Imagine the water, clouds, and other gases lying on top of the Earth’s crust as a big soupy mass that can shift around as the planet rotates on its axis and is tugged on by the Moon, Sun, and other planets. The oceans and atmosphere slosh around due to tidal action, bulging a bit on one side or the other, and this can hamper or speed up the planet’s motion. In comparison to the total weight of our planet, the liquids and gases are fairly light, but the inertia they experience does, in fact, change Earth’s speed. The amount of change is not noticeable to us: a few thousandths of a second each year. Over millions of years, however, this has a cumulative effect.