Precession of the Equinoxes
Precession is the wobble of the earth on its axis, just like a gyroscope slowing down will wobble before it tips over. Because of this motion, as we spin around the sun, our relation to the stars appears to change very slowly over time.
Precession is best measured against the signs of the zodiac in relation to the sun. The sign of a precessional "age" is that sign in which the sun rises heliacally (the last sign seen before sunrise on the vernal equinox, March 20-23). Over the course of the precessional cycle, the "wheel" of the signs appears to rotate backwards due to precession. Thus, over the course of a single year, the zodiac moves from Pisces to Aries to Taurus to Gemini, but in the course of a precessional "year," the sun rises heliacally in Gemini then to Taurus to Aries to Pisces and is now in Aquarius.