Recently, the Earth has been exhibiting a bit of a childish temper tantrum, eagerly wishing to "grow up" just like we did in our childhood.

On June 29th, the Earth's rotation was 1.59 milliseconds shorter than the standard 24 hours, making it the shortest day recorded since humans began using atomic clocks.

July 26th saw a similarly brief day, faster by 1.5 milliseconds. This acceleration in the Earth's rotation suggests that days are getting shorter.

As a member of the vast cosmos, Earth is subject to the gravitational forces exerted by celestial bodies, causing constant adjustments in its rotation and orbit. Currently, the Earth's rotation is accelerating, leading to shorter days. Scientists attribute this acceleration to the Chandler wobble, as reported in the Annual Meeting of the Asia-Oceania Geosciences Society.

But what's the principle behind atomic clock timekeeping? Is the Earth's rotation indeed accelerating? There are three standard time systems: Universal Time (UT), based on Earth's rotation; Ephemeris Time (ET), based on Earth's orbit around the Sun; and Atomic Time (AT), based on the frequency of atomic oscillations.

The development of time systems has transitioned from astronomical to atomic standards. Astronomical time is derived from the periodic movements of celestial bodies like the Sun, Moon, and stars, which includes UT and ET. Nuclear time is based on the precise electromagnetic waves emitted or absorbed during atomic transitions facilitated by nuclear clocks.

Atomic time emerged in the late 1940s and early 1950s, with the definition of the second transitioning from astronomical to atomic standards in 1967. The International Committee for Weights and Measures defined the second as the duration of 9,192,631,770 periods of radiation corresponding to the transition between two hyperfine levels of the ground state of the cesium-133 atom.

This marked the advent of atomic seconds, with January 1st, 1958, as the starting point for nuclear time.

The relationship between UT and Earth's rotation is intimate: if Earth's rotation speeds up, so does UT, and vice versa. Consequently, the discrepancy between atomic time and UT grows over time.

Currently, Coordinated Universal Time (UTC), based on atomic time, is the internationally recognized standard. Leap seconds are periodically added or subtracted to keep UTC in sync with UT. When the accumulated difference between atomic time and UT reaches 0.9 seconds, a leap-second adjustment is made to UTC.

The average duration of Earth's rotation is 23 hours, 56 minutes, and 4 seconds. Studies suggest that its rotation has gradually slowed since Earth's formation, with a predicted increase of half an hour in a day's length in the next billion years. However, over shorter timescales, such as tens or hundreds of thousands of years, factors like the weakening of lunar tidal forces will slow Earth's rotation further.