Leap Seconds Are Dead: Timekeeping Goes Atomic as UTC Drifts from Earth's Spin

Short summary

In the podcast, Science Friday examines why a global committee voted to kill the leap second and what comes next for timekeeping. Time on our clocks is tied to the Earth's rotation, but that rotation varies, so scientists rely on atomic clocks to keep time. Data from over 90 laboratories feeds the International Bureau of Weights and Measures to define Coordinated Universal Time UTC, while the Earth Rotation Service tracks the planet's spin. The leap second has been a troublesome discontinuity for networks and software, causing outages for major services. The podcast explains how ending the leap second means UTC will gradually drift from solar time, reducing outages but raising future questions about navigation, astronomy, and how we keep time.

• The leap second is being retired, and UTC will diverge slightly from Earth's rotation over time.
• Atomic clocks anchored by cesium-133 define the second and UTC, via BIPM.
• Outages and maintenance challenges from leap seconds have driven the change, especially for internet services and airports.
• The transition raises questions about future timekeeping, navigation, and astronomical measurements.

Introduction

The podcast explains that while our sense of time seems simple, it hinges on two different notions: the Earth's rotation and the counting of seconds by atomic clocks. The first is solar time, the second is atomic time. For decades a global committee managed the relationship between these two by inserting leap seconds to keep UTC in line with the Earth's rotation. The latest decision ends this practice, effectively allowing a small but persistent drift between the world’s time and the planet’s spin.

Timekeeping foundations

The program describes the standard time system as a collaboration among laboratories around the world. More than 90 labs contribute precise clock data to a central French institution, the International Bureau of Weights and Measures (BIPM). The BIPM then produces Coordinated Universal Time (UTC), the official time standard. Meanwhile, the International Earth Rotation Service keeps track of how the Earth’s rotation compares with that atomic time and communicates with the BIPM about the need for leap seconds. Atomic clocks are built around a cesium atom, which has a resonant frequency of 9,192,631,770 oscillations per second. This frequency is used to define the second, a definition that does not change over time.

The leap second era

Leap seconds have been added whenever the discrepancy between the atomic timescale and Earth’s rotation threatened to exceed one second. They are inherently unpredictable because they depend on subtle, natural variations in the Earth’s rotation. The discontinuity created by inserting an extra second has caused headaches for time dissemination systems and, importantly, for computer networks. The podcast highlights outages experienced by Cloudflare, DNS providers, Mozilla, Reddit, and Meta as examples of why timekeeping systems and software must handle discontinuities with care. The leap second is also a source of frustration for engineers who must design systems that can tolerate such changes without programming for every possible second of adjustment.

Two sides of the debate

Dr. Judah Levine, a timekeeping expert, explains that the core disagreement was between linking UTC to UT1 (the Earth’s angle relative to the stars) and keeping a stable, continuous time scale. Proponents of maintaining a strong link to astronomical time argued that the sun and Earth’s motions are an ancient and meaningful standard. Critics argued that leap seconds are impractical for modern technologies that require predictable timekeeping and continuous time scales for precise measurements and navigation. A key concern was the possibility of a negative leap second if the Earth’s rotation speeds up. The podcast notes this has happened, contrary to earlier expectations, and underscores why many timekeepers viewed leap seconds as an administrative burden rather than a reliable fix.

What happens next

With the leap second retired, UTC will gradually diverge from the earth’s solar time. The guests point out that since leap seconds began, a total of 37 seconds have been added, amounting to roughly a minute per century. At this rate, a future century could see a longer drift, but the scale remains small on human timescales. In the long run, this drift could necessitate another form of adjustment to keep timekeeping aligned with solar time for applications that depend on it, such as certain navigation and astronomical measurements, though nobody can predict precisely how future societies will manage these issues. The speakers emphasize that this is a deliberate move to simplify infrastructure and software while acknowledging that time, in the larger sense, remains a human construct that must evolve with technology.

Standards, infrastructure and the human element

The discussion closes with reflections on the importance of standards. Levine notes that timekeeping, like other units of measurement, underpins daily life and commerce: calls between continents, weighing goods, and even GPS navigation all rely on consistent time standards. The end of leap seconds is described as kicking the can down the road rather than a complete solution; a new approach will be needed to manage long term discrepancies between UTC and solar time. The podcast ends with appreciation for the people who maintain time standards and the recognition that the global timekeeping system is a collaborative, evolving effort that will continue to adapt to future technologies and needs.