Throughout its history, humanity has measured the passage of time based on the rotation of Earth. However, in 1967 this same humanity—or rather the small group of people responsible for measuring and disseminating the passage of time, the metrologists—took the “revolutionary” decision to change the clock model, replacing the traditional Earth-based clock with a modern atomic clock and changing the classical definition of a second to one based on the transition between two energy levels of an electron in an atom. The reason behind this decision lies in the fact that atomic clocks are much more stable than the Earth-based one, as the speed of the planet’s rotation depends on various factors that can change over time.
Brainteaser 1
For reasons of logistics, history, romance and even biology—we evolved to function at the rhythm set by sunlight—it was decided to adjust both times whenever the difference between Coordinated Universal Time (UTC), determined by a constellation of atomic clocks distributed around the planet, and Universal Time (UT1), defined by Earth’s rotation, reached one second.
Since 1972, this has meant that the two time measurement standards have had to be synchronised, and on 27 occasions a leap second has had to be added to atomic time because of Earth’s tendency to fall behind. But in an increasingly interconnected and computer-dependent world, this is a headache because of the uncertainty about how the addition of this extra second will affect computer systems. So much so that in 2022, metrologists decided to widen the gap between the two standards and only synchronise them when the difference is close to one minute. The change will take effect on or before 2035.
Brainteaser 2
The speed of Earth’s rotation depends on many factors, but three are particularly important. First, the friction between Earth’s crust and the mass of water that covers it has slowed down the rotation. The second is Earth’s shape, which has been changing since the last ice age, becoming increasingly spherical and causing a redistribution of the moment of inertia, which increases the rotation rate. Finally, there is the rotation of Earth’s core. The overall effect is that the Earth’s rotation period is increasing at a rate of 6 millionths of a second per year.
Or at least this was the case until recently. Since 1972, and after some frenetic first years, the need to add seconds to atomic time has become less and less frequent. This indicates that the speed of Earth’s rotation has begun to increase, mainly due to the behaviour of the core.
Brainteaser 3: A clock running ahead of UTC
A clock that runs at a uniform rate shows a delay of 6 minutes with respect to UTC at 17:00 on Monday. That same clock on the following Monday shows 9:10 and 40 seconds, when it should be 9:00. When did the clock give the correct time?
And this is a real problem, because it means that sooner rather than later Earth’s rotation period will be less than 24 hours, and the adjustment will have to be made in the opposite direction: instead of adding an extra second to atomic time, it will have to be subtracted from it. This is an unprecedented change and raises many uncertainties about how it will affect the IT infrastructure.
It is precisely in this uncertain scenario that a recent study has discovered that there is a new factor at play: the accelerated melting of the polar ice caps due to human activity, and the resulting global warming, is causing large masses of water to move from the poles to the equator, a process that is slowing down the speed of Earth’s rotation. There are two sides to this discovery, one negative and one positive. The downside is that it is a new variable to take into account. The positive side is that by slowing down the rotational speed, it partially counteracts the increase caused by the variation in the core. And it gives us more time to prepare for this unprecedented adjustment.
Miguel Barral
Comments on this publication