The precession of the equinoxes is the slow change of management of the axis of rotation of the Ground.

This change of management is caused by the couple which exert the forces of tides of the the Moon and of the Sun on the equatorial bulge of the Earth. These forces tend to bring the excess of mass present at the equator towards the plan of the ecliptic . The Earth being in rotation, these forces cannot change the angle between the equator and the ecliptic but cause a displacement of the axis of rotation of the Earth in a direction perpendicular to this axis and the couple. This displacement is called Précession and has as a result that the axis of rotation of the Earth describes a cone whose full rotation is carried out in approximately 25.800 years.

One of the consequences of this displacement is that the position of the star S on the celestial Sphère changes too. Currently, the brilliant star nearest to the celestial north pole is α Ursae Minoris (m 1,97), known as also the pole star , of which it is distant only from approximately 1 °. In the past, towards 3000 av. J. - C., it was the star α Draconis (Thuban) which was used as pole star but it was much less luminous than pole star. On the other hand, in approximately 12  000 years the very brilliant Alpha Lyrae (Vega, 0,03 m ) will be the pole star. In the southern hemisphere, the night ciels are relatively deprived of brilliant stars in the area of the poles: the star Sigma Octantis, though strong near to the pole is really too weak to be visible (5,5 m ).

As the orbit of the Moon is tilted compared to the ecliptic, the action of the Moon slightly comes to disturb the precession while adding to it of small oscillations of which the period is 18,6 years. This effect is called the Nutation .

Hipparque, with approximately of -130, is the first to have to leave a trace of estimate of the precession of the Earth. By comparing its measures of location of Alpha Virginis (Spica), a star close to the ecliptic, compared to the Not vernal with those carried out by the astronomers of the previous centuries, like Timocharis of Alexandria and the astronomers Babylonian S and chaldéen S, it showed that the vernal Point had moved of 2° compared to α   Virginis. It also showed that this movement related to also other stars and was worth at least 1° per century. The current value is of approximately 1° for 72 years. Because of the precession of the equinoxes, the duration of the cycle of the Saison S (Tropical year) is approximately 20 minutes shorter than time than puts the Earth to occupy the same position compared to the stars (sidereal Année). This difference is important for the Calendrier S and their rules concerning the leap years.

See too

• Parameters of celestial Milanković
• Pole