Earth’s days are mysteriously getting longer — scientists don’t know why

Precise astronomical observations, combined with atomic clocks, have revealed a sudden lengthening of the length of a day. Scientists don’t know why.

This has important implications not only for our timekeeping, but also for GPS and other precision technologies that govern our modern lives.

The speed of rotation of the Earth on its axis has been increasing over the last few decades. This tendency shortens our days because it determines how long a day is. In fact, in June 2022 We made a record The last half century or so has been a shorter day.

However, despite this achievement, since 2020 that steady pace has curiously turned into a slowdown. Now, the days are getting longer again, and the reason is still a mystery.

While the clocks on our phones indicate that there are exactly 24 hours in a day, the actual time it takes for the Earth to complete one cycle is always slightly different. These changes sometimes occur over millions of years, other times almost instantaneously. For example, earthquakes and storm events can also play a role.

It turns out that a day is rarely the magic number of 86,400 seconds.

A changing planet

Earth’s rotation has slowed over millions of years due to frictional effects associated with moon-driven tides. That process adds about 2.3 milliseconds to the length of each day every 100 years. A few billion years ago, there was only one Earth day 19 hours.

For the past 20,000 years, another process has been working in the opposite direction, speeding up Earth’s rotation. As the last ice age ended, the melting polar ice caps reduced surface pressure, and the Earth’s mantle began to move steadily toward the poles.

Just as a ballet dancer spins faster as they bring their arms toward their body — the axis on which they spin — our planet’s rate of rotation increases as it moves closer to Earth’s axis. This process slows down every century by 0.6 milliseconds every day.

Over decades and over long periods of time, interaction between Earth’s interior and surface also takes place. Large earthquakes can change the length of the day, although usually by small amounts. For example, the great Tohoku earthquake in Japan in 2011, with a magnitude of 8.9, is believed to have accelerated the Earth’s rotation by a relatively small amount. 1.8 microseconds.

Apart from these large-scale changes, weather and climate over short periods of time have important effects on the Earth’s cycle, causing variations in both directions.

Fortnightly and monthly tidal cycles move mass around the planet, causing the length of day to change by up to a millisecond in either direction. We can see wave variations In long-term records up to 18.6 years. The movement of our atmosphere has a particularly strong effect, and ocean currents also play a role. Seasonal fog and rain, or groundwater extraction, change things further.

Why is the Earth suddenly slowing down?

Beginning in the 1960s, operators of radio telescopes around the planet began developing the techniques. Simultaneously observe cosmic objects such as quasarsWe have very accurate estimates of the Earth’s rotation rate.

Using radio telescopes to measure Earth’s rotation involves observations of radio sources such as quasars. debt:[{” 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.