Lunisolar precession

   Lunisolar precession is the apparent change in the location of the stars in relation to the observers on Earth. It is caused by the pull of gravity of the Sun and the Moon on the Earth
   Actually, the Earth's axis is almost fixed at an angle of 23½ degrees to the plane of its equator. But the gravitational effect of the Sun and Moon (sometimes supplementing, sometimes counteracting each other) causes the axis of the spinning Earth to wobble slightly, as in the motion of a spinning top. The result is that the stars used as reference points (such as the North star or the constellations) seem to move slowly westward, completing a cycle in about 26,000 years.