估算显示三峡大坝确实让地球自转变慢，但量级极小。大坝将巨量水体抬升并固定在距离自转轴更远的位置，增加地球整体转动惯量，从而在角动量守恒下导致角速度下降。依据三峡库区质量、海拔差、纬度等参数，计算得自转周期延长约 5.3×10⁻⁶ 秒，即约 5.3 微秒，属真实但微弱的可测量效应。由于采用了理想化假设，该值可能略为高估，但数量级判断成立。

原理基于 I 增加导致 ω 降低的比例关系，与“旋转椅伸手变慢”相同。影响幅度取决于库水质量与其到地轴的距离，因而接近赤道的高位水体对日长影响最大。地球作为近似孤立体系，其角动量保持不变，故任何大尺度质量重新分布（如筑坝、地质活动、冰盖变化）都会引起日长的微小漂移。

人类活动对自转的其他潜在影响在原理上类似，包括大规模建造高层建筑、统一抬高地表质量或让全球人口同时站立，皆会增加转动惯量，但变化量均远低于微秒级。该案例主要展示了行星尺度宏观物理的灵敏性，而非提供可用的“延长白天”方案。

Calculations indicate the Three Gorges Dam does slow Earth’s rotation, though by an extremely small amount. By raising and holding a large water mass farther from the rotation axis, the dam increases Earth’s total moment of inertia, which—under conservation of angular momentum—reduces angular velocity. Using reservoir mass, elevation change and latitude, the resulting increase in day length is about 5.3×10⁻⁶ seconds, or roughly 5.3 microseconds. This likely overestimates the effect slightly, but the magnitude is correct: real yet negligible.

The mechanism follows the proportional relationship in which increasing I lowers ω, analogous to extending one’s arms on a spinning chair. The effect scales with how much mass is displaced and how far that mass is from the axis; high-altitude water nearer the equator produces the largest change. Because Earth behaves as a near-isolated system, any major mass redistribution (dams, geological shifts, ice loss) yields slight drifts in day length.

Other human activities could in principle alter rotation similarly—large skyscraper networks, elevating surface mass, or even everyone standing simultaneously—but all would generate changes far below the microsecond scale. The case mainly illustrates the sensitivity of planetary-scale rotational physics rather than offering a practical method to “lengthen the day.”