The length of Earth's days has been mysteriously increasing
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.
has critical impacts not just on our timekeeping, but also things
like GPS and other technologies that govern our modern life
(
https://bit.ly/3BRqoFX).
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.
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 Tohoku 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.
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 Nina 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.
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.
