precession of equinoxes

 The above two explanations explain the physics of the situation from a wide perspective. You can understand the earth-centric effects of precession on the equinoxes without worriying overly about the cause of this effect. For an earth based viewer "Precession of the Equinoxes" expains the phenomenon whereby the stars (and hence the Zodiacal constellations) revolve about the earth once every 20,000 odd years or so.  From an Astrological perspective this expains how the various ages (ie so called age of aquarius) come about. Over the course of a year the sun travels through the 12 astrological constellations, which is how the zodiacal sign of a given day is established. The sign in which the sun resides on the spring equinox establishes the current "age". Due to precession, this equinoxal sign changes every few thousand years as the sun completes its 20,000 year journey round the heavens and through the zodiac.

  The simple way to understand the behaviour of precession from a physics perspective is to use a spinning top. When the top is spinning, two rotations can be observed. Firstly the top itself will be spinning very quickly, this corresponds to the spin of the earth representing days. Look carefully however and you will notice that the spindle of the top traces out a slow circle as the top itself spins. This gradual gradual rotation about the vertical axis of the top's spindle is precession.

To clarify further, imagine two planes: the Earth's equatorial plane (perpendicular to its rotation axis) and the Earth's orbital plane.  These two planes intersect along a line; the Earth crosses this line at the equinoxes.  Because the Earth's rotation axis wobbles (precesses) with respect to the distant stars, so does its equatorial plane, and as a consequence the intersection of this plane with the orbital plane also moves slowly.  That's what's referred to as the precession of the equinoxes.

Astrology (a.k.a. Astromancy) usually doesn't bother with such details (it isn't a science, after all) and simply defines zodiacal signs based on an even division of the calendar into twelve parts.  Originally, the zodiac (the set of constellations lying along the celestial equator) was used to assign a "sign" to a person by determining in which constellation the Sun lay at the moment of that person's birth.  It's easy to quibble with this, of course.  Constellations are arbitrary: get a sky map showing just the stars as dots, and amuse yourself in connecting the dots.  Each culture developed its own set of constellations (the Chinese constellations are *very* different from ours, for example).  The boundaries between the constellations are also arbitrary; the IAU (International Astronomical Union) has divided the celestial sphere into constellations by specifying the boundaries (following the celestial equivalents of lines of longitude and latitude), for convenience's sake.  Looking at those, it is quite clear that some "signs" last much longer than others (Virgo lasts very long, for example).  The zodiac actually should include a thirteenth constellation (Ophiuchus), which the Sun traverses for a few days in early December.  I happen to be "of that sign", so it always amuses me when someone asks for my sign and I answer "Ophiuchus".  ;-)

 For the sake of 40 points I can't be bothered to quibble.