The Earth not only spins on its axis and orbits the Earth, it also slowly wobbles, just like a spinning top wobbles around its axis. This motion is called Precession of the Equinoxes because it causes the apparent shift westward (backwards through the zodiac) of the equinoxes relative to the background stars (by 1 degree every 72 years). (It causes a shift of the Sun relative to the background stars as measured on any day of the year, but the Equinoxes are convenient times to measure this shift).