Atomic clocks, combined with precise astronomical measurements, have shown that the length of a day is suddenly increasing, and scientists don’t know why.
Not only is this having critical implications for how we keep time, but also for things like GPS and other technologies that shape our modern lives.
In recent decades, the Earth’s rotation around its own axis – which determines how long a day is – has accelerated. This trend has made our days shorter; In fact, in June 2022 we set a record for the shortest day in about half a century.
But despite that record, that steady acceleration has curiously turned into a slowdown since 2020 — the days are getting longer again, and why is a mystery so far.
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 revolution varies very slightly. These changes occur over timescales ranging from millions of years to almost instantaneously—even earthquakes and storm events can play a role.
It turns out that a day is very rarely exactly the magic number of 86,400 seconds.
The ever changing planet
For millions of years, the Earth’s rotation has slowed due to frictional effects associated with lunar-driven tides. This process increases the length of each day by about 2.3 milliseconds every century. A few billion years ago, a day on Earth lasted only about 19 hours.
For 20,000 years another process has been working in the opposite direction, accelerating the Earth’s rotation. When the last Ice Age ended, melting polar ice sheets reduced surface pressure and the Earth’s mantle began to move steadily toward the poles.
Just as a ballet dancer spins faster when they bring their arms to their body—the axis about which they rotate—so our planet’s rate of rotation increases as this mantle mass moves closer to Earth’s axis. And this process is shortening by about 0.6 milliseconds per century every day.
For decades and longer, the connection between the earth’s interior and the earth’s surface also comes into play. Major earthquakes can change the length of days, albeit usually by small amounts.
For example, the 2011 magnitude 8.9 Tōhoku earthquake in Japan is believed to have accelerated the Earth’s rotation by a relatively tiny 1.8 microseconds.
Aside from these large-scale changes, weather and climate also have important effects on the Earth’s rotation over shorter periods of time, causing variations in both directions.
The 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 precipitation or groundwater abstraction continue to change things.
Why is the earth suddenly slowing down?
Since the 1960s, when operators of radio telescopes around the planet began developing techniques to simultaneously observe cosmic objects such as quasars, we have had very accurate estimates of the Earth’s rotational speed.
A comparison between these estimates and an atomic clock has revealed an apparently ever shorter day length in recent years.
But there is a surprising revelation once we strip away the spin rate fluctuations that we know occur due to tidal and seasonal effects. Although Earth reached its shortest day on June 29, 2022, the long-term trajectory seems to have shifted from shortening to lengthening since 2020. This change is unprecedented in the last 50 years.
The reason for this change is not clear. It could be due to changes in the weather systems with back-to-back La Niña events, although these have occurred before. It could be an accelerated melting of the ice sheets, although they have not deviated massively from their constant melting rate in recent years.
Could it be related to the massive volcanic explosion in Tonga, which is spewing huge amounts of water into the atmosphere? Probably not as this happened in January 2022.
Scientists have speculated that this recent, mysterious change in the planet’s rotation speed is related to a phenomenon called the “Chandler Wobble” – a small deviation in the Earth’s rotational axis spanning a period of about 430 days.
Observations from radio telescopes also show that wobble has decreased in recent years; the two can be linked.
A final possibility, which we think is plausible, is that nothing notable has changed inside or around the Earth. It could simply be long-term tidal effects acting in parallel with other periodic processes to produce a transient change in the rate of Earth’s rotation.
Do we need a “negative leap second”?
Precise knowledge of the Earth’s rotational speed is crucial for a variety of applications – without it, navigation systems such as GPS would not work. Also, timekeepers add leap seconds to our official time scales every few years to ensure they stay in sync with our planet.
If Earth switched to even longer days, we might need to incorporate a “negative leap second” — this would be unprecedented and could destroy the internet.
The need for negative leap seconds is currently considered unlikely. For now, we can welcome the news that we all have a few extra milliseconds every day, at least for a while.
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 republished by The Conversation under a Creative Commons license. Read the original article.