What are leap seconds?
About every one and a half years, one extra second is added to Universal Coordinated Time (UTC) and clocks around the world. This leap second accounts for the fact that the Earth’s rotation around its own axis, which determines the length of a day, slows down over time while the atomi clocks we use to measure time tick away at almost the same speed over millions of years. So, leap seconds are a means to adjust our clocks to the Earth’s slowing rotation.
How leap seconds work
(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.
How many leap seconds have been added so far?
Since 1972, a total of 24 seconds have been added. This means that the Earth has slowed down 24 seconds compared to atomic time since then.
This does not mean that days are 24 seconds longer nowadays. Only the days on which the leap seconds are inserted have 86,401 instead of the usual 86,400 seconds.
When are leap seconds added?
Leap seconds are inserted at the end of the last day in June or December. When that is the case, UTC ticks from 23:59:59 to 23:59:60 before reverting to 00:00:00 (in the 12-hour format, this corresponds to 11:59:59 pm – 11:59:60 pm – 12:00:00 midnight). When that happens the last minute of the month has 61 instead of 60 seconds.
Who decides when leap seconds are added?
The International Earth Rotation and Reference System Service (IERS) observes the Earth’s rotation and compares it to atomic time. When the difference between the two approaches 0.9 seconds, they order a leap second to be added worldwide.