Science Focus
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If the length of a day changed by fractions of a millisecond, it probably wouldn’t make you late for work—that’s still on you. But the speed of Earth’s rotation does very subtly change over time, influenced by a number of factors. Some of these are geological, as you might expect. But the Earth's rotation is also influenced by something we don't think of as being that substantial: the weather.
We’ve all seen an Olympic figure skater begin spinning, tuck his or her arms in, and accelerate to a nauseating velocity. This is conservation of momentum. The farther the skater's mass is spread from the axis of rotation, the greater the moment of inertia, and the more angular momentum they need to spin at a given rate. Pulling in your arms reduces the moment of inertia but not the angular momentum—the result is an increased rate of rotation.
The same thing can happen (with far less dramatic results, thankfully) when the mass of the Earth is redistributed, as happens when the crust moves during a particularly large earthquake. On longer time scales, the motion of tectonic plates, circulation in the mantle, or circulation in the outer core can also affect Earth’s moment of inertia.
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