An international group of scientists, diplomats and timekeepers voted for abolish the leap second, used for over 50 years to maintain aligned The Universal Coordinated Time (UTC), i.e. the time scale on which clocks around the world are based, with the astronomical timebased on the speed of rotation of the Earth.
Until now the legislation established that, as soon as astronomical time “was delaying” Of at least 1 second compared to civil time, a was introduced leap second (in English leap second), that is, a minute of 61 seconds that realigned the two time counts. The preferential “slots” were two per year: June 30th and December 31st, always at 11.59pm. The last leap second was introduced on December 31, 2016.
Starting from May 20, 2027therefore, UTC will become a continuous time scale. But why is this decision coming now?
There are mainly two reasons: leap seconds, which are unpredictable by nature, create first and foremost problems increasingly greater than digital infrastructuresto the navigation systems satellite and telecommunications, which depend on a continuous and precise flow of time. The second motivation, however, is more urgent: the Earth’s rotation is accelerating and, for the first time in history, a leap second negative (i.e. removing a second instead of adding it), an untested operation, with some engineers comparing it to the millennium bug.
What is the leap second, what was it for and why was it abolished
To better understand the reasons for this decision, we must know that there are two different ways of measuring time.
The first is the astronomical timewhich is based on the rotation of the Earth on its axis. In the past the Sun was used as a reference, while today it is defined through the relative earth rotation to distant objects like quasars or galaxies. Today this time is officially known as UT1 (Universal Time 1).
The second, however, is the atomic time. In 1955, Britain’s National Physical Laboratory put the first cesium atomic clock into operation. These instruments measure time based on the resonance frequency of the cesium-133 atom and, over the decades, their precision has grown enormously: the most advanced models in use today have a margin of error of approx a second in the span of 138 million years.
The problem is that the rotation of the Earth is irregular: slow down gradually due to the gravitational attraction of the Moon, but also undergoes additional variations due toseismic activityto the interaction between the Earth’s core and mantle, to the redistribution of mass due for example to the melting of ice and the variation in atmospheric patterns. In practice, therefore, this means that time measured by atomic clocks and time based on the Earth’s rotation are they progressively misalign.
This is why, to reconcile these two time scales, the Universal Coordinated Time (UTC), then officially recognized as a standard in 1963. At this point, whenever the difference between astronomical time and universal coordinated time exceeded 0.9 seconds, the International Earth Rotation and Reference Systems Service (IERS) announced the inclusion of a leap second, with approximately six months’ notice. Midnight, instead of taking place after 11.59.59pm, arrived after 11.59.60pm. In practice, a minute of 61 seconds.
They have been added from 1972 to today 27 leap secondsthe last of which was on 31 December 2016. Today, however, scientists have decided to abolish this mechanism, for two main reasons.
The first concerns the impact on modern technology: i computer systems more advanced, in fact, they have need of a continuous flow of time and predictable, and the inclusion of a leap second (however rare and brief) has caused real problems over the years. For example, platforms like Reddit And Cloudflare they suffered interruptions of service, some airlines have had difficulty planning flights and high-frequency financial transactions have been complicated.
The second reason instead has to do with the earth rotation which, since 2016, has started to accelerate. Expert projections indicated an increasing probability of having to apply for the first time negative leap second (i.e. subtract a second, rather than add it). The Advisory Committee on Time and Frequencies (CCTF) estimated that this chance would have reached 30% by 2035.
The problem is that a negative second it was never implemented in the history of the UTC: no system has been designed or tested to handle it and the necessary preparation would require investments comparable to those incurred for the so-called “millennium bug”.
What changes now: from leap second at the intercalary hour
At this point the question is: will this misalignment no longer be correct?
The General Congress of Weights and Measures (CGPM) has established that, starting from May 20, 2027, UTC will become a continuous time scale. Simply put, the new upper limit for the difference between astronomical time (UT1) and Coordinated Universal Time (UTC) has been raised to 3,600 seconds, or one hour. In short, we have gone from the “leap second” to what one might define “leap hour”: instead of correcting the difference one second at a time, the difference will be allowed to accumulate slowly over centuries, until it reaches a maximum of one hour.
And how long will it take for this gap to accumulate? According to scientists’ estimates, a leap hour will not be needed for over a thousand years. The text of the BIPM resolution speaks of “long-term continuity guaranteed for several centuriesThis means, in practical terms, that the abolition of the leap second solves the problem for the extremely distant future.
In fact, for people’s daily lives nothing will changegiven that the deviation between UTC and UT1, which accumulates extremely slowly, will remain completely imperceptible over the course of an entire human lifetime.
Who will need to know the astronomical time with precision (like astronomers to point telescopes) will be able to continue doing it. The resolution requires the BIPM to collaborate with the IERS and other bodies to continue to monitor and disseminate the updated value of the UT1-UTC difference, available through the navigation messages of the GNSS (Global Navigation Satellite Systems) systems and the IERS internet services.
