Leap seconds have helped keep clocks synchronized with Earth’s rotation, but their time may be over – here’s why
If you think about leap years, it may seem like humans have a very odd way of measuring time. Sometimes, timekeepers even have to add extra seconds to clocks.
Leap seconds have been utilized to maintain synchronization between clocks and Earth's rotation, but their future may be uncertain. Leap seconds serve a similar purpose to leap years, which add an extra day to February every four years to account for Earth completing one orbit around the Sun in approximately 365.25 days. If leap seconds were not added, equinoxes and solstices would gradually shift, altering the seasons.
Originally, a day was defined as a full rotation of Earth. However, Earth's rotation is not entirely consistent, requiring enhanced precision in modern timekeeping. This necessitated a departure from relying on Earth's rotation as the basis for the definition of a second. This shift resulted in a discrepancy between the definition of a second in the past and its modern definition.
Over time, this discrepancy can accumulate, causing the systems to become a few seconds out of sync with Earth's movement. To rectify this, leap seconds are employed. The leap second was first introduced in the 1970s, when the General Conference on Weights and Measures defined a second based on the cesium-133 atom. Today, Coordinated Universal Time (UTC) relies on atomic clocks using cesium-133.
Despite the small difference, it can accumulate to a significant fraction of a second annually. Since 1972, 27 leap seconds have been added to UTC, aiming to keep the discrepancy between UTC and UT1, the time scale based on Earth's rotation, below 0.9 seconds. The last leap second was added at the end of 2016. However, there is speculation that this practice may come to an end in October 2026.
The addition of leap seconds can disrupt high-precision computer systems, as most operating systems assume a day consists of 86,400 seconds and time advances in a predictable manner. When a leap second is added, clocks jump by one second on the designated day, causing inconsistencies. In 2012, Reddit experienced an outage following a leap second addition on June 30.
Despite concerns, leap seconds have been crucial in maintaining the synchronization between clocks and Earth's rotation throughout history.
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