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The Leap Second Explained

Leap seconds are added to Universal Coordinated Time (UTC) - and clocks worldwide - in order to compensate for the slowing of the Earth's rotation.

How leap seconds work

Illustration image

A second is defined by exactly 9,192,631,770 oscillations of a cesium atom.
(illustration does not show cesium atom)


UTC is the time standard used to determine local times in time zones worldwide. It is primarily based on the combined output of several highly precise atomic clocks, a statistical time scale called International Atomic Time (TAI). Although a normal day has 86,400 seconds, in this time scale one second is not defined as one 86,400th of the time it takes Earth to rotate around its axis but as the time it takes a Cesium-133 atom at the ground state to oscillate precisely 9,192,631,770 times.

The advantage of this definition is that it is extremely precise: atomic clocks deviate only approximately one second in 20 million years. On the other hand, the Earth's rotation, which is expressed by the time standard UT1, is far less reliable. It slows down over time, which means that days get longer. On average, an Earth day is about 0.002 seconds longer than the daily sum of the 86,400 seconds measured by the atomic clocks. This makes for a discrepancy between TAI and UT1 of around 1 second every 1.5 years.

Leap seconds are added to our clocks (UTC) so this discrepancy does not get too large over time and the time we use is synchronized as much as possible with the Earth's rotation. Before the difference between UTC and UT1 exceeds 0.9 seconds, one second is added to UTC. This means that the time difference between TAI and UTC amounts to an integral number of seconds because whole seconds are added, while the time difference between UTC and UT1 is always less than 0.9 seconds.


Did you know...?

Leap seconds can be positive (one second added) or negative (one second omitted) - at least in theory: so far, all leap seconds were positive, and given the slowing of the Earth's rotation it is unlikely that a negative leap second will ever occur.

The speed of the Earth's rotation differs from day to day and from year to year, so the difference between UT1 and TAI varies accordingly. For example, the accumulated discrepancy over one year was 0.28 seconds in 2011, but only 0.02 seconds in 2001 (based on data from IERS).

Not only do days become longer, but the rate at which day lengths increase also grows over time – but only by about two thousandths of a second per century, according to Dr Bruce Warrington, from Australia’s National Measurement Institute (NMI). This means that at the moment days are 0.002 seconds longer than the sum of 86,400 seconds measured by atomic clocks; in 100 years, they are expected to be 0.004 seconds “too long”.

Some scientists suggest abolishing leap seconds, effectively redefining the way we measure time.

Will leap seconds be used in the future?