Explanation:
As the Earth spins on its axis, the sky seems to rotate around us.
This motion, called
diurnal motion,
produces the beautiful concentric trails traced
by stars during time exposures.
In the middle of the picture is the North Celestial Pole (NCP), easily identified as the point in the sky at the center of all the star trail arcs.
The star Polaris,
commonly known as the
North Star, made the very short bright circle near the NCP.
Full circle star trails are pictured over Vienna,
Austria.
This image, a relatively short exposure followed by a
digital trick, could not have been
taken during a single night because 24-hours are needed for
one full rotation, and the
Sun is sure
to dominate the frame at some time.
