The Leap Second Era is officially coming to a close.

Time seems to fly, doesn't it? But did you know that by 2035, the concept of a 'leap second,' which is a theoretical second that has never truly existed, will be a thing of history? It’s because the practice of adding a leap second will be halted for at least the next century.

In November 2022, the General Conference on Weights and Measures (CGPM), the global authority on timekeeping standards, passed a resolution allowing clocks to tick forward without any adjustments for leap seconds.

The decision represents a shift toward atomic timekeeping, where time is measured with extraordinary precision using the combined output of around 400 atomic clocks. These clocks are powered by atoms suspended by lasers in Earth's gravitational field. This move marks a departure from the traditional reliance on universal time, or astronomical time, which tracks time based on Earth's rotation and the sun's movement across the sky.

Why Were Leap Seconds Introduced to Keep Time?

The leap second was originally added to address the gap between atomic and astronomical time. The reason for this is that Earth doesn't spin consistently at the same speed in each 24-hour cycle, even though our timekeeping systems use a fixed 86,400-second day. The discrepancy arises because Earth's rotation doesn't always align with the 24-hour clock.

Earth's daily and seasonal cycles can result in variations in the length of a day. For instance, a chart tracking Earth's rotation since 2000 shows an increase in speed after nearly two decades of gradual slowing. On June 29, 2022, Earth set a new record for the shortest day, completing a full rotation 1.59 milliseconds faster than usual.

Natural disasters can also cause changes in Earth's rotation. Seismic events, for example, can impact the planet's speed. After the 2004 earthquake in Indonesia, scientists from NASA's Jet Propulsion Laboratory recorded a three-microsecond acceleration in Earth's rotation. Similarly, following the 2011 Tōhoku earthquake in Japan, Earth's rotation sped up by 1.8 microseconds.

Although it may seem paradoxical, despite Earth's accelerating rotation speed over recent decades and following various natural calamities, the planet's spin has actually been slowing down. This subtle, yet significant deceleration over millions of years is primarily attributed to Earth's gravitational pull, which causes a slight bulge on the planet's surface directly beneath the moon. This discrepancy between the highest point of Earth's bulge and the moon's gravitational force generates torque, a force that draws two objects toward each other, thereby slowing Earth's rotation.

Other contributors to the gradual reduction in Earth's rotation include the movement of our oceans and the winds that gust across the surface. For example, during El Niño events, Earth's rotation decelerates slightly due to the shift in winds.

Leap Seconds Have Been Added at Irregular Intervals

It might be tempting to assume that, considering the precise nature of timekeeping, leap seconds would be added at fixed intervals. However, this is not the case. In reality, leap seconds are inserted irregularly.

Since the initial leap second was introduced in 1972 to adjust the global measurement of atomic time, only 27 more have been added. These additional leap seconds have been inserted at unpredictable intervals, with the most recent one occurring in 2016.

Although the General Conference on Weights and Measures will continue to add leap seconds as necessary up until 2035, these adjustments won't fully close the gap between Earth's rotation and our timekeeping. While physicists aren't entirely certain about the exact scale of the difference, they are confident it won't align perfectly.

It may take anywhere from 50 to 100 years for the discrepancy between Earth's rotation and atomic time to accumulate into a full minute. At that point, it might be better to extend the last minute of the year to two minutes, rather than adding a leap minute.