Star trails. View of star trails around the north celestial pole. Star trails are formed on a long- exposure photograph as the rotation of the Earth causes the stars to appear to move around the sky, centred on the celestial poles. As the Earth takes 24 hours to complete one revolution, star trails extending for 90 degrees (as here) require a six- hour exposure to produce. The brightest trail here is that of Polaris, the Pole Star (Alpha Ursae Minoris), in the constellation Ursa Minor.