Can a Hydroelectric Dam Really Make the Days Longer?

"To answer these questions, we need three basic physics ideas: (1) angular velocity, (2) angular momentum, and (3) the moment of inertia. I'll explain each of these. First, here's a little experiment that you can do yourself. If you have a rotating desk chair, get it spinning with your feet off the ground and your arms tucked in. Now, before you throw up, extend your arms. It slows you down, right? Here's a version with a human on a rotating platform:"

"The speed of rotation is what we call angular velocity, represented by the Greek letter omega ( ω). Why "angular"? Here's a two-dimensional example below. Think of this circle as centered on an xy coordinate plane. I've drawn a radius ( R) to a point on the circle where it cuts the horizontal axis: Now imagine this point moving counterclockwise around the circle. As it does, the radius line sweeps around like the second hand on a (backward) clock-and at any point in time,"

Construction of the Three Gorges Dam moved large volumes of water to higher elevation behind the reservoir. Shifting mass outward from Earth's axis increases the planet's moment of inertia. Conservation of angular momentum requires a compensating decrease in angular velocity when the moment of inertia increases. A lower angular velocity corresponds to a slower rotation and thus longer days. Simple rotating-chair experiments illustrate that extending mass away from the axis slows rotation. The actual change in day length from the dam is extremely small, measurable in microseconds. Building additional large reservoirs would similarly alter Earth's rotation, but the cumulative time gains would remain negligible for daily life.

Read at WIRED