Scientists may have finally found the mystery behind the length of days changing. Our units of time are not really as constant as we might think them to be. The length of a day depends on how long it takes the Earth to spin on its axis. But there are several other things that affect how long it takes our planet to do that. Starting from gravitational interactions with other planets, activity in the oceans and the atmosphere, to the exchange of angular momentum between the Earth’s core and its mantle, all of this affects how long it takes the Earth to complete its rotation.
In recent research, however, what scientists did not expect to find was that the length of a day oscillates back and forth. New evidence has suggested that instead of consistently rotating faster than Earth’s spin, the solid inner core oscillates – spinning first in one direction with respect to the surface far above, then the other, changing direction every six years. And this oscillation of the inner core is also the reason for shifts in the length of a day, according to the same six-year cycle.
Recently, geophysicists Wei Wang and John Vidale from the University of Southern California unearthed a clue about why this might occur when they looked at old seismic data from underground Soviet nuclear tests. The data revealed strange workings in the Earth’s inner core. “From our findings, we can see the Earth’s surface shifts compared to its inner core, as people have asserted for 20 years,” said Vidale. “However, our latest observations show that the inner core spun slightly slower from 1969-71 and then moved the other direction from 1971-74. We also note that the length of a day grew and shrank as would be predicted. The coincidence of those two observations makes oscillation the likely interpretation.” In earlier research from 2013, scientists discovered a six-year cycle in the length of the day, increasing and decreasing by just over 0.1 milliseconds. A fraction of a millisecond may not seem like much, but that had led scientists to give this more thought. Earth’s days undergo time variations of more or less 0.2 seconds approximately every six years or so, too, and Earth’s magnetic field also oscillates over a six-year period. In amplitude and phase, they match the periodicity of the model Vidale and Wang derived for the oscillations of Earth’s inner core.
The new evidence about the inner core’s oscillation came as a total surprise. “We expected to see fairly steady rotation, maybe changing slowly over decades, not a reversal of motion,” Vidale said. Previously, scientists believed that the Earth’s inner core rotated slightly faster than the crust. “The oscillation [we see] resembles a pendulum,” Vidale explained. “It goes one way for a while, then reverses, repeating the cycle for many iterations. There is a preferred position for the inner core, a gravitational well, and it oscillates about that position.” This oscillation has a period of about six years and it may explain why the length of the day changes slightly over this time period.
In order to understand this rotation reversal, the scientists have come up with a simple model that includes the gravitational interaction between the mantle and the core, as well as the topographies where the core meets the mantle, and where the inner core meets the outer core. If the Earth’s inner core rotates at a different speed than the mantle and crust, there is an exchange of angular momentum between the two. The “differential rotation angle,” as it is called in scientific terms, corresponds to the change in length of day.
While the scientists have mentioned that this is not the only possible explanation, they added that the reversal of direction does indeed help to explain the changes in time.
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