Leap Seconds, Time Drift, and the Future of Civil Time

Summary

Leap seconds are occasional one-second adjustments made to Coordinated Universal Time (UTC) to account for Earth's irregular rotation. The leap second was first introduced in our article on atomic time. Leap seconds were intended to keep civil time in sync with astronomical time. The problem is that they also introduce technical complications for modern digital systems. International timekeeping authorities plan to eliminate them by 2035. This article is a follow-up to our article on atomic time. It explores why leap seconds exist, the problems they cause and how emerging technologies like optical atomic clocks are reshaping the future of timekeeping.

Key Takeaways

• Leap seconds are used to reconcile the difference between atomic time and Earth's slowing rotation.
• Since 1972, 27 leap seconds have been added to UTC.
• Their irregularity complicates time-sensitive technologies.
• Major internet platforms and financial systems have experienced disruptions during leap second insertions.
• International Atomic Time (TAI) continues uninterrupted; UTC includes leap seconds to align with Earth time.
• Global standards bodies agreed to abolish leap seconds in 2022 starting in 2035.
• The long-term impact of this change on daily life is minimal, but it marks a philosophical shift in how we define time.
• Optical atomic clocks promise unprecedented precision and may lead to a redefinition of the SI second.

Why Time Is Not As Stable As You Think

Time is one of the few remaining constants of the digital age. Our phones, computers, and global networks all appear perfectly and effortlessly synchronized. But beneath this seamless appearance lies a complex and evolving system.

Even our most precise clocks need occasional corrections. The edifice of time requires a human hand to orchestrate its passage. The concept of the leap second is closely connected to this theme. Understanding leap seconds requires stepping back to see how civil time evolved and what might come next in our pursuit of perfect timekeeping.

What Is a Leap Second and Why Do We Need It?

A leap second is a one-second adjustment applied to Coordinated Universal Time (UTC). They are used to keep UTC in sync with Earth's irregular and gradually slowing rotation. Our planet does not spin at a constant rate. Tidal forces, core-mantle interactions and other geophysical phenomena conspire to create subtle variations in the rotational axis of the Earth.

These variations create differences between precise time (International Atomic Time (TAI) measured by atomic clocks and solar time. Solar time is also known as Universal Time (UT1). UTC is widely used for international timekeeping and is the reference for civil time in most countries. UTC is based on International Atomic Time (TAI). This means it would run ahead of observed solar time unless it is reset to UT1 as required. The leap second provides this adjustment.

How Often Are Leap Seconds Added?

Twenty-seven leap seconds have been added since their introduction in 1972. Leap seconds typically occur on June 30 or December 31 and are scheduled by the International Earth Rotation and Reference Systems Service (IERS). Leap seconds are irregular. They are typically announced about six months in advance.

Timeline of Leap Second Implementation

Their need is determined by how much Earth's rotation deviates from atomic time. Some years require a leap second; others do not. Their unpredictability is what makes them difficult to handle within technology systems requiring consistent and uniform progressions of time.

Why Are Leap Seconds a Problem for Technology?

Seemingly insignificant adjustments to time increments can have potentially significant impacts on the functioning of technologies powering our daily lives. Digital infrastructure thrives on regularity. Inserting a leap second disrupts the regularity of computers and networks built to handle seconds. They are not built to handle anomalies.

Technical Impacts of Leap Second Events

Systems like GPS, high-frequency trading platforms, airline databases and global communication networks have all experienced problems due to poorly handled leap second events. Reddit and Cloudflare suffered outages during the 2012 and 2016 leap second insertions. Linux systems in particular have experienced problems when internal clocks don't know how to process a 61-second minute. Such complications have made the leap second a liability in certain corners of the digital world.

What Is Time Drift and Why Does It Happen?

Time drift refers to the gradual divergence of atomic time and Earth rotation-based Universal Time (UT1). Atomic time is based on the resonance of frequencies of atoms. It is governed by TAI and progresses with near-perfect consistency. But the Earth's rotation fluctuates unpredictably. It is influenced variably by phenomena such as tidal braking, glacial rebound, and seismic activity. This causes Universal Time to drift away from TAI. Leap seconds are used to realign UTC (which tries to split the difference) to UT1. This keeps our clocks roughly within 0.9 seconds of true solar time.

Time Drift At a Glance

• Earth's rotation is gradually slowing by about 1.7 milliseconds per century
• The gap between atomic time and astronomical time grows by roughly 1 second every 1.5 years
• Civil time would drift from sun-based time by 25 minutes over 1,000 years without leap seconds
• The decision to eliminate leap seconds prioritizes technological stability over astronomical alignment

What Is the Difference Between UT1, UTC, TAI and GMT?

Understanding leap seconds requires distinguishing between four key time standards:

Key Characteristics:

• Universal Time (UT or UT1) is a time standard based on Earth's rotation. It is computed from a measure of the Earth's angle with respect to the International Celestial Reference Frame (ICRF).

• UTC (Coordinated Universal Time) is the basis for civil time worldwide. It is a hybrid system that follows atomic time but adjusts with leap seconds to stay close to UT1.

• TAI (International Atomic Time) is a continuous and uniform time scale without any leap seconds. It is derived from the readings of hundreds of atomic clocks worldwide.

• GMT (Greenwich Mean Time) is the older standard based on the Earth's rotation. GMT has largely been replaced by UTC in official contexts but the term continues to be used colloquially.

What is the Problem With Leap Seconds?

The technical burden of implementing leap seconds has led some international organizations to advocate for their elimination. The International Telecommunications Union (ITU) and the International Bureau of Weights and Measures (BIPM) have long warned about the risk to critical infrastructure.

The BIPM voted in 2022 to phase out leap seconds by 2035 following discussions on the issue which had been ongoing since the early 2000s. UTC would no longer track astronomical time so closely under this arrangement. Corrections after 2035 will occur only when the discrepancy becomes significant. This could mean decades or even centuries between adjustments.

Will We Notice If Leap Seconds Disappear?

Not in the near term. The difference between solar time and atomic time will accumulate slowly. It would take thousands of years before noon drifts significantly from the sun's position in the sky. Daily activities from lunch to rocket-launches would proceed unaffected.

The implications are more philosophical than practical. Should civil time remain aligned with nature or reflect the atomic precision of our machines? Some astronomical and religious traditions may need to adjust in order to answer this question. But society-at-large will barely notice.

The Roadmap to Redefining the Second

Timekeeping continues to evolve even as the leap seconds debate continues. The next frontier of time is the optical atomic clock. These devices use elements like strontium or ytterbium and operate at even higher frequencies than caesium-based clocks. Optical clocks offer accuracy to within one second over billions of years.

They are so precise they can detect slight differences in gravity between two floors of a building. Researchers are working to redefine the SI second based on these technologies within the next decade. Optical clocks could revolutionize not just timekeeping. They may herald a new golden age of navigation, geology and even fundamental physics.

Evolution of Timekeeping Precision

Applications Beyond Timekeeping

• Relativistic Geodesy: Measuring the Earth's gravitational field with unprecedented precision
• Quantum Computing: Providing ultra-precise timing for quantum operations
• Fundamental Physics: Testing Einstein's theories and exploring quantum mechanics
• Deep Space Navigation: Enabling more accurate positioning for interplanetary missions
• Financial Networks: Achieving sub-nanosecond transaction synchronization globally
• Telecommunications: Supporting future ultra-high bandwidth networks

Time Moves On Even When We Don't Feel It

Leap seconds are a reminder that timekeeping is a human construct. It must constantly adapt to the conflicting rhythms of nature, society and technology. The move toward abandoning leap seconds and embracing ultra-precise optical clocks reflects a choice of stability over tradition; it reflects a choice of accuracy over astronomical alignment. Time remains as elusive and essential as ever even in a world governed by atomic vibrations and laser traps. Whether by stars or atoms, our pursuit of perfect timekeeping continues.

FAQs

What is a leap second?

A leap second is a one-second adjustment added to Coordinated Universal Time (UTC) to keep it in sync with Earth's rotation. It compensates for the gradual slowing of Earth's rotation.

How often are leap seconds added?

Leap seconds are added irregularly. They typically occur on June 30 or December 31. Twenty-seven leap seconds have been added since their introduction in 1972. The need for a leap second is determined by how much Earth's rotation deviates from atomic time.

Why are leap seconds a problem for technology?

Leap seconds disrupt the regularity of time-sensitive systems. They can cause issues in network infrastructure, financial trading, GPS receivers and other technologies that rely on precise timekeeping. Notable incidents include outages at Reddit and Cloudflare during leap second events.

What is the difference between UTC, TAI, UT1 and GMT?

UTC (Coordinated Universal Time) is the time standard used for civil time worldwide. It is based on International Atomic Time (TAI) but includes leap seconds to stay close to UT1, which is based on Earth's rotation. TAI is a continuous time scale without leap seconds. GMT (Greenwich Mean Time) is an older standard based on Earth's rotation and has largely been replaced by UTC in official contexts.

What is the future of leap seconds?

The International Bureau of Weights and Measures (BIPM) has voted to phase out leap seconds by 2035. UTC will no longer track astronomical time so closely after this date. Leap seconds will only be added when the discrepancy becomes significant. This could take decades or even centuries.

Further Reading

Arias, E. F., & Guinot, B. (2011). The Leap Second: History and Possible Future

Lombardi, A., Heavner, T. A. & Jefferts, S. R. (2007) NIST Primary Frequency Standards and the Realization of the SI Second

Petit, G. & Luzum, B. (eds.) (2010). International Earth Rotation and Reference Systems Service Conventions

Quantum Enabled Position, Navigation & Timing. (n.d.) Optical Atomic Clocks: A Quantum Leap in Precision Timing