Earth's deep secret: Why your day isn't quite 24 hours
You might think the length of a day never varies—exactly 24 hours. You'd be wrong, at least if you count days in milliseconds, as physicists do.
Earth's day length is not always exactly 24 hours, as physicists measure time in milliseconds. For about 30 years, scientists have observed that the Earth's liquid core rotation speeds up for a few decades, then slows down over another few decades. However, the mantle, a 3,000-kilometer thick rocky shell, also changes its rotational speed to compensate, as the planet's angular momentum must stay constant.
This alteration in the mantle's rotational speed can alter the length of a day by a few milliseconds over time. The exact mechanism of how the core and mantle exchange angular momentum has been a mystery, but a new study in Nature reveals that small changes in the rotation of the inner core, the solid central part of the core, can exert a gravitational torque on mass anomalies within the mantle, causing slight fluctuations in the length of a day.
Another form of torque, called core-mantle boundary torque, arises at the boundary between the core and mantle and resists the gravitational pull, limiting the changes in the length of a day. The balance between these gravitational and core-mantle torques ultimately explains the observed variations in day length. This research also highlights the viscous deformation of the inner core over a time scale of about 10 years.
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