It directly depends on the Earth 's axial tilt , which fluctuates within a margin of 2 ° over a 40,000-year period , due to tidal forces resulting from the orbit of the Moon .
It directly depends on the Earth 's axial tilt . The axial tilt fluctuates within a margin of 2 ° over a 40,000-year period , mostly due to tidal forces from the orbit of the Moon .