Leap seconds face a possible rule change from an international timekeeping body in October 2026
International timekeepers may change how leap seconds are added in October 2026, a space scientist wrote in The Conversation. Leap seconds keep atomic time in step with Earth's uneven spin, so a new strategy changes how clocks and planet are reconciled.
The Scientist · Science desk
What happened
- Since 1967 the second has been defined as exactly 9,192,631,770 oscillations of radiation from the cesium-133 atom, a standard set by the General Conference on Weights and Measures.
- Coordinated Universal Time runs on cesium atomic clocks, so UT1, the time scale based on Earth's rotation, drifts relative to it.
- Over several years the two scales can fall a few seconds out of step, and timekeepers insert a leap second to keep clock time aligned with Earth's daily rotation.
- Leap seconds do for Earth's daily rotation what leap days do for its orbit, which takes about 365 and one-quarter days.
Why it matters
- Who has to decide decision Systems that keep UTC will inherit whatever leap-second strategy is adopted, because UTC is the atomic scale that leap seconds adjust to track Earth.
- What it limits constraint Part of the UTC-UT1 gap comes from short-term wobbles in Earth's spin, so any new rule has to handle a drift that does not run at a steady rate.
- What it sets up precedent The second was redefined several times before cesium, so a change to how it is reconciled with Earth would follow a long habit of revising timekeeping rules as measurement improves.
The drift has a physical cause. Earth's rotation is not completely uniform. Once timekeeping stopped using that rotation as its basis, the second on the clocks and the second implied by the planet no longer matched [13]. Part of the resulting gap comes from short-term irregularities in how fast Earth turns [7].
The correction itself is small. The 86,400-second day comes from dividing a mean solar day into 24 hours of 60 minutes of 60 seconds, a scheme built on the base-60 subdivisions Ptolemy used for planetary motion [8]. One second is about 0.0012 percent of that day [14]. Leap seconds exist because those few seconds keep adding up over several years [3].
On the decision itself, the author writes that in October 2026 an international organization governing timekeeping "may change its strategy for adding leap seconds" [1]. The headline goes further and says the leap second's "time may be over" [2]. The first phrasing would fit a revised rule for when to insert the extra second. The second suggests stopping insertions altogether. The thing this account doesn't tell you is the terms of the proposal, so for now the two readings carry equal weight.
Revising the second has a long history. Christiaan Huygens invented the pendulum clock in 1657, which made it possible to measure time down to the second [9]. Over the next three centuries the definition of the second changed several times before scientists settled on cesium [11]. The General Conference on Weights and Measures, which adopted the cesium definition, sets measurement standards and today has more than 100 member nations [5].
"The precision of physicists' measurements has gotten better as the way we measure time has changed," the author, a space scientist, wrote [12].
What to watch
- The text of the proposal before the timekeeping body, including how large a gap between UTC and UT1 it would tolerate.
- Whether a decision is actually taken in October 2026 or deferred to a later meeting.
Clarity's read
What the record supports and how the coverage leans. The claims behind it follow.
Reality
- Evidence45
- Adoption
- Insufficient
- Hype gap+15
- Incentives
- Insufficient
- Confidence40
Claim ledger
Ranked by verification strength, evidence, and original report placement.
- [1]
In October 2026, an international organization governing timekeeping may change its strategy for adding leap seconds.
- [2]
The Conversation's headline says leap seconds have helped keep clocks synchronized with Earth's rotation, "but their time may be over."
- [3]
Over the course of several years, modern time systems can become a few seconds out of sync with Earth's movement; timekeepers use a leap second to solve this, keeping timekeeping aligned with Earth's daily rotation.
- [4]
In 1967 the General Conference on Weights and Measures adopted a definition of the second based on the cesium-133 atom: exactly 9,192,631,770 oscillations of the radiation produced when the atom switches between two particular energy states.
- [5]
The General Conference on Weights and Measures is an intergovernmental organization that sets measurement standards and has over 100 member nations today.
- [6]
Coordinated Universal Time (UTC) is based on atomic clocks that use cesium-133; as a result UT1, the time scale based on Earth's rotation, can drift relative to UTC.
- [7]
The divergence between UT1 and UTC reflects, in part, short-term irregularities in how fast Earth rotates.
- [8]
Based on systems devised by Ptolemy, who used base-60 subdivisions while measuring planetary motions around 150 C.E., a mean solar day was divided into 24 hours of 60 minutes of 60 seconds: 86,400 seconds.
- [9]
Measuring time down to the second only became possible after Christiaan Huygens invented the pendulum clock in 1657.
- [10]
Leap days are added because Earth completes one orbit around the Sun in about 365 and one-quarter days; leap seconds keep timekeeping aligned with Earth's daily rotation in the same way leap years keep calendars aligned with its orbit.
- [11]
Scientists spent three more centuries after the pendulum clock developing precise, standardized time measurement, and the definition of the second changed several times before they settled on the cesium atom.
- [12]
"The precision of physicists' measurements has gotten better as the way we measure time has changed."
- [13]
Earth's rotation is not completely uniform; to be more precise, modern timekeeping systems stopped using Earth's rotation as their basis, creating a discrepancy between how a second used to be defined and how it is defined today.
- [14]
One second is about 0.0012 percent of a 86,400-second day.
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