Nuclear timekeepers joined with exact galactic estimations have uncovered that the length of a day is unexpectedly getting longer, and researchers don’t have the foggiest idea why.
This has significant implications not only for our timekeeping, but also for things like GPS and other technologies that drive our modern lives.
Over the past few decades, the Earth’s rotation on its axis – which determines how long a day is – has been accelerating. This trend is shortening our days. In fact, in June 2022 we set the record for the shortest day in the last half century or more.
But despite this record, the steady speed-up since 2020 has curiously slowed down – the days are getting longer again, and the reason is still a mystery.
Although our phone clocks tell us that there are exactly 24 hours in a day, the time it takes for the Earth to complete one rotation varies slightly. These changes occur almost instantaneously over millions of years – even earthquakes and storm events can play a role.
Incidentally, a day is seldom the enchanted number of 86,400 seconds.
The ever-changing planet
Over millions of years, the Earth’s rotation has been slowing down due to 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.
Two or quite a while back, Earth’s day was just 19 hours in length.For the past 20,000 years, another process has been operating in the opposite direction, accelerating the Earth’s rotation. When 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 rapidly when they bring their arms toward their body – the axis around which they spin – so does the rate of rotation of our planet as this mass of mantle moves closer to Earth’s axis. increases. And this process shortens by about 0.6 milliseconds per day per century.
Over decades and longer periods, the relationship between the Earth’s interior and surface also plays out. Huge tremors can change the length of the day, albeit as a rule just barely.
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.
In addition to these large-scale changes, short-term weather and climate also have significant effects on the Earth’s rotation, causing variations in both directions.
Fortnightly and monthly tidal cycles move the planet around the planet, causing changes in day length of up to a millisecond in either direction. We can see tidal variations in the diurnal record over periods 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 cover and rainfall, or groundwater abstraction, change things further.
Why is Earth suddenly slowing down?
Since the 1960s, when operators of radio telescopes around the planet began to develop techniques to simultaneously observe cosmic objects such as quasars, we have had fairly accurate estimates of Earth’s rotation speed.
A comparison between these estimates and an atomic clock has revealed an apparently ever-shrinking day length over the past few years.
But a surprising revelation occurs when we remove the fluctuations in rotational speed that we know are caused by tides and weather effects. Despite Earth reaching its shortest day on June 29, 2022, the long-term trajectory appears to have shifted from shorter to longer since 2020. This change is unprecedented in the last 50 years.
The reason for this change is not clear. This could be due to changes in the climate system, with back-to-back La Niña events, although these have occurred before. This can be attributed to the melting of the ice sheets, although they have not deviated much from their steady rate of melting in recent years.
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Could it be related to a large volcanic eruption in Tonga that is releasing large amounts of water into the atmosphere? Probably not, happened in January 2022.
Researchers have hypothesized that this new, baffling change in the planet’s pivot speed is connected with a peculiarity known as the “Chandler Wobble” — a little deviation in Earth’s turn hub with a time of around 430 days. .
Observations with radio telescopes also show that the shaking has decreased in recent years. Both can be linked. A final possibility, which we think is plausible, is that nothing significant has changed in or around the Earth. These may be long-term tidal effects acting in parallel with other periodic processes to produce temporary changes in the Earth’s rotation rate.
Do we need a ‘negative leap second’?
Accurately understanding the Earth’s rotation rate is vital for many applications – navigation systems such as GPS would not work without it. Also, every few years timekeepers add leap seconds to our official time scales to make sure they don’t get out of sync with our planet.
If the Earth moved for even longer days, we might need to add a “negative leap second” – that would be unprecedented, and could break the Internet.
The need for negative leap seconds is currently considered impossible. Until further notice, we can invite the news that – for some time – we as a whole have a couple of additional milliseconds every day. The discussion
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 Special Sciences, University of Tasmania.
