A salutary reminder from Y. Ryabov’s An Elementary Survey of Celestial Mechanics:
There is of course no sense in asking why the planets rotate or why they have motion in general. Everything in the universe, from the smallest dust particle to colossal cosmic bodies, is in constant motion. There is no such thing as matter without motion. The matter that later went to form the planets was also in motion. In the process of their “birth” the planets acquired both translational orbital motion about the sun and rotational motion on their axes.
I give my students a hard time about this, but the mentality that things aren’t supposed to move is deeply embedded in the mentality of most civil engineers. To some extent it is understandable; their job is to insure that nothing does move, or at least doesn’t move to a place it isn’t supposed to. As I like to say, when things move, civil engineers freak out, but in many cases when this happens to a structure everybody else does too.
On the other hand, that static mentality has made bringing understanding to pile dynamics an uphill battle, from the early days of pile dynamics to the present. And that’s not a good thing; it has helped to promote the “black box” attitude many civil engineers take to pile dynamics and all that goes with it. But engineers of all kinds need to make an effort to understand what is in front of them, and part of the purpose of this site is to help in that process.