On 27 days in the past century, time has gone backwards for a second.
asserted
time → go → second
These extra "leap seconds" were added to keep our ultra-precise atomic clocks aligned with the Earth's slightly less reliable rotation.
But we have probably already leapt our last second.
asserted
we → add → rotation
Later this week, delegates will meet near a French palace in Versailles to decide whether to abandon the leap second for good.
asserted
delegates → meet → second
In its place, if the vote succeeds, it will ask people centuries in the future to observe a "leap hour".
asserted
it → succeed → hour
How Earth's spin affects clocks
asserted
spin → affect → clocks
Our 24-hour days can vary in length by a thousandth of a second (a millisecond).
asserted
days → vary → second
Earth's rotation is generally slowing down, thanks mostly to the pull of the Moon.
asserted
rotation → slow → Moon
Astronomers see this in historical records of solar eclipses from more than 3,000 years ago.
asserted
Astronomers → see → eclipses
But over shorter time periods, our planet occasionally spins faster.
asserted
planet → spin → periods
Since about 2020, Earth has sped up.
asserted
Earth → speed → ?
Lucia McCallum, a geodesist at the University of Tasmania, said a number of things can affect the Earth's spin.
asserted
number → say → spin
"Atmosphere, wind, big ocean currents, glaciers melting, big volcanoes, redistribution of mass in the inner Earth," Dr McCallum said.
asserted
McCallum → melt → Earth
This slight variation means the traditional definition of a second isn't precise enough for technologies like GPS, which needs to measure fractions of milliseconds to work.
asserted
which → mean → milliseconds
In the late 1960s, the governing body of the International Bureau of Weights and Measures (BIPM), which standardises the world's measurements, adopted a more precise definition of a second based on measurements of caesium atoms.
asserted
which → govern → atoms
This is what underpins coordinated universal time (UTC).
asserted
what → underpin → time
UTC is tracked by atomic clocks, currently the most accurate measures of time, although newer "nuclear clocks" may soon surpass them.
uncertain
clocks → track → them
Michael Wouters, leader of time standards at Australia's National Measurement Institute (NMI), said the atomic second was as close to the traditional second as possible.
"
asserted
second → say → second
There was no exact value to begin with, because it was changing all the time.
asserted
it → be → ?
"
Dr McCallum helps measure the speed of Earth's rotation, using telescopes pointed at extremely distant black holes.
asserted
McCallum → help → holes
"We use those black holes as anchor points," Dr McCallum said.
asserted
McCallum → use → points
These observations are vital for satellite navigation, and they are also used to tell when Earth's rotation falls out of sync with UTC, prompting a leap second.
asserted
rotation → use → second
Since their introduction in 1972, leap seconds have been added to our clocks 27 times, most recently at the end of 2016.
asserted
seconds → add → 2016
How leap seconds mess with modern life
Leap seconds have become increasingly difficult to deal with in the computer age.
asserted
seconds → mess → age
Darryl Veitch, a researcher in computer networking at the University of Technology Sydney, said leap seconds break some of software's assumptions about time, such as that it only moves forwards and there are 60 seconds in a minute.
asserted
it → say → minute
But Dr Wouters said people "forgot about this problem, and they wrote software that then crashed when time went backwards".
asserted
time → say → software
Unlike time zones or daylight saving, leap seconds are irregular, scheduled only six months in advance.
asserted
seconds → schedule → advance
Previously, they have caused platforms like internet technology provider Cloudflare and social media site Reddit to crash.
asserted
platforms → cause → Cloudflare
In 2022, the BIPM's governing body decided to abandon the leap second by 2035 and start discussing a different method of keeping Earth's days in sync with UTC.
asserted
body → decide → UTC
But the decision needed to be fast-tracked when Earth started speeding up, because it meant the next leap second may need to be "negative".
uncertain
second → need → ?
Dr Wouters said a negative leap second was less likely to cause problems because it involves time jumping forwards rather than backwards, but Professor Veitch was not so sure.
asserted
Veitch → say → time
"It's never happened before, and therefore the software for that has never been tested," Professor Veitch said.
asserted
Veitch → happen → that
Since leap seconds will soon be retired, introducing a negative one seems an unnecessary risk.
asserted
introducing → retire → one
"It's being cautious about an unusual event that's prompting the accelerated timetable," Dr Wouters said.
asserted
Wouters → prompt → timetable
"Some people have a philosophical objection to [losing leap seconds], and that's because we're disconnecting the way we count time from our actual experience of it," Dr Wouters said.
asserted
Wouters → have → it
"
Vote may create 'leap hour'
uncertain
Vote → create → hour
The governing body of the BIPM will vote on leap seconds at its meeting on 12 –15 October.
asserted
body → govern → October
The leading proposal is to let UTC drift from Earth's rotation until it is an hour out of sync, and then observe a "leap hour" to bring it back.
asserted
it → lead → it
Earth's rotation currently loses less than a second per year, so it could take more than a millennium for this leap hour to become necessary.
uncertain
hour → lose → millennium
Our current timekeeping systems are less than a century old, and it is likely they will look very different by then.
asserted
they → look → less
"I would imagine in 100 years, software would be completely automated, and we just won't have the kinds of problems we have now," Professor Veitch said.
asserted
Veitch → imagine → problems
…and 4 more, not listed.
The Aporia