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
flagFrance
7%
0%
64%
28%
14
flagAustralia
8%
18%
54%
18%
37
flagUnknown
20%
12%
40%
28%
25
flagthe United Kingdom
12%
22%
33%
30%
93
flagthe United States
23%
15%
31%
29%
263
flagPoland
16%
12%
30%
39%
7108
flagGermany
16%
9%
30%
43%
62
flagPakistan
33%
11%
29%
22%
27
flagIndia
33%
19%
28%
16%
283
flagGreece
14%
14%
28%
42%
14
flagthe United Arab Emirates
36%
18%
27%
18%
11
flagthe Philippines
32%
28%
25%
10%
28
flagAustria
0%
0%
25%
75%
12
flagCanada
37%
18%
18%
25%
16