The Length of Earth’s Days Has Been Mysteriously Increasing, and Scientists Don’t Know Why – Singularity Hub

Atomic clocks, combined with precise astronomical measurements, have revealed that the length of a day is suddenly getting longer, and scientists dont know why.

This has critical impacts not just on our timekeeping, but also things like GPS and other technologies that govern our modern life.

Over the past few decades, Earths rotation around its axiswhich determines how long a day ishas been speeding up. This trend has been making our days shorter; in fact, in June 2022 we set a record for the shortest day over the past half a century or so.

But despite this record, since 2020 that steady speedup has curiously switched to a slowdowndays are getting longer again, and the reason is so far a mystery.

While the clocks in our phones indicate there are exactly 24 hours in a day, the actual time it takes for Earth to complete a single rotation varies ever so slightly. These changes occur over periods of millions of years to almost instantlyeven earthquakes and storm events can play a role.

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

Over millions of years, Earths rotation has been slowing down due to friction effects associated with the tides driven by the moon. That process adds about 2.3 milliseconds to the length of each day every century. A few billion years ago an Earth day was only about 19 hours.

For the past 20,000 years, another process has been working in the opposite direction, speeding up Earths rotation. When the last ice age ended, melting polar ice sheets reduced surface pressure, and Earths mantle started steadily moving toward the poles.

Just as a ballet dancer spins faster as they bring their arms toward their bodythe axis around which they spinso our planets spin rate increases when this mass of mantle moves closer to Earths axis. And this process shortens each day by about 0.6 milliseconds each century.

Over decades and longer, the connection between Earths interior and surface comes into play too. Major earthquakes can change the length of a day, although normally by small amounts. For example, the Great Thoku Earthquake of 2011 in Japan, with a magnitude of 8.9, is believed to have sped up Earths rotation by a relatively tiny 1.8 microseconds.

Apart from these large-scale changes, over shorter periods weather and climate also have important impacts on Earths rotation, causing variations in both directions.

The fortnightly and monthly tidal cycles move mass around the planet, causing changes in the length of day by up to a millisecond in either direction. We can see tidal variations in length-of-day 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. Seasonal snow cover and rainfall, or groundwater extraction, alter things further.

Since the 1960s, when operators of radio telescopes around the planet started to devise techniques to simultaneously observe cosmic objects like quasars, we have had very precise estimates of Earths rate of rotation.

A comparison between these estimates and an atomic clock has revealed a seemingly ever-shortening length of day over the past few years.

But theres 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 Nia 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 volcanic explosion in Tonga injecting huge amounts of water into the atmosphere? Probably not, given that occurred in January 2022.

Scientists have speculated that this recent, mysterious change in the planets rotational speed is related to a phenomenon called the Chandler wobblea small deviation in Earths rotation axis with a period of about 430 days. Observations from radio telescopes also show that the wobble has diminished in recent years; the two may be 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 Earths rotation rate.

Precisely understanding Earths rotation rate is crucial for a host of applications navigation systems such as GPS wouldnt work without it. Also, every few years timekeepers insert leap seconds into our official timescales to make sure they dont drift out of sync with our planet.

If Earth were to shift to even longer days, we may need to incorporate a negative leap secondthis 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 thatat least for a whilewe all have a few extra milliseconds each day.

This article is republished from The Conversation under a Creative Commons license. Read the original article.

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The Length of Earth's Days Has Been Mysteriously Increasing, and Scientists Don't Know Why - Singularity Hub