The year was 1971, and something extraordinary was about to change forever. For the first time in human history, our planet would no longer be the ultimate timekeeper. The leap second was introduced in 1972. This wasn’t just another adjustment to our clocks – it was humanity officially admitting that the Earth itself had become unreliable for keeping perfect time. But what was timekeeping like before we started adding these mysterious extra seconds? The answer reveals a fascinating story of astronomical precision, human ingenuity, and our eternal struggle to synchronize with the cosmos.
The Ancient Dance Between Earth and Time
For millennia, humans had no choice but to dance to Earth’s rhythm. The rotation of the Earth on its axis has served as the basis for timekeeping since the dawn of history. The day was divided into 24 hours, each of 60 minutes, each of 60 seconds. This wasn’t just convenience – it was survival. Our ancestors needed to know when to plant crops, when to hunt, and when to navigate by the stars.
The concept was beautifully simple. One day equaled one complete rotation of our planet. For much of history, the chosen periodic phenomenon was the apparent motion of the Sun and stars across the sky, caused by the Earth spinning about its own axis. One of the earliest known timekeeping methods – dating back thousands of years – involved placing a stick upright in the ground and keeping track of its moving shadow as the day progressed. What our ancestors didn’t realize was that they were betting everything on a spinning top that wasn’t quite as steady as it seemed.
The Greenwich Mean Revolution
The industrial age demanded precision, and Greenwich Mean Time (GMT), a time scale in which the mean position of the sun at noon, averaged over the year, is above the Greenwich meridian (longitude zero). This wasn’t just British imperialism at work – it was a genuine attempt to create order from chaos. Before GMT, every town kept its own time based on when the sun reached its highest point locally.
Four years later, at the International Meridian Conference held in Washington DC in the US, GMT was adopted as the reference standard for time zones around the globe and the second was formally defined as a fraction (1/86,400) of the mean solar day. Finally, the world had agreed on something. But there was a problem lurking beneath this apparent precision. Until the 1950s, broadcast time signals were based on UT, and hence on the rotation of the Earth.
The Earth, as it turns out, doesn’t spin like a perfectly balanced clock. It wobbles, speeds up, slows down, and generally behaves like the massive, dynamic planet it is rather than the precise timepiece humanity needed it to be.
When Scientists Discovered Earth’s Imperfections
The first cracks in Earth-based timekeeping appeared as measurement technology improved. As our ability to measure this unit of time improved, it became clear that the Earth’s period of rotation is not constant. The period is not only gradually slowing down due to tidal friction, but also varies with the season and, even worse, fluctuates in unpredictable ways. This was devastating news for anyone who needed precise time.
Scientists discovered that Earth’s rotation wasn’t just imperfect – it was chaotic. Processes such as earthquakes, ocean currents, and energy losses due to the changing tides, can all make minute changes to the Earth’s rotation speed. Imagine trying to set your watch by a clock that randomly gained or lost seconds based on weather patterns, geological activity, and the gravitational pull of the moon.
Analysis of historical astronomical records shows a slowing trend; the length of a day increased by about 2.3 milliseconds per century since the 8th century BCE. Over centuries, these small variations accumulated into significant differences. Ancient eclipse records, when compared with modern calculations, revealed that Earth’s rotation had been gradually but persistently changing throughout history.
The Atomic Revolution Changes Everything
Then came the game-changer. In 1955, the caesium atomic clock was invented. For the first time in human history, we had a timekeeper more precise than the Earth itself. By definition, a SI second is the duration of 9,192,631,770 periods of radiation from the transition between two hyperfine levels of the ground state of the cesium-133 atom, a physical phenomenon distinct from the rotation of Earth. Before atomic timekeeping, clocks were set to the skies.
This wasn’t just an improvement – it was a revolution. By contrast, atomic clocks, and other highly accurate physical processes, proceed at a constant rate which is set by physics unrelated to the Earth’s rotation. In modern times, we have chosen to define time using atomic clocks, rather than measurements of the Sun’s position. This is essential for modern science, so that highly precise laboratory measurements of how long processes do not produce different results depending on how fast the Earth is rotating at the time.
Suddenly, scientists had access to timekeepers that could measure intervals so precisely that Earth’s irregularities became glaringly obvious. The planet that had been humanity’s timekeeper for millennia was revealed to be frustratingly unpredictable.
The Growing Gap Between Earth and Atomic Time
As atomic clocks ticked away with unwavering precision, Earth continued its erratic dance. Since then, leap seconds have occasionally been added to that stream of atomic seconds to keep the signals synchronized with the actual rotation of Earth. Adjustments are needed because Earth’s rotation is slightly less regular and a bit slower on average than cesium-133’s quantum-scale rhythms.
The gap between atomic time and Earth time wasn’t just growing – it was becoming a serious problem for modern technology. UTC is based on TAI (International Atomic Time, abbreviated from its French name, temps atomique international), which is a weighted average of hundreds of atomic clocks worldwide. UTC is within about one second of mean solar time at 0° longitude, the currently used prime meridian, and is not adjusted for daylight saving time.
By the early 1970s, the discrepancy had reached nearly ten seconds. Scientists faced a choice: let atomic time drift away from Earth time completely, or find a way to keep them synchronized. The solution they chose would change timekeeping forever.
The Historical Variations Nobody Expected
Before leap seconds, scientists began uncovering the true extent of Earth’s rotational variations throughout history. Records of ancient and medieval eclipses, spanning from 720 BC to AD 1600, along with lunar occultations of stars from AD 1600 to 2015, have been analyzed to investigate variations in the Earth’s rotation rate. These studies reveal that the length of the mean solar day (LOD) increases at an average rate of +1.8 milliseconds per century, which is less than the rate predicted by tidal friction.
The picture that emerged was startling. Based on data such as this, the graph below shows Stephenson’s best estimate of the historical lengths of days over the past three-thousand years, expressed as an offset in milliseconds from their present length of 86,400 modern seconds. Extending the data back further still, it is likely that days lasted only 22 modern hours at the time of the dinosaurs, 65 million years ago.
These weren’t just academic curiosities. Additionally, fluctuations in the LOD occur on time scales ranging from decades to centuries, with evidence of periodic oscillations. Scientists realized that Earth’s rotation had been varying on multiple timescales throughout history, from daily fluctuations to long-term trends spanning geological eras.
Tidal Forces and the Moon’s Gravitational Drama
The primary culprit behind Earth’s slowing rotation was hiding in plain sight: our own moon. Earth’s rotation is slowing slightly with time; thus, a day was shorter in the past. This is due to the tidal effects the Moon has on Earth’s rotation. This celestial dance between Earth and moon has been going on for billions of years, gradually stealing rotational energy from our planet.
The main reason for the slowing down of the Earth’s rotation is tidal friction, which alone would lengthen the day by 2.3 ms/century. But tidal friction wasn’t the only player in this cosmic drama. These processes change the Earth’s moment of inertia, affecting the rate of rotation due to the conservation of angular momentum. Some of these redistributions increase Earth’s rotational speed, shorten the solar day and oppose tidal friction. For example, glacial rebound shortens the solar day by 0.6 ms/century and the 2004 Indian Ocean earthquake is thought to have shortened it by 2.68 microseconds.
The moon’s influence created a delicate balance of forces, some speeding up Earth’s rotation and others slowing it down. This cosmic tug-of-war made Earth’s rotation fundamentally unpredictable over human timescales.
Climate and Ice Ages: The Planetary Time Modulators
Long before humans worried about precise timekeeping, Earth’s rotation was being influenced by massive climate cycles. A century ago, Serbian scientist Milutin Milankovitch hypothesized the long-term, collective effects of changes in Earth’s position relative to the Sun are a strong driver of Earth’s long-term climate, and are responsible for triggering the beginning and end of glaciation periods (Ice Ages). Specifically, he examined how variations in three types of Earth orbital movements affect how much solar radiation (known as insolation) reaches the top of Earth’s atmosphere as well as where the insolation reaches. These cyclical orbital movements, which became known as the Milankovitch cycles, cause variations of up to 25 percent in the amount of incoming insolation at Earth’s mid-latitudes.
These climate cycles didn’t just change temperatures – they dramatically affected Earth’s rotation. They found that 90% of recurring fluctuations between 1900 and 2018 could be explained by changes in groundwater, ice sheets, glaciers, and sea level. The remainder mostly resulted from Earth’s interior dynamics, like the wobble from the tilt of the inner core with respect to the bulk of the planet.
During ice ages, massive ice sheets redistributed Earth’s mass, changing how fast the planet spun. When the ice melted, the redistribution happened again. Days on Earth are growing slightly longer, and that change is accelerating. The reason is connected to the same mechanisms that also have caused the planet’s axis to meander by about 30 feet (10 meters) in the past 120 years. These weren’t small changes – they were dramatic enough to affect the length of days by measurable amounts.
The Core-Mantle Connection: Earth’s Hidden Timekeeper
Deep beneath our feet, another drama was unfolding that affected Earth’s rotation. Observations of the Earth’s rotation have shown irregular variations of rate which have characteristic times of decades. These have been attributed to transfer of angular momentum between core and mantle by some mechanism such as inertial coupling, viscous stress, electromagnetic coupling or stresses produced by topographic features on the core mantle boundary.
Scientists discovered that Earth’s liquid outer core doesn’t rotate at exactly the same speed as the solid mantle above it. Many investigators now seem to believe that the “decade variations„ in the Earth’s rotation rate are caused by torques between the core and mantle caused by the uneven motions at the core-mantle boundary. This internal dance between different layers of our planet created rotational variations that lasted for decades.
These core-mantle interactions were completely invisible to ancient timekeepers, but they caused variations in day length that accumulated over years and decades. The Earth wasn’t just a spinning ball – it was a complex system of interacting layers, each influencing the planet’s rotation in subtle but measurable ways.
The 1972 Compromise: Birth of the Leap Second
By 1972, the gap between atomic time and Earth time had reached a critical point. The first leap second was inserted into the UTC time scale on June 30, 1972. This wasn’t just a technical adjustment – it was humanity’s attempt to have the best of both worlds: the precision of atomic time and the astronomical relevance of Earth-based time.
On 1 January 1972, GMT as the international civil time standard was superseded by Coordinated Universal Time (UTC), maintained by an ensemble of atomic clocks around the world. The leap second system was designed to keep UTC within 0.9 seconds of Earth’s rotation, ensuring that civil time remained connected to the planet’s actual movement.
This compromise seemed elegant at the time. Since then, leap seconds have occurred on average about once every 19 months, always on 30 June or 31 December. As of July 2022, there have been 27 leap seconds in total, all positive, putting UTC 37 seconds behind TAI. But the seemingly simple solution would create its own set of problems as technology advanced.
The End of an Era and What We Lost
Before leap seconds, timekeeping was simpler but less precise. Nevertheless, the Earth’s rotation was still the “master clock” against which other clocks were calibrated and adjusted on a regular basis. As technology progressed, the need for higher-resolution timing increased. The transition to atomic time marked the end of an era when humanity lived in sync with planetary rhythms.
After many years of discussions by different standards bodies, in November 2022, at the 27th General Conference on Weights and Measures, it was decided to abandon the leap second by or before 2035. The decision to eventually eliminate leap seconds represents the final break between human timekeeping and Earth’s rotation.
What we’re losing is more than just technical precision. For thousands of years, human civilization was directly connected to Earth’s rotation. Our days, our seasons, and our understanding of time itself were tied to the planet beneath our feet. The leap second was the last thread connecting modern timekeeping to this ancient relationship.
Conclusion: From Cosmic Dance to Atomic Precision
The time before leap seconds wasn’t just a different era of timekeeping – it was a fundamentally different relationship between humanity and our planet. For millennia, we trusted Earth to keep time for us, adjusting our clocks and calendars to match the rhythm of rotation and revolution. We built sundials, developed mechanical clocks, and created global time zones, all based on the assumption that Earth was our most reliable timekeeper.
The discovery that Earth’s rotation was irregular, unpredictable, and influenced by everything from ice ages to earthquakes marked a turning point in human civilization. When we started adding leap seconds in 1972, we were acknowledging that human technology had finally surpassed the precision of our planetary timekeeper.
Now, as we prepare to abandon leap seconds entirely, we’re completing a journey that began with ancient humans placing sticks in the ground to track shadows. The irony is striking: in our quest for perfect time, we’ve disconnected ourselves from the very celestial mechanics that shaped our understanding of time in the first place.
Did you ever imagine that something as simple as keeping time could tell such a complex story about our planet’s past and our technological future?
