[How, and whether, to keep atomic time in sync with Earths
rotation is still up for debate. ]
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THE LEAP SECOND’S TIME IS UP: WORLD VOTES TO STOP PAUSING CLOCKS  
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Elizabeth Gibney
November 18, 2022
Nature [[link removed]]

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_ How, and whether, to keep atomic time in sync with Earth's rotation
is still up for debate. _

From 2035, no extra seconds will be added to bring clocks in sync
with astronomical time., Adam Smigielski/Getty

 

The practice of adding ‘leap seconds’ to official clocks to keep
them in sync with Earth’s rotation will be put on hold from 2035,
the world’s foremost metrology body has decided.

The decision was made by representatives from governments worldwide at
the General Conference on Weights and Measures (CGPM) outside Paris on
18 November. It means that from 2035, or possibly earlier,
astronomical time (known as UT1) will be allowed to diverge by more
than one second from coordinated universal time (UTC), which is based
on the steady tick of atomic clocks. Since 1972, whenever the two time
systems have drifted apart by more than 0.9 seconds, a leap second has
been added.

Stopping the adjustments is “a leap forward” for researchers who
work on time and frequency, says Georgette Macdonald, director general
of the Metrology Research Centre in Halifax, Canada. “I’m pleased
their efforts got us to this moment.”

Leap seconds aren’t predictable, because they depend on to Earth’s
natural rotation. They disrupt systems based on precise timekeeping,
Macdonald says, and can wreak havoc in the digital age. Facebook’s
parent company, Meta, and Google are among the tech companies that
have called for leap second
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The CGPM — which also oversees the international system of units
(SI) — has proposed that no leap second should be added for at least
a century, allowing UT1 and UTC to slide out of sync by about 1
minute. But it plans to consult with other international organizations
and decide by 2026 on what upper limit, if any, to put on how much
they be allowed to diverge.

Time for change

Representatives from Canada, the United States and France were among
those at the CGPM who called for the leap second to be scrapped before
2035. But Russia, which voted against the proposal, wants to push it
back to 2040 or later to deal with technical issues within its
satellite-navigation system, GLONASS.

The Russian system incorporates leap seconds, while the Global
Position System (GPS) and others already effectively ignore them. The
decision means that Russia might need to install new satellites and
ground stations, says Felicitas Arias, former director of the Time
Department at the International Bureau of Weights and Measures (BIPM)
in Sèvres, France.

Astronomers who rely on UT1 to align their telescopes will also need
to adjust, says Elizabeth Donley, who leads the Time and Frequency
division at the National Institute of Standards and Technology, in
Boulder, Colorado. But the current situation is unsustainable and
would continue to get worse, she adds. Different organisations handle
the leap second differently (Google, for example, smears out the extra
second in the 24 hours around midnight UTC). This creates an ambiguity
between time sources of as much as half a second, she says, “which
is huge.”

Although in the long term Earth’s rotation slows due to the pull of
the Moon, a speed-up since 2020 has also made the issue more pressing,
because for the first time, a leap second might need to be removed,
rather than added. UTC has only ever had to slow a beat to wait for
Earth, not skip ahead to catch up with it. “It's kind of being
described as a Y2K issue, because it's just something that we've never
had to deal with,” Donley says.

There is a chance that the International Telecommunications Union
(ITU) could stymie plans to make the switch in 2035. The body
effectively ceded decision making about the leap second to the CGPM in
2015 [[link removed]], and Arias
says its working group agreed with the CGPM’s proposal. But the ITU
remains in control of disseminating UTC, and could argue that the time
is not right to make the change, she says. “This is the thing that
makes us a little bit nervous.”

Subtle difference

Although human timepieces have been calibrated with Earth’s rotation
for millennia, most people will feel little effect from the loss of
the leap second. “In most countries, there is a one hour step
between summertime and winter time,” says Arias. “It is much more
than one second, but it doesn't affect you.”

Future metrologists might find more elegant ways than the leap second
to realign UTC and UT1. At the point where the difference becomes
significant, “our ability to reconcile it will be better than what
our ability is right now”, says Macdonald.

Or they might not bother, Arias adds. When the difference becomes big
enough, countries could permanently shift their legal time zone by one
hour, she says. Or we could even decouple our sense of time from the
Sun entirely, to create a single world time zone in which different
countries see the Sun overhead at different times of day or night.
“It could be a solution,” she says. “Science already doesn’t
use local times, we talk in UTC.”

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_ELIZABETH GIBNEY is a senior physics reporter at Nature. She has
written for Scientific American, BBC and CERN_

_NATURE is the foremost international weekly scientific journal in the
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