Do You Know? The first leap hour is not expected for thousands of years. The change approved by international timekeepers is intended to reduce the risk that computer systems will fail when clocks are adjusted by a single second.
What happened?
Delegates from 69 countries voted on Thursday at a conference in Versailles, France, to replace the current leap-second approach with a system that permits a much larger adjustment: a leap hour.
The decision was taken by the International Bureau of Weights and Measures, the body responsible for helping maintain common international standards for time, weight and distance. The new approach is due to be adopted from next year.
Under the decision, coordinated universal time, or UTC, may drift by up to one hour from astronomical time before a leap hour is required. The source report says that such a correction is unlikely to be needed for thousands of years.
Why replace leap seconds?
Atomic clocks provide exceptionally stable and precise timekeeping, but Earth’s rotation is less predictable. The planet’s molten core, changing ocean movements, global jet streams and other natural forces can affect its spin.
Since 1972, officials have added 27 leap seconds to keep UTC aligned with astronomical time. A leap second can make a minute last 61 seconds. A negative leap second would instead remove one second, producing a 59-second minute.
These irregular adjustments create challenges for software and technical systems that expect minutes to contain exactly 60 seconds. The systems affected can include telecommunications networks, electricity grids, navigation systems, satellites, trading systems and other computer infrastructure.
“This will reduce the risks to the systems and national infrastructure that depend on accurate time while still preserving the fundamental link between our global time system and the rotation of the Earth,” said Dr Setnam Shemar, principal scientist in the time and frequency department at the UK’s National Physical Laboratory.
Why has Earth’s changing rotation mattered now?
Over long periods, Earth’s rotation has generally slowed. That difference from atomic time led to the addition of leap seconds. But the source report says Earth’s rotation sped up during the past decade, raising the possibility that officials might have needed to introduce the first negative leap second.
The report attributes a 30% probability of a negative leap second in the next decade to the decision to change course. Such an adjustment has never previously been applied, and software may not have been tested to handle it.
Shemar said leap seconds could be applied at the wrong time by software updates, potentially knocking networks out of sync. The report also says some systems froze or went offline after a leap second was applied in 2012; websites including Reddit and LinkedIn were among those affected, while Qantas flights experienced delays.
How timekeeping reached this point
- In 1955, Louis Essen and Jack Parry at the National Physical Laboratory built the first practical high-precision caesium atomic clock.
- In 1967, the astronomical definition of a second based on Earth’s rotation was abandoned.
- Since then, one second has been formally defined by 9,192,631,770 vibrations of a caesium atom.
- Since 1972, 27 leap seconds have been added to keep UTC aligned with astronomical time.
- From next year, the new system will allow UTC to drift farther before a larger correction is needed.
Who could be affected?
The change is primarily relevant to organisations that rely on highly accurate and synchronised timing. These include telecommunications providers, electricity-grid operators, navigation services, satellite systems, financial trading systems and other computer networks.
The source report says testing every computer system that communicates with others around the world would be extremely costly. That concern helped influence the vote.
The United Kingdom has historically objected to ending leap seconds because of their connection with Greenwich Mean Time. Russia has also opposed the change, citing possible effects on its Glonass satellite-navigation system.
What happens next?
The International Earth Rotation and Reference Systems Service’s Earth Orientation Centre in Paris will continue tracking changes in Earth’s rotation and other movements that feed into UTC. It also issues notices when leap seconds must be added under the current system.
Dr Christian Bizouard, director of the centre, supported the vote but said it moves the decision far into the future. The source report describes the change as effectively postponing the need to decide how a leap hour would eventually be implemented.
Bottom Line
International timekeepers have approved a future system that replaces disruptive one-second clock adjustments with the possibility of a leap hour. The practical change is not expected to affect daily life soon: the first leap hour is considered unlikely for thousands of years. Its immediate purpose is to reduce the risk that precise timing adjustments will disrupt critical computer and communications systems.
Source
This report is based on information published by The Guardian Science.
