 h 19 min	7 h	53 min	0 min	56%
Thu Sep 23	10 h 24 min	7 h 7 min	2 h 7 min	5 min	45%
Fri Sep 24	10 h 29 min	7 h 13 min	3 h 42 min	1 h 37 min	34%
Sat Sep 25	10 h 34 min	7 h 19 min	5 h 26 min	3 h 18 min	23%
Sun Sep 26	10 h 39 min	7 h 25 min	7 h 11 min	4 h	14%
Mon Sep 27	10 h 44 min	7 h 31 min	8 h 54 min	4 h	7%
Tue Sep 28	10 h 50 min	7 h 37 min	10 h 36 min	4 h	2%
Wed Sep 29	10 h 55 min	7 h 43 min	10 h 55 min	4 h	0%
Thu Sep 30	11 h	7 h 49 min	11 h	4 h	1%

Moon lit: SunCalc illuminated fraction at the middle of the 10 PM to 2 AM window.

Questions

Can you see the northern lights in Viljandi in September?: On dark, clear nights with enough activity. 30 of 30 nights at Viljandi have civil darkness, and 5% of past September days reached Kp 6.
How dark is Viljandi in September?: Viljandi averages 9.7 hours of civil night a night in September 2027, from 8 h 27 min on the 1st to 11 h on the last night.
How cloudy is Viljandi in September?: NASA POWER's 2001 to 2020 climatology gives Viljandi's 1 degree cell 68% mean cloud in September, against 71% for the whole year. It is an all-hours average, not a forecast.
What is the best night in September 2027 for aurora in Viljandi?: By darkness and moon alone, Sep 1, with 4 h of moon-free darkness inside 10 PM to 2 AM at Viljandi. Activity on that date will only be known hours ahead.
Is September or October better in Viljandi?: By civil night times the clear share, September scores 3.1 hours at Viljandi and October 3.1. 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.