Precession
The direction of the rotational axis of the Earth is not really fixed in space. Over time it undergoes a slow drift, or precession, much like that of a spinning top. This effect stems from the gravitational attraction of the Sun and the Moon on the Earth's equatorial bulge. Due to the precession, the northern celestial pole (presently situated near the star a Ursae Minoris, or Polaris) slowly turns around the pole of the ecliptic with a period of about 26000 years. As a consequence, the vernal equinox, the intersection of equator and ecliptic, regresses by about 50" per year along the ecliptic. Moreover, the plane of the ecliptic itself is not fixed in space. Due to the gravitational attraction of the planets on the Earth, it slowly rotates around a "line of nodes", the speed of this rotation being presently 47" per century. The plane of the ecliptic and that of the equator, and the vernal equinox, are the fundamental planes and the origin of two important coordinate systems on the celestial sphere: the ecliptical coordinates (longitude and latitude) and the equatorial coordinates (right ascension and declination). So, due to the precession, the coordinates of the "fixed" stars are continuously changing. Star catalogues, therefore, list the right ascensions and declinations of stars for a given epoch, such as 1900.0, or 1950.0, or 2000.0.