地球的自转周期并不完全相同；科学家们几十年来一直在记录这些微小的变化。这些变化非常微小（有时仅持续几毫秒），几乎难以察觉，但科学家们非常重视它们，因为它们暗示着一个更为重大的变化：地球的自转速度正在发生变化。
Earth's days are not exactly the same length. Scientists have been recording small variations in for decades. These changes are minuscule—sometimes just a few milliseconds—and mostly imperceptible, but scientists are keen to understand them as they point to a much more monumental change: Our planet’s rotation speed is shifting.
科学家们已经找出了导致这些变化的一些原因：大气和海洋的波动会略微加快或减慢地球的自转速度；大地震会重新分配地球的部分质量；而大陆冰层融化后向海洋中注入大量水，也是影响地球自转速度的一个因素。
Scientists have already pinpointed a handful of compounding causes. Fluctuations in the atmosphere and oceans can slightly speed up or slow down the Earth’s rotation; major earthquakes can redistribute part of the planet’s mass; and by depositing enormous amounts of water into the oceans, melting continental ice is also a factor.
这些因素可以解释一些较为突然和近期的自转变化，但在几十年的时间尺度上，一些自转速度的变化似乎源自地球内部更深处。在地球的岩石地幔之下，存在着外核——这是一层厚度达1400英里（约2250公里）的超高温液态金属层，它始终处于运动状态。当这些液态金属的流动发生变化时，它们会略微影响地球整体的自转速度。
These influences account for some of the more sudden and recent changes, but on a several decades-long timescale, some of the variation appears to originate from a source much deeper within the Earth. Beneath our planet’s rocky mantle lies the outer core—a 1,400 mile thick layer of ultra-hot liquid metal that is in constant motion. As these flows change, they can slightly alter the speed at which the rest of the planet rotates.
然而，这些力量是如何从一个区域传递到另一个区域的，至今仍是个谜。地核与地幔之间的摩擦力实在太弱了，因此地质学家一直在寻找地球内部其他可能影响自转速度的机制。
However, how these forces are transmitted from one region to another has remained a mystery. Friction between the core and the mantle is too weak, so geologists have been searching for other mechanisms operating in the Earth’s interior.
在本周发表于《自然》杂志（Nature）的一项新研究中，研究人员认为他们已经找到了其中一个潜在的机制：内核的引力会促使地球自转加速，而地核与地幔之间的其他相互作用则会阻碍这种加速。这种相互作用的结果，经过数十年的累积，最终会导致地球自转速度的微小变化（即加速或减速）。
In a new study published this week in Nature, researchers believe they have identified one of these hidden gears. They propose that the gravity of the inner core pushes rotation in one direction, while other interactions between the core and the mantle oppose it. The result of this competition, accumulated over decades, would be small accelerations and decelerations in the planet’s spin.
这一想法基于这样一个事实：内核的旋转速率与地球其他部分不同，导致其与地幔中某些质量不规则性略微错位。但内核“想要对齐”，参与该研究的阿尔伯塔大学地球物理学家马修·邓伯里在新闻稿中表示。这意味着引力试图将地球内部各层拉回到更和谐的位置，从而影响——尽管极其微小——地球表面的旋转。
The idea is based on the fact that the inner core does not rotate at the same rate as the rest of the planet, causing it to become slightly misaligned with certain mass irregularities in the mantle. But the core “wants to be aligned,” Mathieu Dumberry, a geophysicist at the University of Alberta involved in the study, said in a press release. What this means is that gravity tries to pull the layers of the inner Earth back into their more harmonic position, which influences—albeit minutely—the rotation of the planet’s surface.
作者利用内核旋转的地震估算和外核液态金属流动模型，重建了这种相互作用在1964年至2019年间如何影响地球自转。他们的引力模型非常接近地复现了几十年来观测到的真实渐变变化的时间和幅度。
The authors reconstructed how this interaction influenced Earth’s spin between 1964 and 2019 using seismic estimates of the inner core’s rotation and models of liquid metal flows in the outer core. Their gravitational model closely reproduced both the timing and the magnitude of real gradual changes that had been observed over the decades.
其他尚未知的因素很可能也参与其中，但这一假说揭示了我们多变的昼夜长度的一个主要成分。该模型也未计及所有相互竞争的力量，但这并不妨碍它得出令人信服的结果。邓伯里表示，因为这可能是一场拔河比赛，但“引力依然获胜。”本文原刊登于《WIRED en Español》，并从西班牙语翻译而来。
Other yet-unknown factors are likely involved, but this proposal unravels a major component of our variable days. The model also didn’t account for all the forces competing involved, but it didn’t need to to produce a compelling result. Because it might be a tug-of-war, Dumberry said, but “gravity still wins.” This story originally appeared in WIRED en Español and has been translated from Spanish.