Atomic clocks, combined with precise astronomical measurements, have revealed that the length of the day is getting longer, without scientists understanding why. This has critical implications not only for how we measure time, but also for GPS and other technologies that govern our modern lives.
In recent decades, the rotation of the Earth around its axis, which determines the length of the day, has accelerated. This trend has shortened our days. Indeed, in June 2022 we set a record shortest day in the last half-century.
But despite this record, since 2020 the trend has changed and it seems that the rotation of the Earth has slowed down: the days are longer again, and the reason is, until now, a mystery.
Although our phone clocks show that there are exactly 24 hours in a day, a day is rarely exactly the magic number of 86,400 seconds. The actual time it takes Earth to complete a single rotation varies slightly. These changes occur over periods ranging from millions of years to almost instantaneously; even earthquakes and storms can play a role.
The ever-changing planet
For millions of years, the Earth’s rotation has slowed due to the frictional effects associated with the tides driven by the Moon. This process adds about 2.3 milliseconds to the length of each day per century. A few billion years ago, an Earth day lasted only a few 19 hours.
Over the past 20,000 years, another process has worked in the opposite direction, accelerating the Earth’s rotation. We mean that when the last ice age ended, the melting of the polar ice caps reduced the pressure on the surface and the Earth’s mantle began to move steadily towards the poles.
Just as a ballet dancer spins faster as she brings her arms closer to her body – the axis around which she spins – our planet’s rotational speed increases as this mantle mass gets closer to the axis of the planet. Earth. And this process is shortening each day by about 0.6 milliseconds per century.
For decades and even longer, the connection between the Earth’s interior and surface also comes into play. Large earthquakes can change the length of the day, although usually by small amounts. For example, the 2011 Greater Tohoku earthquake in Japan, with a magnitude of 8.9, would have accelerated the Earth’s rotation by a relatively small amount: 1.8 microseconds.
Besides these large-scale changes, over shorter time periods, weather and climate also have major impacts on the Earth’s rotation, causing variations in both directions.
Bi-weekly and monthly tidal cycles move mass around the planet, causing changes in day length of up to a millisecond in any direction. We see the variations of the tides in daylength records for periods up to 18.6 years.
The movement of our atmosphere has a particularly strong effect, with ocean currents also playing a key role. Snow cover and seasonal rainfall or groundwater extraction further complicate matters.
Why is the Earth suddenly slowing down?
Since the 1960s, when radio telescope operators around the world began devising techniques to simultaneously observe cosmic objects such as quasarswe have very precise estimates of the speed of rotation of the Earth.
A comparison between these estimates and an atomic clock revealed increasingly shorter day lengths in recent years.
But there’s a startling discovery once we remove the rotational rate fluctuations we know occur due to tidal and seasonal effects. Even though Earth reached its shortest day on June 29, 2022, the long-term trend appears to have shifted from shorter to longer since 2020. This change is unprecedented in the past 50 years.
The reason is unclear. This could be due to changes in weather systems, with back-to-back La Niña events, although they have happened before. It could be a new melting of the ice sheets, although they have not deviated much from their regular rate of melting in recent years. Could it be linked to the huge explosion of the Tonga volcano which injected huge amounts of water into the atmosphere? Probably not, considering it happened in January 2022.
Scientists have hypothesized that this mysterious recent change in the planet’s rotation rate is related to a phenomenon called the Chandler Oscillation, a small deviation of the Earth’s axis of rotation with a period of about 430 days. Observations from radio telescopes also show that the oscillation has diminished in recent years. The two could be linked.
A final possibility, which seems plausible to us, is that nothing specific has changed in or around the Earth. They could simply be long-term tidal effects working in parallel with other periodic processes to produce a temporary change in the Earth’s rate of rotation.
Do we need a “negative leap second”?
Knowing precisely the speed of rotation of the Earth is crucial for many applications: navigation systems like GPS would not work without it. Additionally, every few years, timekeepers introduce leap seconds into our official time scales to ensure they don’t get out of sync with our planet.
If the Earth were to have even longer days, a “negative leap second” would have to be incorporated, which would be unprecedented and could break the internet.
The need for negative leap seconds is considered unlikely at this time. For now, we can be content with learning that – at least for a while – we all have a few more milliseconds every day.
Matt Kingdirector of the ARC Australian Center for Excellence in Antarctic Science, University of Tasmania there Christopher WatsonLecturer, School of Geography, Planning and Space Sciences, University of Tasmania