Precise measurements show that the Earth’s rotation has been mysteriously slowing since 2020, making the day longer.
Accurate astronomical observations, combined with atomic clocks, have revealed that the length of a day is suddenly getting longer. Scientists don’t know why.
This has critical impacts not only on our timing, but also on things like GPS and other precision technologies that govern our modern lives.
The Earth’s rotation around its axis has accelerated in recent decades. As this determines the length of a day, this trend has shortened our days. Indeed, in June 2022 we set a record for the shortest day in about half a century.
However, despite this record, since 2020 this steady acceleration has curiously turned into a slowdown. Now the days are getting longer again and the reason remains a mystery so far.
While our phone clocks say there are exactly 24 hours in a day, the actual time it takes Earth to complete a single rotation can vary very slightly. These changes sometimes occur over periods of millions of years, and other times almost instantaneously. For example, even earthquakes and storms 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
The Earth’s rotation has been slowing for millions of years 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 100 years. A few billion years ago, an Earth day was only about 19 hours.
For 20,000 years, another process has been acting in the opposite direction, accelerating the rotation of the Earth. At the end of the last ice age, the melting of the polar ice caps reduced the surface pressure and the Earth’s mantle began to move gradually towards the poles.
Just as a ballet dancer spins faster by bringing their arms towards their body – the axis around which they spin – our planet’s rotational speed increases as this mantle mass gets closer to Earth’s axis. This process has shortened each day by about 0.6 milliseconds each century.
As decades and more go by, the connection between the Earth’s interior and surface also comes into play. Major earthquakes can alter the length of the day, although normally by small amounts. For example, the 2011 Great Tōhoku earthquake in Japan, with a magnitude of 8.9, is said to have accelerated the Earth’s rotation by a relatively minute rate. 1.8 microseconds.
In addition to these large-scale changes, over shorter time periods, weather and climate also have large impacts on the Earth’s rotation, causing variations in both directions.
Bi-monthly and monthly tidal cycles move mass around the planet, causing changes in day length of up to a millisecond in either direction. You can see tidal variations in daylength records over periods as long as 18.6 years. The movement of our atmosphere has a particularly strong effect, and ocean currents also play a role. Snow cover and seasonal precipitation, or groundwater extraction, further alter things.
Why is the Earth suddenly slowing down?
Since the 1960s, when radio telescope operators around the planet began devising techniques to simultaneously observe cosmic objects like quasarswe had very accurate estimates of the Earth’s rotation rate.
Using radio telescopes to measure the Earth’s rotation involves observations of radio sources like quasars. Credit:[{” attribute=””>NASA Goddard
A comparison between these measurements and an atomic clock has revealed a seemingly ever-shortening length of day over the past few years.
But there’s a surprising reveal once we take away the rotation speed fluctuations we know happen due to the tides and seasonal effects. Despite Earth reaching 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 over the past 50 years.
The reason for this change is not clear. It could be due to changes in weather systems, with back-to-back La Niña events, although these have occurred before. It could be increased melting of the ice sheets, although those have not deviated hugely from their steady rate of melt 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 occurred in January 2022.
Scientists have speculated this recent, mysterious change in the planet’s rotational speed is related to a phenomenon called the “Chandler wobble” – a small deviation in Earth’s rotation axis with a period of about 430 days. Observations from radio telescopes also show that the wobble has diminished in recent years. Perhaps the two are linked.
One final possibility, which we think is plausible, is that nothing specific has changed inside or around Earth. It could just be long-term tidal effects working in parallel with other periodic processes to produce a temporary change in Earth’s rotation rate.
Do we need a ‘negative leap second’?
Precisely understanding Earth’s rotation rate is crucial for a host of applications – navigation systems such as GPS wouldn’t work without it. Also, every few years timekeepers insert leap seconds into our official timescales to make sure they don’t drift out of sync with our planet.
If Earth were to shift to even longer days, we may need to incorporate a “negative leap second” – this would be unprecedented, and may break the internet.
The need for negative leap seconds is regarded as 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.
Written by:
- Matt King – Director of the ARC Australian Centre for Excellence in Antarctic Science, University of Tasmania
- Christopher Watson – Senior Lecturer, School of Geography, Planning, and Spatial Sciences, University of Tasmania
This article was first published in The Conversation.