It's the precession of the equinoxes, or axial precession:spice a dit:
"Axial precession is the movement of the rotational axis of an astronomical body, whereby the axis slowly traces out a cone. In the case of the Earth, this type of precession is also known as the precession of the equinoxes or precession of the equator. The Earth goes through one such complete precessional cycle in a period of approximately 26,000 years, during which the positions of stars as measured in the equatorial coordinate system will slowly change; the change is actually due to the change of the coordinates. Over this cycle the Earth's north axial pole moves from where it is now, within 1° of Polaris, in a circle around the ecliptic pole, with an angular radius of about 23.5 degrees (or approximately 23 degrees 27 arcminutes). The shift is 1 degree in 72 years, where the angle is taken from the observer, not from the center of the circle."
In the above image the circle looks small, but when projected onto the entire cosmos, it makes an anti-clockwise motion through the different zodiac signs, completing one cycle in 26.000 years. That's why we are in the Age of Pisces, slowly moving into the Age of Aquarius. There is some debate as to when the next age starts exactly. Some believe we are already in the Age of Aquarius, but I think it will take another 300 or 400 years.
Western astrology doesn't take axial precession into account, but Vedic astrology does (in which this principle is called ayanamsa, the "drifting part").
The principle of precession is often related to gravitational pull and so on. An alternative explanation is that our Sun is actually part of a binary system, meaning that its movement through space is related to another star with which it performs an eternal dance. There are numerous binary stars in the universe. You can read more about this particular theory here: http://www.binaryresearchinstitute.org/ An overview of the issue (problems with the current theory) can be seen in this (large) image or PDF.
From the same wikipedia page: "The precession of the Earth's axis has a number of observable effects. First, the positions of the south and north celestial poles appear to move in circles against the space-fixed backdrop of stars, completing one circuit in 25,771.5 years (2000 rate). Thus, while today the star Polaris lies approximately at the north celestial pole, this will change over time, and other stars will become the "north star". As the celestial poles shift, there is a corresponding gradual shift in the apparent orientation of the whole star field, as viewed from a particular position on Earth."
