Earth's days are not a constant 24 hours, according to a WIRED report on a new study in Nature. Scientists record differences of a few milliseconds, and while imperceptible, they reflect real shifts in the planet's rotation. Known causes include atmospheric and oceanic fluctuations, major earthquakes, and meltwater from continental ice entering the oceans.
The new research focuses on a deeper cause. Beneath the mantle, a 1,400-mile-thick outer core of liquid metal moves constantly, and changes in those flows can alter the rotation of the surface. The authors propose that gravitational alignment between the inner core and mass irregularities in the mantle creates a tug-of-war, with gravity pulling the inner Earth back into position and minutely speeding or slowing the planet's spin.
Using seismic estimates of inner-core rotation and models of outer-core flows, the team reconstructed spin changes from 1964 to 2019. Their model matched both the timing and magnitude of observed gradual day-length changes. The single source notes that other unknown factors are likely involved, but says the proposal accounts for a major component of variable day length.