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.
Partial-circle star trails are pictured above over Oregon, USA last month. Near the middle of the circles 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.
Source: Astronomy Picture of the Day
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