Atomic clocks, combined with precise astronomical measurements, have revealed that the length of a day on Earth is suddenly getting longer, and scientists don’t know why. If in recent decades the rotation of the Earth has accelerated, since 2020 this constant acceleration has curiously turned into a slowdown: the days are getting longer again and the reason remains a mystery until now.
Two researchers from the University of Tasmania, Australia, claim in a recently published article in The Conversation that the Earth has passed through the past two years a process of deceleration of its rotation, resulting in longer days. However, the acceleration that had been seen over the past twenty years led to the recording of the shortest day on June 29, 2022. Now the trend has reversed and promises to intensify in the long term.
According to scientists, this change in trend unprecedented in the last 50 years: If it were to increase, it could jeopardize key systems for the daily functioning of our planet, such as GPS navigation or the Internet. What is the cause of this variation, which takes us from the shortest days on record to a further lengthening of their duration?
Every day is not 24 hours
Although our smartphone clock tells us that each day has exactly 24 hours, in reality each day does not last exactly 86,400 seconds. Slight changes lead the Earth to change the time it takes to complete a single rotation, a rotation around its own axis. Variations occur over periods of millions of years or almost instantaneously: phenomena such as earthquakes and large storms can play an important role in these changes.
In fact, any small change in the masses that affect the interior and surface of the Earth produces variations in the exact time of rotation. If we look at a ballet dancer, we can see that she can spin faster when she brings her arms back to her body, the axis around which she spins. Something similar happens with the Earth: the Mass distribution affecting it increases or decreases its rotation speed.
The connection between the interior and the surface of the Earth also comes into play: the earthquake of greater impact can change the length of the day, although usually small amounts. For example, the Great Tohoku earthquake in Japan in 2011, which reached a magnitude of 8.9 on the Richter seismological scale, accelerated the Earth’s rotation by about 1.8 microseconds. It seems insignificant for our vision, but for the planet, it is not.
Along the same lines, the fortnightly and monthly cycles of the tides they move masses around the planet, causing changes in day length of up to a millisecond in either direction. The movement of our atmosphere and the ocean currents also play an important role, even the seasonal layers of snowfinally rainy or the extraction of underground water can change the length of the day.
A sudden and inexplicable turn
Since around the 1960s, radio telescopes around the planet have begun to get very accurate estimates of the Earth’s rotational speed. A comparison between these estimates and an atomic clock revealed an apparently shorter day length in recent years.
However, a new fact surprises scientists: by removing the fluctuations in rotational speed that are known to occur due to tides and seasonal effects, the trend of shorter days seems to reverse. Although the Earth has reached its shortest day in 2022, the long-term trajectory seems to be exceeded from shorter days to longer days since 2020.
This sudden variation, unprecedented in the last five decades, has not yet been explained by scientists. A possible explanation would be the so-called “Chandler wobble”, a small deviation from the axis of rotation of the Earth with a period of about 430 days. There is some data showing that this process might be related, but Australian researchers have another idea.
Apparently, nothing specific would have changed on or around the Earth. These would simply be long-term tidal effects, which, working in parallel with other periodic processes, would have led to a temporary change in the rate of rotation of the Earth. If the planet transitions to even longer days in the future, we may need to incorporate a “negative leap second”: Periodically, timekeepers insert leap seconds into our official time scales so as not to lose synchronization with the Earth, but to do so in the negative is unprecedented and could put the Internet in failure.