The planet’s axis and speed of rotation naturally varies over time, like a wobbly top, slightly slowing down with each spin. Many different factors affect this, but the moon in particular plays a large role by interacting with Earth’s tides. The moon’s gravitational pull drags the tides along behind the moon’s orbit. This momentum creates a kind of natural brake pad against the Earth.
Interestingly, we can track this change over time and actually pinpoint drastic changes. Anything with enough mass or inertia can create a wobble. For instance, the 2004 tsunami had enough of a punch behind it that it sped up the rotation of the earth by three microseconds.
We have actually changed the Earth’s rotation too, although somewhat by accident. Within the last century there has been a massive increase in dam construction, both to sate the world’s demand for cheaper electrical power and also to create emergency water supplies. Most of this construction has occurred in developing countries such as China and the former Soviet Union territories and some of it has been quite impressive. China’s Three Gorges Dam holds back nine cubic miles of water that weigh about 40 billion tons. And this is one of nearly ninety large dams built since the 1950’s.
So we’ve got ourselves an interesting little phenomenon. Billions and billions of tons of what had been free-flowing water is now sequestered in set locations around the middle latitudes, moving it away from the equator. Furthermore, most of the dams are at high altitudes, not sea level. This shift of mass towards the axis has fundamentally changed the wobble of the earth. It’s not a drastic shift, something on the order of one tenth of a microsecond, but it has been proved that we, in our hubris, have the power to change even the motion of the earth itself.