Just like a spring is first loaded, then released, and the act of loading the spring is driving the whole mechanism, the Earth spin was set up by the primordial

Just like a spring is first loaded, then released, and the act of loading the spring is driving the whole mechanism, the Earth spin was set up by the primordial configuration of the solar system. As the Earth rotates, its molten core gets continually mixed (as it doesn't rotate at the same rate as the crust[0]) and this generates heat. Likewise, the atmosphere doesn't rotate at the same speed as the crust, and this generates heat too. And, well, since perfectly rigid bodies can't exist[1], rotating bodies will always release heat, even those who are solid [2]. On the interaction of many bodies, tidal forces generate heat too, again because the bodies aren't rigid. Those are all dissipative forces. I think it's not possible to have an orbital system composed of purely reversible processes. In this case, any orbital system counts as a clock in the sense of the article. [0] https://en.wikipedia.org/wiki/Inner_core_super-rotation https://en.wikipedia.org/wiki/Inner_core_super-rotation [1] due to relativity, https://physics.stackexchange.com/a/201499 https://physics.stackexchange.com/a/201499 [2] actually.. I'm not so sure. I was searching a bit and https://phys.libretexts.org/Bookshelves/Classical_Mechanics/Classical_Mechanics_(Tatum)/04%3A_Rigid_Body_Rotation/4.07%3A_Nonrigid_Rotator https://phys.libretexts.org/Bookshelves/Classical_Mechanics/... says the non-rigid body eventually settles into rotating around its principal axis of rotation, which makes it not have any internal stresses, and thus not release heat. I think Earth revolves around its principal axis of rotation though (it had billions of years to settle). At this point I'm trying to read about orbital mechanics of non-rigid bodies, like this https://impa.br/wp-content/uploads/2017/04/30CBM_03.pdf https://impa.br/wp-content/uploads/2017/04/30CBM_03.pdf - the only thing I can gather is that indeed there are dissipative forces even in solids.