NIGHT and day are at the mercy of Earth鈥檚 twisted heart. Fluctuations at the core subtly shift the planet鈥檚 rotation over a six-year cycle.
of the University of Joseph Fourier in Grenoble, France, and colleagues, modelled fluid behaviour in the Earth鈥檚 core based on measurements of fluctuations in the magnetic field.
The innermost region of the Earth鈥檚 outer core periodically flows faster or slower, and this action 鈥渢ugs鈥� at the planet鈥檚 magnetic field, says Gillet. Like an array of rubber bands, the field then pulls the region back towards its original position. But the effect ripples outward, changing the core鈥檚 rate of rotation layer by layer.
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The researchers calculated how this would affect the rotation rate of the whole planet, which would compensate to conserve angular momentum. They found the day length varies by 0.4 milliseconds over a six-year period (Nature, ). This fits with day-length measurements taken between 1925 and 1997.
The twisting had previously been blamed for a 60-year cycle in day length, but the rate of rotation found in this study suggests it is not the culprit, says Jon Mound , UK. 鈥淭hings we believed for 40 years, we don鈥檛 actually understand.鈥�
