The Length of a Day Oscillates Every 6 Years

It's possible that our understanding of our planet's core has to be revised.

According to new data, the solid inner core oscillates, spinning first in one direction with regard to the surface far above, then the other, changing direction every six years.

This not only has consequences for our knowledge of the inner workings of our home planet, but it also neatly explains a riddle that has confused scientists for some time: a 5.8-year cyclical change in the length of Earth's day.

"From our findings, we can see the Earth's surface shifts compared to its inner core, as people have asserted for 20 years," said geophysicist John E. Vidale of the University of Southern California, Los Angeles (UCLA).

"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."

Although our knowledge of the Earth's core has advanced significantly in recent decades, there is still much we don't know. We can't directly travel there and look around; all we know comes from indirect observations, such as seismic waves spreading and bouncing around the globe.

However, this is still a powerful instrument. Scientists have determined that Earth's inner core is most likely a hot, compact ball of solid iron with a diameter of 2,440 kilometers (1,516 miles), somewhat larger than Pluto. It also appears to have superrotation, which means it rotates faster than Earth.

Researchers originally documented this phenomena in 1996, estimating a 1 degree per year superrotation rate. Using data from underground nuclear tests conducted at the Russian Novaya Zemlya testing site in the 1970s, Vidale and his UCLA colleague Wei Wang recalculated the pace down to 0.29 degrees per year.

They traveled back in time for the current study, adding two experiments done beneath Amchitka Island in 1971 and 1969. And this showed an oddity. The findings indicated that, rather than superrotating, Earth's inner core was subrotating, or spinning at a slower rate than Earth's rotation, by around 0.1 degrees per year.

The researchers concluded that this was compatible with oscillation. The inner core superrotates when it is fully spinning, but then slows down before speeding up again.

"The idea the inner core oscillates was a model that was out there, but the community has been split on whether it was viable," Vidale added.

"We went into this expecting to see the same rotation direction and rate in the earlier pair of atomic tests, but instead we saw the opposite. We were quite surprised to find that it was moving in the other direction."

The oscillation's six-year regularity fits other oscillations for which we have no established explanation.

Every six years or so, Earth's days experience temporal changes of plus or minus 0.2 seconds, and the magnetic field oscillates with a six-year cycle. They fit the periodicity of the model Vidale and Wang developed for the oscillations of Earth's inner core in magnitude and phase.

This will need additional data, which might be difficult to come by. The US Air Force's Large Aperture Seismic Array, which captured data from nuclear tests, ended in 1978, and underground nuclear testing is no longer as common as it once was.

However, developments in sensor technology may imply that the comprehensive data needed to explore Earth's inner core isn't so far away; the results thus far suggest that Earth's inside are a little more complex than we thought. 

"The inner core is not fixed – it's moving under our feet, and it seems to [be] going back and forth a couple of kilometers every six years," Vidale explained.

"One of the questions we tried to answer is, does the inner core progressively move, or is it mostly locked compared to everything else in the long term? We're trying to understand how the inner core formed and how it moves over time – this is an important step in better understanding this process."