Today, official time is kept by atomic clocks – and technologies such as the Internet, positioning systems and mobile-phone networks depend on the clocks’ extraordinarily accurate time signals.
These atomic clocks define the second in terms of the frequency of light that is involved in a specific transition in atomic caesium. The definition was chosen so that 86,400 atomic seconds corresponds very closely to the length of a day on Earth – which is the traditional definition of the second.
However, the correspondence is not exact. Between 1970 and 2020, the average length of a day on Earth (the period of Earth’s rotation) was about 1–2 ms longer than 86,400 s. This means that every few years, a second-long discrepancy builds up between time as measured by Earth’s rotation and time measured by an atomic clock.
Since 1972 this deviation has been corrected by the insertion of 27 leap seconds into co-ordinated universal time (UTC).
Complicated process
This correction process is complicated by the fact that various factors cause Earth’s period to vary on a number of different time scales. So leap seconds are inserted when needed – not according to a regular schedule like leap years. Nine leap seconds were inserted in 1972–1979, for example, but none have been inserted since 2016.
Indeed, since about 2020 Earth’s average period has dipped below 86,400 s. In other words, Earth’s rotation appears to be speeding up. This bucks the long-term trend of the rotation slowing, and is probably related to interactions deep within the Earth. As a result, metrologists face the unprecedented prospect of “negative leap seconds” – which could be even more disruptive to computer systems than leap seconds.
Time traders
But now, Duncan Agnew of the Scripps Institution of Oceanography and the University of California, San Diego has identified a new process that may be countering this increase in rotational speed – something that could postpone the need for negative leap seconds.
Writing in Nature, he shows that the increased melting of ice in Greenland and Antarctica is decreasing the Earth’s angular velocity. This is because water from the poles is being redistributed throughout the oceans, thereby changing our planet’s moment of inertia. Because angular momentum is conserved, this change results in a decrease in angular velocity – think of a spinning ice skater who slows down by extending their arms.
Agnew reckons that this will postpone the need for a negative leap second by three years. A negative leap second could be needed in 2029, but it could be one of the last because metrologists have voted to get rid of the leap-second correction in 2035.
