Isaac Newton, and others since his day, have shown that the Earth's precession is caused by the gravity of the Moon and Sun acting on the oblateness of the Earth. Since it can be calculated so accurately I have to believe it is true. The obliquity, or angle between the plane of the Earth's orbit and the axis, is about 23 degrees but varies between 22.1 and 24.5 degrees, in-phase with the precession circuit.