Atomic clocks, combined with precise astronomical measurements, have revealed that the length of a day is suddenly lengthening, and scientists don’t know why.
This has critical impacts not only on our timekeeping, but also on things like GPS and other technologies that govern our modern lives.
Over the past few decades, the rotation of the Earth on its axis, which determines how long a day is, has been accelerating. This trend has been making our days shorter; in fact, in June 2022 we set a record for the shortest day in the last half century or so.
But despite this record, since 2020, this constant acceleration has curiously turned into a slowdown: the days are getting longer again, and the reason is still a mystery.
Although the clocks on our phones indicate that there are exactly 24 hours in a day, the actual time it takes the Earth to complete a single rotation varies slightly. These changes occur over periods of millions of years to almost instantaneously, even earthquakes and storms can play a role.
It turns out that a day is rarely exactly the magic number of 86,400 seconds.
The constantly changing planet
For millions of years, the Earth’s rotation has been slowing 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 every century. A few billion years ago a day on Earth lasted only about 19 hours.
For the past 20,000 years, another process has been working in the opposite direction, speeding up the Earth’s rotation. As the last ice age ended, the melting of the polar ice sheets reduced the surface pressure and the Earth’s mantle began to move steadily poleward.
Just as a ballet dancer spins faster as they bring their arms toward their body, the axis around which they spin, so our planet’s rotation speed increases as this mantle mass approaches the axis of the Earth. And this process shortens every day by about 0.6 milliseconds every century.
For decades and more, the connection between Earth’s interior and surface also comes into play. Major earthquakes can change the length of the day, although usually by small amounts.
For example, the great 2011 Tōhoku earthquake in Japan, with a magnitude of 8.9, is believed to have accelerated the Earth’s rotation by a relatively small 1.8 microseconds.
Apart from these large-scale changes, over shorter periods weather and climate also have significant impacts on the Earth’s rotation, causing variations in both directions.
Fortnightly and monthly tidal cycles move mass around the planet, causing changes in day length of up to a millisecond in either 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, and ocean currents also play a role. Seasonal snow cover and rainfall, or groundwater extraction, alters things further.
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 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 apparently shortened day length in recent years.
But there is a surprising revelation once we remove the fluctuations in rotation rate that we know occur due to tides and seasonal effects. Although Earth reached its shortest day on June 29, 2022, the long-term trajectory appears to have changed from shortening to lengthening since 2020. This change is unprecedented for the past 50 years .
The reason for this change is not clear. It may be due to changes in weather systems, with back-to-back La Niña events, although they have occurred before. Melting of the ice sheets could be increased, although the ice sheets have not deviated much from their constant melting rate in recent years.
Could it be related to the huge volcano explosion in Tonga injecting huge amounts of water into the atmosphere? Probably not, given that it happened in January 2022.
Scientists have speculated that this recent and mysterious change in the planet’s rotation speed is related to a phenomenon called the “Chandler wobble,” a small deviation in the Earth’s rotation axis with a period of about 430 days.
Radio telescope observations also show that the wobble has decreased in recent years; the two may be linked.
A final possibility, which we think is plausible, is that nothing specific has changed in or around the Earth. It could just be long-term tidal effects working in parallel with other periodic processes to produce a temporary change in the Earth’s rotation rate.
Do we need a “negative leap second”?
Accurately understanding the speed of the Earth’s rotation is crucial for a number of applications: navigation systems such as GPS would not work without it. Also, every few years timekeepers insert leap seconds into our official timescales to ensure they are out of sync with our planet.
If Earth were to shift to even longer days, we might have to incorporate a “negative leap second,” which would be unprecedented and could break the Internet.
The need for negative leap seconds is considered unlikely right now. For now, we can welcome the news that, at least for a while, we all have a few extra milliseconds each day.
Matt King, Director of the ARC Australian Center for Excellence in Antarctic Science, University of Tasmania and Christopher Watson, Senior Lecturer in the School of Geography, Planning and Space Sciences, University of Tasmania.
This article is republished from The Conversation under a Creative Commons license. Read the original article.