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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Very interesting!
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Answers by country

about 10 seconds
about 10 minutes
about 1 hour
about 8 hours
answers
flagAustralia
7%
17%
56%
17%
39
flagFrance
5%
0%
52%
41%
17
flagUnknown
20%
12%
40%
28%
25
flagSwitzerland
0%
0%
40%
60%
10
flagGermany
11%
10%
35%
42%
114
flagthe United Kingdom
13%
23%
33%
29%
103
flagthe United States
22%
15%
32%
29%
279
flagPoland
16%
12%
31%
39%
7541
flagIndia
33%
19%
28%
17%
320
flagthe Netherlands
27%
0%
27%
45%
11
flagPakistan
36%
10%
26%
23%
30
flagAustria
0%
10%
25%
65%
20
flagGreece
18%
12%
25%
43%
16
flagthe United Arab Emirates
41%
16%
25%
16%
12
flagthe Philippines
32%
29%
22%
12%
31
flagCanada
36%
15%
21%
26%
19
flagRepublic of Ireland
30%
10%
20%
40%
10