Precession is the effect of bodies in motion upon other bodies in motion -- or, more simply, a ripple effect, like when you throw a stone into a still pond and the waves emanating from it overlap.

• 所谓径动，就是某个运动中的物体与另一个也在运动中的物体间的交互作用。或者简单一点，涟漪效应。当你丢一颗石头进池塘，会产生一圈圈涟漪，而这涟漪反弹回来后又会彼此交叠产生复杂的交互作用。