Time Precise

Home / Learning / Leap Seconds

Leap Seconds Explained

Why occasionally we add an extra second to our clocks

What is a Leap Second?

A leap second is an added second to UTC (Coordinated Universal Time) to keep it synchronized with Earth's rotation. It's added when Earth's rotation slows down slightly, making our atomic time slightly ahead of solar time. The extra second is inserted at the end of the day, creating a minute with 61 seconds instead of the usual 60.

Unlike leap years, which follow a predictable calendar pattern, leap seconds are inherently unpredictable. Earth's rotation varies due to tidal forces, climate changes, and geological events. Scientists at the International Earth Rotation and Reference Systems Service (IERS) monitor Earth's rotation and announce leap seconds when the difference between atomic time and solar time approaches 0.9 seconds.

Quick Fact

Unlike leap years (add 1 day), leap seconds are unpredictable because Earth's rotation varies due to tidal forces, climate changes, and geological events.

Why Do We Need Leap Seconds?

Our modern world runs on two different concepts of time. Atomic time (TAI) is based on the incredibly precise vibrations of cesium atoms, which never change. Earth rotation time is based on how long it takes our planet to complete one full spin. These two systems were once perfectly aligned, but they've gradually drifted apart.

Earth's rotation is slowing down. Tidal friction caused by the Moon's gravitational pull acts as a brake on our planet's spin. Additional factors include post-glacial rebound, atmospheric circulation patterns, and even major earthquakes. As a result, days are getting about 1.7 milliseconds longer each century.

Without leap seconds, atomic time would eventually diverge from solar time. After just a few centuries, noon on atomic clocks wouldn't match noon on the sun. Navigation systems, astronomical observations, and everyday life would be affected. Leap seconds bridge this gap, keeping our clocks within 0.9 seconds of Earth's actual rotation.

History of Leap Seconds

The modern system of leap seconds was introduced on January 1, 1972, when Coordinated Universal Time (UTC) was redefined. Since then, 27 leap seconds have been added. The pace has varied: some decades saw frequent additions, while others had none.

The 1970s and 1980s required the most adjustments as the system was calibrated. The longest gap without a leap second was from 1999 to 2005, a seven-year stretch where Earth's rotation happened to speed up slightly. More recently, leap seconds were added in 2012, 2015, and 2016.

The Controversy: Should We Abolish Leap Seconds?

Leap seconds have become increasingly controversial in the technology world. Major tech companies like Google, Amazon, and Meta have pushed for their elimination. The primary concern is that leap seconds create complications for computer systems, financial markets, and telecommunications networks.

In 2012, a leap second caused outages at major websites including Reddit, LinkedIn, and Yelp. The gaming network PlayStation Network also experienced disruptions. These incidents highlighted how deeply integrated precise timekeeping has become in our digital infrastructure.

Proponents of keeping leap seconds argue they maintain an important connection between human timekeeping and astronomical reality. They point out that without adjustments, the drift would accumulate over centuries. However, opponents note that the civil timekeeping system has already drifted from solar time many times throughout history.

In November 2022, at the General Conference on Weights and Measures, delegates voted to abolish leap seconds by 2035. Under the new plan, UTC will be allowed to drift further from solar time, potentially accumulating an error of a minute or more over centuries. The decision reflects the priority of digital systems over astronomical precision.

How Leap Seconds Are Added

Leap seconds can only be added on two specific dates each year: June 30 or December 31. The decision is announced at least six months in advance by the IERS. When a leap second occurs, it's inserted at 23:59:60 UTC, creating a minute with 61 seconds.

Different systems handle leap seconds in various ways. GPS time doesn't use leap seconds at all. Some computer systems "smear" the extra second across the final minute, gradually slowing down clocks. Others simply ignore the leap second entirely. This inconsistency can cause synchronization issues between systems that handle leap seconds differently.

Frequently Asked Questions

How often do leap seconds occur?

There's no fixed schedule. Leap seconds have been added between once every few years to multiple times in a single year. The IERS makes the decision based on measurements of Earth's rotation speed.

What happens during a leap second?

The final minute of the day has 61 seconds. In UTC, the time goes from 23:59:59 to 23:59:60 before midnight. Most people don't notice it, but computer systems must be designed to handle this extra second.

Will leap seconds stop after 2035?

Yes, the plan approved in 2022 will eliminate leap seconds by 2035. After that, UTC will gradually drift from solar time, with corrections potentially made once every century or so.

Do leap seconds affect my clock?

Most smartphones and computers receive time updates that handle leap seconds automatically. You may not notice anything, but the devices connected to internet time servers will adjust accordingly.