23,000 Year Ice Age Cycle
Earth's polar axis precesses very slowly over a period of about 23,000 to 26,000 years. The term precession simply refers to a change in the direction of the axis of a rotating object. As this occurs, the pole of the Earth inscribes an arc in the heavens called the precessional arc. Earth's pole aligns with different pole stars throughout its precessional period. Polaris is Earth's current north-pole star. Earth's ~25,000-year precessional cycle is also referred to as "Earth's Great Year."
Precession of the Equinoxes
Because Earth's axis is tilted 23° 27' with respect the plane created by Earth's orbit around the Sun (the ecliptic plane), Earth's equatorial plane (green) is also tilted at this same angle with respect to the ecliptic plane (blue). These two intersecting planes (Earth's equatorial plane and the ecliptic plane) create an intersecting line in the ecliptic called the vernal axis. Looking from Earth, we see one point along this axis lying in one direction in the ecliptic, and the other lying in the opposite direction. One point is the location in the ecliptic where we see the sun on the March equinox. This location is the vernal point (VP). The other point is the location where the sun resides on the September equinox. This is the anti-vernal point.
As Earth's pole precesses, so to does Earth's equatorial plane and subsequently the vernal axis (moving in a counter-clockwise motion). Thus, the vernal axis (the vernal equinox points) move around the ecliptic at a rate of precession, which is currently measured to be about 1° in 72 years, making one complete cycle in about 23,000-26,000 years. The precessional movement of the vernal points is called the "Precession of the Equinoxes."