Explained | Earth's record of shortest rotation and its impact

Negative leap second, proposed to compensate for faster planetary spin in the UTC, could wreak havoc on IT systems and software that rely on schedulers

Planet Earth recently hit a new record - no, not just reaching overshoot day, effectively putting planetary resources at a deficit - but rather of having its shortest day thus far.

The planet, whose rotation is monitored by high-accuracy atomic clocks, took 1.59 milliseconds less than 24 hours to complete one rotation on June 29, followed by another near-record of 1.5 milliseconds on July 26.

Now, a millisecond doesn't seem like a lot in 24 hours, which has 86,400 seconds and consequentially, 86.4 million milliseconds. However, the reduction in time taken for Earth's rotations to complete indicates that the planet has sped up, if only a tiny bit.

The planet has seen 28 shortest days ever since atomic clocks, high-accuracy clocks that measure time by monitoring radiation of atoms, were introduced in the 1960s. In 2020, the shortest day recorded was 1.47 milliseconds less than 24 hours on July 19, while it fluctuated in 2021.

Scientists have speculated various possible causes for this differing in time, including melting and refreezing of polar and mountainous ice caps, Earth's climate, or the "Chandler wobble," a deviation of the planet's axis of rotation, which can be represented by the subtle wobbling seen on a toy as it spins.

To compensate for the change in rotational speeds, scientists monitoring atomic clocks have used the concept of 'leap seconds' to ensure the Universal Coordinated Time, the time standard used to adjust time zones around the world. It essentially involves adding, or 'leaping' 1 second in the UTC.

The leap second concept was first introduced in 1972 by the International Earth Rotation and Reference Systems Service (IERS) in an attempt to periodically update the UTC due to imprecise observed solar time (UT1) and the long-term slowdown in the Earth’s rotation.

But scientists are proposing a negative leap second to compensate for the faster spin of the Earth, which could potentially wreak havoc on IT systems around the world.

In the current system, IT clocks update from 23:59:59 to 23:59:60 before resetting to 00:00:00, which itself is problematic due to potential of data corruption and crashes due to timestamps on data storage, according to a blog post by Meta, but introducing a negative leap second could cause the timestamp to go from 23:59:58 directly to 00:00:00, skipping an entire second negatively.

“The impact of a negative leap second has never been tested on a large scale; it could have a devastating effect on the software relying on timers or schedulers,” the blog post stated.

“In any case, every leap second is a major source of pain for people who manage hardware infrastructures.”