While it is moving in this way, due to varying gravitational forces, the axis wobbles (precesses) in a clockwise circle. Just imagine the way the axis of a top spins as it begins to fall. So, the angle of the earth stays the same (or somewhere within its three degree variance), but the direction in which it points changes. For example, our current North Star is Polaris (or Ursae Minoris), as the North Pole points towards this star. However, approximately 13,000 years ago, the North Pole would have pointed towards the star Vega, as it will do again in about another 13,000 years. It takes about 25,776 years to complete one precessional cycle.