I found an image of the night sky taken by James Sainty at http://500px.com/photo/11615131.
It has the following EXIF metadata:
Camera Canon 5D Mk II
Lens Canon 16-35mm L
Focal Length 16mm
Shutter Speed 140 sec
As I know there is a rule of "600" and night sky photographing, which says that if I don't want to shoot star trails, the max shutter speed should be 600/Focal Length. In this case, based on this rule, shutter speed should be 37s. But it is 140s, and there are not any star trails.
How could it be? Is this rule incorrect or are there some other aspects to it?