World's timekeepers weigh dropping leap seconds: What would change?
Theo Al Jazeera English (Priyanka Shankar)
Global timekeeping authorities are meeting at Versailles to decide whether to abolish the leap second, a change that could decouple our clocks from Earth's rotation. Supporters say it would end costly disruptions to financial markets and data centers, while critics warn of a growing mismatch between civil time and the natural day.
The world's timekeeping authorities are meeting near Paris on Thursday to decide whether to abolish the "leap second" — a move that could detach our clocks from Earth's rotation and realign global time scales.
Clocks worldwide are built on the assumption that Earth completes one rotation on its axis in exactly 24 hours. In reality, Earth wobbles, so its rotation time is not truly exact. Over the long term, the planet's spin is slowing.
To compensate, since 1972 timekeepers have periodically inserted a leap second every few years — most recently in 2016 — to keep clocks and Earth's rotation in sync.
Pressure to scrap the leap second, however, is mounting because it complicates operations for financial markets and data centers, which depend on time accurate to the millisecond.
So what is a leap second? Can it be abolished? And how might that change the way we understand time?
What is a leap second?
Clocks run on Coordinated Universal Time (UTC), a global standard based on atomic clocks that have delivered far greater precision over the past seven decades. Atomic clocks measure time by observing the vibrations of atoms.
But Earth's rotation operates independently of atomic clocks.
A leap second is a second added periodically to UTC to keep it in sync with Earth's rotation. The first leap second was added to the world's clocks in 1972.
To date, 27 leap seconds have been added, the most recent on December 31, 2016. None has been added since.
According to the U.S. National Institute of Standards and Technology (NIST), leap seconds occur at the end of December or the end of June. NIST also cites the gap between the precision of the atomic second and Earth's rotation to explain the need for leap seconds.
"The rate of Earth's rotation is not constant... This gradual slowing causes the length of the mean solar second to grow relative to the atomic second," NIST says.
Why is pressure to scrap the leap second growing?
Although leap seconds narrow the gap between Earth's rotation and UTC, inserting them creates headaches for technology companies, financial markets and businesses that rely on precise time.
In 2012, LinkedIn and Reddit went down after a leap second was added. The same year, the booking system of Australian airline Qantas was also crippled after the leap second was inserted.
Three years later, the Australian Securities and Investments Commission also issued a warning on the issue. And in July 2022, Meta engineers said: "Every leap second is a major source of pain for people managing hardware infrastructure."
Faced with mounting pressure, the International Bureau of Weights and Measures (BIPM) voted in 2022 to abolish the leap second by 2035. But some countries, such as Russia, want to push the implementation date back to 2040. Russia builds leap seconds directly into its GLONASS satellite navigation system, so delaying abolition would give it more time to update its satellites.
Despite Moscow's concerns, the 69 BIPM member states are meeting on Thursday at Versailles, on the outskirts of Paris, to debate and vote on a resolution to scrap the leap second by 2027.
What is on Thursday's agenda?
According to BIPM, the national timekeeping bodies will discuss three proposals at Versailles.
First, they will consider whether atomic clocks remain the most accurate device for measuring a second. Optical frequencies are now in practice more accurate than atomic clocks. If the proposal to change how time is measured is approved, the second could get a new definition, characterized by optical frequency rather than atomic vibration.
Second, the most important proposal concerns abolishing the leap second.
Third, a proposal to synchronize timekeeping on Earth with a new time standard NASA is developing for the Moon.
The approved proposals are expected to take effect on May 20, 2027, World Metrology Day.
What happens if the leap second is abolished?
If the leap second is scrapped, our clocks will no longer be tied to Earth's rotation.
According to NIST, if that happens, "adjustments will have to be made to timestamps and to legacy devices and software that are synchronized to UTC for astronomical observations."
"Some proposals have been made to make larger corrections, such as leap hours, which would occur far less frequently than leap seconds," NIST added.
Scientists say people's daily routines will not be affected, while software bugs, data server crashes and network failures in financial markets will be eliminated once the leap second is dropped.
But over centuries, the gap between Earth's cycles and clocks will become ever clearer: by 2100, clocks will run 54 seconds ahead of Earth. After another two millennia, that gap will widen to several hours. The result? Sunrise and sunset times will drift away from the rhythms of human life as we know them today.
