Atomic clocks, combined with precise astronomical measurements, have
revealed that the length of a day is suddenly getting longer, and scientists
don't 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, Earth's rotation around its axis—which determines
how long a day is—has 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-century or so.
But despite this record, since 2020 that steady speedup has curiously
switched to a slowdown—days 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
instantly—even earthquakes and storm events can play a role.
It turns out a day is very rarely exactly the magic number of 86,400
seconds.
The ever-changing planet
Over millions of years, Earth's rotation has been slowing down due to
friction effects associated with the tides driven by the Moon. That process
adds about 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 Earth's rotation. When the last ice age ended,
melting polar ice sheets reduced surface pressure, and Earth's mantle
started steadily moving toward the poles.
Just as a ballet dancer spins faster as they bring their arms toward their
body—the axis around which they spin—so our planet's spin rate increases
when this mass of mantle moves closer to Earth's axis. And this process
shortens each day by about 0.6 milliseconds each century.
Over decades and longer, the connection between Earth's interior and surface
comes into play too. Major earthquakes can change the length of day,
although normally by small amounts. For example, the Great Tōhoku Earthquake
of 2011 in Japan, with a magnitude of 8.9, is believed to have sped up
Earth's 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 Earth's 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.
Why is Earth suddenly slowing down?
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 Earth's 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 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; 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 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.
This article is republished from The Conversation under a Creative Commons
license. Read the
original article.