The precession of the equinoxes
Opera Laboratori Fiorentini

Around 130 B.C. Hipparchus discovered the precession of the equinoxes, a movement of the Earth that causes the orientation of its axis to change slowly but continuously in relation to the ideal sphere of the fixed stars. The Earth's axis completes one full rotation around the vertical every 25,800 years. This movement causes the position of the celestial poles to change. In 13,000 years' time the north celestial pole will no longer be indicated by the Pole Star but by Vega.