Atomic clocks combined with precise astronomical measurements have revealed that the length of the day has suddenly gotten longer, and scientists don’t know why.
This has a major impact not only on our timekeeping, but also on technologies such as GPS and other technologies that govern our modern lives.
Earth’s rotation around its axis — which determines how long a day is — has been accelerating over the past few decades. This trend has made our days shorter; in fact, we set a record in June 2022 for our shortest day in over half a century.
But despite this record, since 2020, the steady acceleration has turned bizarrely into a slowdown—and the days are getting longer, and the reason for that remains a mystery to this day.
While the clocks in our phones show that there are exactly 24 hours in a day, the actual time it takes for the Earth to complete a single rotation is slightly different. These changes occur over millions of years, almost instantaneously—even earthquake and storm events can play a role.
It turns out that a day rarely happens to be the magic number of 86,400 seconds.
Earth’s rotation has been slowing down for millions of years due to frictional effects associated with moon-driven tides. This process adds about 2.3 milliseconds to the length of each day in each century. Billions of years ago, the Earth day had only about 19 hours.
For the past 20,000 years, another process has been going in the opposite direction, accelerating Earth’s rotation. When the last ice age ended, melting polar ice caps lowered surface pressure and the mantle began to move steadily towards the poles.
Just like ballet dancers spin faster when they extend their arms toward their bodies — the axis of their rotation — so as this mantle moves closer to Earth’s axis, our planet’s rotation speed increases. This process is shortened by about 0.6 milliseconds per century per day.
Over decades or more, the connections between the Earth’s interior and its surface also come into play. Large earthquakes can change the length of a day, although usually by a small amount.
For example, the magnitude 8.9 earthquake in Japan in 2011 is thought to have accelerated the Earth’s rotation by a relatively tiny 1.8 microseconds.
In addition to these large-scale changes, over shorter periods of time, weather and climate also have important effects on the Earth’s rotation, leading to changes in both directions.
Bi-weekly and monthly tidal cycles move mass around Earth, causing the length of a day to vary by up to a millisecond in either direction. We can see tidal changes over a day length record of up to 18.6 years.
The movement of our atmosphere has a particularly strong influence, and ocean currents also play a role. Seasonal snow and rain, or groundwater extraction, further changes things.
Why did the earth suddenly slow down?
Since the 1960s, when radio telescope operators around Earth began devising techniques to simultaneously observe cosmic objects such as quasars, we have had very precise estimates of the Earth’s rotational speed.
Comparing these estimates with atomic clocks, the length of the day appears to have been decreasing over the past few years.
But once we removed the rotational speed fluctuations we know occur due to tidal and seasonal influences, a surprising discovery emerged. Although Earth had its shortest day on June 29, 2022, the long-term trajectory appears to have shifted from shortening to lengthening since 2020. This change is unprecedented in the past 50 years.
The reason for this change is unclear. This could be due to changes in weather systems, as well as back-to-back La Niña events, although these have already happened. This could increase the melting of the ice sheet, although the rate of melting of the ice sheet has not significantly deviated from its steady rate in recent years.
Could it have something to do with Tonga’s huge volcanic eruption that pumped a lot of water into the atmosphere? Probably not, because that happened in January 2022.
Scientists speculate that the recent mysterious changes in Earth’s rotational speed are related to a phenomenon known as “Chandler wobble” — small deviations from the Earth’s rotational axis, with a period of about 430 days.
Radio telescope observations also show that this wobble has decreased in recent years. The two may be connected.
The last possibility we think is plausible is that nothing concrete has changed in or around the Earth. It may just be that long-term tidal effects and other periodic processes simultaneously produce temporary changes in Earth’s rotational speed.
Do we need “negative leap seconds”?
Accurate knowledge of the Earth’s rotation rate is critical for many applications – without it, navigation systems such as GPS would not work. Also, every few years, timekeepers insert leap seconds into our official timescale to ensure they don’t get out of sync with our planet.
If the planet were to shift to longer days, we might need to include “negative leap seconds” — something that would be unprecedented and could disrupt the internet.
Negative leap seconds are currently considered unlikely to be required. Now, we can welcome news that — at least for a while — we have a few milliseconds of extra time every day.
Matt King, Director of the ARC Australian Centre of Excellence for 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 text.