Earth’s inner core oscillates, gently swaying and swirling from one direction to another in a six-year cycle that even affects the length of a day on our planet, new research suggests.

As reported in the journal Science Advances, this new theory of Earth’s inner workings contradicts previous ideas about the planet’s innermost geological layer rotating alongside the rest of the planet at a slightly faster rate than the surface.

Earth's inner core is a red-hot ball of dense solid iron surrounded by a liquid outer core, then the mantle and the crust. 

In the 1990s, research suggested that Earth’s inner corner undergoes a super-rotation, in which the inner core rotates slightly faster than the rest of the planet. This theory was backed up by work that studied waves generated from underground nuclear bomb tests carried out by the USSR between 1971 and 1974 in the Novaya Zemlya archipelago of northern Russia.

In the latest study, scientists from the University of Southern California (USC) used the same methodology to study a pair of earlier underground atomic tests beneath Amchitka Island at the tip of the Alaskan archipelago in 1969 and 1971.

Surprisingly, the results suggested that the inner core was moving slowly in a different direction between 1969 and 1971, sub-rotating at least a tenth of a degree per year, compared to the direction it was moving between 1971 and 1974.

“From our findings, we can see the Earth’s surface shifts compared to its inner core, as people have asserted for 20 years,” John E Vidale, study co-author and Dean’s Professor of Earth Sciences at USC Dornsife College of Letters, Arts and Sciences, said in a press release. “However, our latest observations show that the inner core spun slightly slower from 1969-71 and then moved the other direction from 1971-74.”

“The inner core is not fixed — it’s moving under our feet, and it seems to going back and forth a couple of kilometers every six years,” Vidale added. 

The inner workings of our planet also have a small but significant effect on the length of our days. We often think of Earth’s days as constant, but they can be subject to a fair amount of variation. Around 300 million years ago, for instance, a day on Earth would last about 21 hours. It can even vary within the space of a decade. 

The length of a day is dictated by the speed at which the planet rotates. This can be influenced by a number of factors, but it’s believed to be linked to changes in the Earth’s magnetic field, generated in the Earth’s core.

The new study notes that the length of days grew and shrank as they expected it to, plus or minus 0.2 seconds over six years, based on the changes within the inner core. 

Many questions remain, however. Much of this research is made possible through data revealed by underground nuclear tests. Since the number of nuclear tests has significantly dropped since the Cold War, this data has become increasingly scarce and scientists are forced to work with earthquake data, which is comparatively inaccurate. 

Nevertheless, the researchers are keen to dig deeper into the mysteries of Earth's inner architecture to better understand how and why the inner core appears to behave in such a strange manner. 

“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,” concluded Vidale.