It is caused by the torque of the Sun and Moon on the Earth's rotational bulge, which is tilted with respect to the orbital plane.  
It occurs with a rate of about 50 arcseconds per year and has a period of about 26,000 years.  To account for the effect astronomical coordinates are given in terms of the epoch.  It is caused by the torque of the Sun and Moon on the Earth's rotational bulge, which is tilted with respect to the orbital plane.  As a result, Vega will be the Pole Star in another 13,000 years.  FTP name this effect discovered by Hipparchus of Rhodes
It occurs with a rate of about 50 arcseconds per year and has a period of about 26,000 years.  
precession of the equinoxes or equivalents (prompt on precession)