What was the exposure time of this photo?

about 10 seconds
about 10 minutes
about 1 hour
about 8 hours
The length of stars trails is pretty much the same as the distance travelled by an hour hand of clock during 30 minutes. But the Earth rotates twice as slow compared to the hour hand of a clock (one rotation per day, while clock's hour hand makes two).
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Answers by country

about 10 seconds
about 10 minutes
about 1 hour
about 8 hours
answers
flagAustralia
7%
19%
52%
21%
42
flagFrance
11%
0%
50%
38%
18
flagSwitzerland
6%
0%
43%
50%
16
flagUnknown
20%
12%
40%
28%
25
flagGermany
12%
13%
34%
39%
182
flagthe United Kingdom
12%
23%
32%
30%
111
flagPoland
16%
12%
31%
39%
7968
flagthe United States
22%
15%
31%
29%
299
flagIndia
33%
19%
28%
18%
336
flagthe Netherlands
27%
0%
27%
45%
11
flagPakistan
35%
9%
25%
25%
31
flagGreece
18%
12%
25%
43%
16
flagthe United Arab Emirates
41%
16%
25%
16%
12
flagthe Philippines
31%
31%
21%
12%
32
flagAustria
2%
12%
20%
64%
39
flagCanada
35%
15%
20%
30%
20
flagRepublic of Ireland
30%
10%
20%
40%
10
flagNigeria
16%
58%
16%
8%
12