Atomic clocks, combined with precise astronomical measurements, have revealed that the length of the day on Earth is getting longer.
which has critical implications not only for how we measure time, but also for GPS and other technologies that govern our lives. modern life.
In recent decades, the Earth’s rotation around its axis, which determines the length of the day, has accelerated. This trend has made our days shorter. In fact, in June 2022 we set a record for the 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 the clocks on our phones indicate that there are exactly 24 hours in a day, a day rarely corresponds exactly to the magic number of 86,400 seconds. The actual time it takes for the Earth to complete a single rotation varies slightly. These changes occur in periods ranging from millions of years to almost instantaneously; even earthquakes and storms can play a role.
The constantly changing planet
Over millions of years, the Earth’s rotation has been slowing down 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 about 19 hours .
For the last 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 toward the poles.
Just as a ballet dancer spins faster when she brings her arms closer to her body – the axis around which she spins – our planet’s rate of spin increases as this mantle mass approaches Earth’s axis. And this process shortens each day by about 0.6 milliseconds every century.
For decades and even longer, the connection between the interior and the surface of the Earth also comes into play. Large earthquakes can change the length of the day, although usually by small amounts. For example, the 2011 Great Tōhoku earthquake in Japan, with a magnitude of 8.9, is believed to have sped up the Earth’s rotation by a relatively small amount: 1.8 microseconds .
Apart from these large-scale changes, over shorter periods weather and climate also have major impacts on the Earth’s rotation, causing variations in both directions.
Biweekly and monthly tidal cycles move mass around the planet, causing changes in day length of up to a millisecond in any direction. We can see tidal variations in day length records over periods of 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 precipitation or groundwater extraction mess things up even more.
Why does the Earth suddenly slow down?
Ever since the 1960s, when radio telescope operators around the globe began devising techniques to simultaneously observe cosmic objects like quasars , we have had very precise estimates of the Earth’s rotation rate.
A comparison between these estimates and an atomic clock has revealed an increasingly shorter day length in recent years.
But there is a surprising finding once we remove the rotational speed fluctuations that we know occur due to tidal and seasonal effects. Even though the Earth reached its shortest day on June 29, 2022, the long-term trend appears to have shifted from getting shorter to longer since 2020. This change is unprecedented in the last 50 years.
The reason is not clear. It could be due to changes in weather systems, with consecutive La Niña events, although they have happened before. It could be a further melting of the ice sheets, although they have not deviated much from their steady rate of melting in recent years. Could it be related to the huge explosion of the Tonga volcano that injected huge amounts of water into the atmosphere ? Probably not, given that it happened in January 2022.
Scientists have speculated that this mysterious recent change in the planet’s rotation rate is related to a phenomenon called Chandler wobble, a small deviation in the Earth’s axis of rotation with a period of about 430 days. Observations from radio telescopes also show that the wobble has decreased in recent years. Both could be related.
One last possibility, which seems plausible to us, is that nothing specific has changed in or around the Earth. It 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 the Earth’s rotation is crucial for a number of applications: navigation systems such as GPS wouldn’t work without it. Also, every few years timekeepers introduce leap seconds into our official time scales to make sure they don’t get out of sync with our planet.
If the Earth happened to have even longer days, it would be necessary to incorporate a “negative leap second”, 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 satisfied with the news that – at least for a while – we all have a few more milliseconds each day.
Matt King , Director of the ARC Australian Center for Excellence in Antarctic Science, University of Tasmania and Christopher Watson , Senior Lecturer, School of Geography, Planning, and Spatial Sciences, University of Tasmania
This article was originally published on The Conversation . Read the original .