Leap second

A leap second, sometimes called an intercalary second, is a one-second adjustment applied to Coordinated Universal Time (UTC). This system is the international standard for civil timekeeping used in most countries. The purpose of a leap second is to keep our precise clocks in sync with the Earth's rotation. Without these adjustments, our highly accurate atomic clocks would eventually drift away from the actual position of the sun in the sky.

To understand why this is necessary, we must look at two different ways of measuring time. The first is International Atomic Time, or TAI. TAI is measured by incredibly precise atomic clocks that count the vibrations of atoms. The second is observed solar time, known as UT1. UT1 is based on the Earth's actual rotation on its axis. Because the Earth's rotation is not perfectly steady, UT1 varies over time.

The Earth's rotation speed changes due to several complex geological and climatic factors. One major cause is tidal friction, which acts to slow the Earth's rotation down. Other factors include the movement of the Earth's crust relative to its core and changes in mantle convection. Any event that redistributes mass within the planet can change its moment of inertia. This redistribution affects the rate of rotation because of the conservation of angular momentum. For example, the 2004 Indian Ocean earthquake is thought to have shortened the solar day by 2.68 microseconds.

Because these planetary changes are irregular, leap seconds are not precisely predictable. The International Earth Rotation and Reference Systems Service (IERS) manages this process. The IERS typically decides to insert a leap second about six months in advance. They aim to ensure the difference between UTC and UT1 does not exceed ±0.9 seconds. They usually make this decision when the difference approaches 0.6 seconds. The IERS announces these decisions every six months in a publication called "Bulletin C."

The history of timekeeping shows a long journey toward precision. In about AD 140, the astronomer Ptolemy subdivided the solar day into small fractions. By the year 1000, the scholar al-Biruni helped create the modern second by subdividing the day into minutes and seconds. In 1967, the definition of the second was revolutionized. It was redefined based on the duration of 9,192,631,770 periods of radiation from the transition between two hyperfine levels of the cesium 133 atom. The leap second system itself was officially introduced in 1972 to bridge the gap between atomic time and solar time.

Since 1972, there have been 27 leap seconds added to UTC. All of these have been positive leap seconds, which add one second to the length of a day. The most recent leap second occurred on December 31, 2016. By the time of that leap second, the difference between TAI and UTC was 37 seconds. This total comes from the original 10-second offset plus the 27 added leap seconds. While positive leap seconds are common, a negative leap second is theoretically possible if the Earth spins faster.

While leap seconds keep us in sync with the sun, they can cause problems for modern technology. Many digital systems, computer networks, and satellite-based systems require continuous and precise timestamping. Because not all systems implement leap-second adjustments in the same way, these adjustments can cause computational anomalies. This has led engineers to discuss other possible timekeeping measures that might eventually eliminate the need for leap seconds altogether. It remains a vital link between the microscopic world of atomic physics and the massive scale of planetary movement.