Time Is Up for the Leap Second

Time goes by quickly, doesn't it? Sometimes it seems that way, but did you know that in 2035 time will literally stop skipping ahead? That's because the practice of adding a "leap second," which is a theoretical second of time that has never actually happened, will end for at least 100 years.

In November 2022, the General Conference on Weights and Measures (CGPM), which is the international timekeeping body that determines global standards for measuring time, passed a resolution to let clocks run in the future without adding or subtracting time.

The decision is a move toward atomic timekeeping, a practice in which time is measured by the extraordinarily precise collective output of about 400 atomic clocks powered by clouds of atoms tossed upward by lasers into Earth's gravitational field. It's a move away from timekeeping's connection with universal time, also known as astronomical time, which tracks time using the speed of Earth's rotation around its axis and the movement of the sun across Earth.

Why Were Leap Seconds Added to Track Time?

Why were leap seconds added to our timekeeping process in the first place? The addition of a leap second here and there was meant to bridge the difference between measurements of atomic and astronomical time. The truth is, Earth doesn't actually spin around its axis at the same rate in every 24-hour cycle, yet our timekeeping devices track the same exact 24 hours in a day, down to the same measurement of 86,400 seconds. The problem between our measure of timekeeping and the reality of time passing occurs because Earth's rotation is rarely completed at the same rate as our 24-hour clocks.

There are daily and even seasonal cycles that cause Earth to have more — or less — time in each day. Since the year 2000, for example, a chart tracking changes to Earth's rotation rate shows that in the last few years it has been rotating faster after spending nearly two decades slowing down. On June 29, 2022, Earth even set a record for the shortest known day, completing one rotation in 1.59 milliseconds less time.

In addition, natural disasters can cause Earth's rotation to speed up. Seismic activity can be another factor. After the 2004 earthquake in Indonesia and neighboring countries along the coast of the Indian Ocean, NASA's Jet Propulsion Laboratory scientists recorded a three-microsecond increase in Earth's rotation. In 2011, after the Tōhoku earthquake that shook Japan with a magnitude of 8.9, Earth's rotation sped up by 1.8 microseconds.

Confusing as it seems, despite Earth's increase in rotation speed over the last several decades and after a series of natural disasters, Earth's rotation has actually been slowing down. The reason for this miniscule-yet-impactful slowdown over millions of years is largely due to Earth's gravitational pull, which creates a slight convex "bump" on Earth directly underneath the moon. This difference between the highest point of our planet's bump and the moon's pull creates torque — the force that attracts two objects closer — on both Earth and the moon, slowing Earth's spin.

Other factors that can impact Earth's rotation and cause it to become slower include the sloshing of our oceans and the gusts of our winds. During the years in which El Niño winds occur, Earth spins slightly slower because of the shift in winds.

Leap Seconds Have Been Added at Irregular Intervals

One might be tempted to think that, given the regularity with which time is measured, leap seconds would be added at predetermined and regular intervals. This actually isn't the case.

After the first leap second was used to pad our global measurement of atomic time in 1972, only 27 have been added since — and they were only added at irregular intervals at that. The most recent leap second was introduced in 2016.

Although the General Conference on Weights and Measures will continue to add occasional leap seconds as needed until 2035, these additions won't entirely bridge the gap between Earth's rotation and our measurement of it. Physicists aren't entirely sure how significant the difference will be, but they are certain it won't match up.

It could take anywhere between 50 and 100 years for the difference between Earth's rotation and atomic time to equal a one-minute discrepancy, at which point it may be best to simply allow the final minute of the final day of the year to stretch to two minutes — instead of adding a leap minute.