Can a Hydroelectric Dam Really Make the Days Longer?

The article explains that the construction of China’s Three Gorges Dam has caused a measurable slowing of the Earth's rotation, effectively making the days slightly longer. This phenomenon occurs because the dam shifts a massive amount of water to a higher elevation, altering the distribution of Earth's mass. This change affects the planet's moment of inertia—a measure of how mass is distributed relative to the axis of rotation—and, due to the conservation of angular momentum, results in a decrease in the Earth's angular velocity, or rotational speed. To understand this, the article introduces key physics concepts: angular velocity (the rate of rotation), moment of inertia (resistance to rotational change depending on mass distribution), and angular momentum (the product of angular velocity and moment of inertia). When mass moves farther from the axis of rotation, the moment of inertia increases, causing the rotation to slow down if angular momentum is conserved. In the case of the Three Gorges Dam, the large volume of water held at a higher elevation increases Earth's moment of inertia