Leap second

Sometimes we add one second to our clocks.

This is called a leap second. It helps our clocks match the Earth.

The Earth does not always spin at the same speed. It can change a little bit. This can make our clocks get ahead.

We add a second to fix the time. This keeps our clocks and the Earth in sync. It has happened 27 times since 1972.

Most leap seconds happen at the end of June or December. This helps us stay on track.

Sometimes we add one second to our clocks. This is called a leap second.

We use atomic clocks to tell time. These clocks are very precise. They use atoms to keep perfect time. This is called International Atomic Time, or TAI. But the Earth does not spin at a steady speed. Its rotation changes because of many things.

Earth's spin changes due to things like tides. Changes in the Earth's crust also matter. These changes can speed up or slow down the spin. Because of this, atomic time can get ahead of solar time. Solar time is based on the Earth's rotation.

To fix this, we add a leap second. This makes the clocks match the Earth again.

A group called the IERS decides when to add one. They usually tell us six months in advance. They do this to keep the time difference small. Since 1972, there have been 27 leap seconds. The most recent one was on December 31, 2016. Most happen at the end of June or December.

Have you ever wondered why our clocks might occasionally need an extra second? This tiny adjustment is called a leap second.

Most of our world uses Coordinated Universal Time, or UTC, to keep track of the day. This system relies on International Atomic Time, known as TAI. TAI uses very precise atomic clocks to measure time. However, the Earth does not always spin at a perfectly steady speed. Because of this, our precise clocks can get out of sync with the sun.

To understand how this works, we must look at two different ways of measuring time. TAI is based on the steady vibrations of atoms. On the other hand, solar time, or UT1, is based on the Earth's actual rotation. The Earth's spin changes because of many natural events. For example, tidal friction can slow the rotation down. Changes in the Earth's crust or moving mass can also affect the speed.

When the difference between atomic time and solar time gets too large, a leap second is added. This step brings the two time scales back together.

People have been studying how to divide time for a very long time. Around the year 140, an astronomer named Ptolemy worked with solar days. Later, in the year 1000, a scholar named al-Biruni helped create the modern second. In 1874, the second was proposed as a base unit for measurement. By 1967, scientists redefined the second using the cesium 133 atom. This made timekeeping much more accurate than ever before. The leap second system itself was officially introduced in 1972.

Since that time, there have been 27 leap seconds added to our clocks.

All of these have been positive leap seconds, which means they added one second to the day. The most recent leap second happened on December 31, 2016. These adjustments are not predictable like a calendar. A group called the IERS makes the decision about six months in advance. They try to keep the difference between the two time scales under 0.9 seconds. They usually announce these changes in their "Bulletin C" publication.

Leap seconds are a great way to see how science connects to our daily lives. They link the tiny world of atoms to the huge world of planets. Even though a single second seems small, it matters for many modern tools. Digital systems, satellites, and computer networks all need very precise timing to work. If these systems do not handle the extra second correctly, they can run into errors. It is a fascinating way that we keep our human clocks in step with the spinning Earth.

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.