Star trails. Time-exposure view of star trails above cirrus clouds. Star trails are formed due to the apparent motion of the stars at night caused by the Earth's rotation. All northern stars appear to circle the north celestial pole, and the southern stars circle the southern one. At the celestial equator, as seen here, the trails have a biconcave shape. At lower left, the stars occupy the southern hemisphere, and appear to be circling a point further down to the left. At upper right, the stars are in the northern hemisphere, and appear to be circling a point beyond the top right corner.