Atomic clocks combined with precise astronomical measurements have recently revealed that the length of a day on Earth is suddenly getting longer. This phenomenon has critical implications not only for our measurement of time, but also for things like GPS and other technologies that govern our modern lives.
In recent decades, the Earth’s rotation around its axis, which determines the length of a day, has accelerated. This trend has shortened our days; in fact, in June 2022 we hit the shortest record day in about half a century.
But despite this record, since 2020, this constant acceleration has strangely turned into a slowdown: the days are getting longer again, and the reason is currently unknown.
While our phone clocks show that a day has exactly 24 hours, the actual time it takes the Earth to complete a single rotation varies very little. These changes occur over time periods ranging from millions of years to almost instantaneous – even earthquakes and storms can play a role. So it turns out that a day very rarely equals the magic number of 86,400 seconds.
The ever-changing planet
For millions of years, the Earth’s rotation has slowed due to the frictional effects associated with the tides caused by the Moon. This process adds about 2.3 milliseconds to the length of each day every century. A few billion years ago, an Earth day lasted only 19 hours.
Over the past 20,000 years, another process has worked in reverse, accelerating the Earth’s rotation. At the end of the last ice age, the melting of the polar caps reduced the pressure on the surface and the Earth’s mantle began to move steadily towards the poles.
Just as a ballet dancer spins faster when he brings his arms closer to his body—the axis around which he spins—our planet’s rotation speed increases as this mass of mantle moves closer to Earth’s axis. And this process shortens every day by about 0.6 milliseconds per
Over decades and longer, the connection between the Earth’s interior and surface also comes into play. Large earthquakes can change the length of the day, although usually by small amounts. For example, the 2011 magnitude 8.9 Tōhoku earthquake in Japan is said to have accelerated the Earth’s rotation by a relatively small 1.8 microseconds.
In addition to these major changes, weather and climate over shorter periods of time also greatly influence the Earth’s rotation, causing variations in both directions.
Bi-monthly and monthly tidal cycles move mass around the planet, causing day length changes of up to a millisecond in either direction. We can observe tidal variations in day length records for periods of up to 18.6 years. The movement of our atmosphere has a particularly strong effect, and ocean currents also play a role. Snow cover and seasonal rainfall or groundwater extraction change things further.
Why is the earth suddenly slowing down?
Since the 1960s, when operators of radio telescopes around the planet began devising techniques to simultaneously observe cosmic objects such as quasars, we have had very accurate estimates of the Earth’s rotation rate.
A comparison between these estimates and an atomic clock revealed an apparently increasingly shorter day length in recent years.
But there is a startling revelation when we remove the fluctuations in rotation rate that we know occur due to tidal and seasonal effects. Although Earth reached 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 last 50 years.
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The reason for this change is unclear. This could be due to changes in weather systems, such as back-to-back La Niña weather events, although these have happened before. It could be increased melting of the ice caps, although these have not deviated much from their regular melting rate in recent years. Could it be related to the huge explosion of the Tonga volcano injecting huge amounts of water into the atmosphere? Probably not, considering it happened in January 2022.
Scientists have speculated that this recent and mysterious change in the planet’s rotation rate is related to a phenomenon called the “Chandler wobble” — a slight deviation of Earth’s rotation axis with a period of about 430 days. Observations from radio telescopes also show that the oscillation has slowed in recent years; the two phenomena could be linked.
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A final possibility that seems plausible to us is that nothing specific has changed in or around Earth. They could simply be long-term tidal effects working in parallel with other periodic processes to produce a temporary change in the Earth’s rotational speed.
Do we need a “negative leap second”?
Accurate knowledge of the Earth’s rotation rate is essential for a wide range of applications – navigation systems such as GPS would not work without it. In addition, every few years timekeepers insert leap seconds into our official time scales to ensure they do not get out of sync with our planet.
If Earth were to transition 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 at this time. For now, we can rejoice in the news that – at least for a while – we all have a few extra milliseconds each day.