So what on Earth is a leap second?
We used to use the Earth’s dutiful rotation as a way of measuring time. It pirouettes on its axis once every 24 hours, which can then be divided into minutes and seconds. But the Earth’s rotation is annoyingly irregular, with some days ending up being a tiny bit longer or shorter than others.
There’s nothing science hates more than unpredictability, so in the 1950s atomic clocks were introduced to keep time.
By measuring the regular atomic vibration in the element cesium (which oscillates exactly 9,192,631,770 times a second), we ended up with a clock that can be used to score off seconds with remarkable accuracy. Multiple atomic clocks work in unison to precisely calculate world time.
But that leaves a problem. If we lived on atomic time it’d very slowly gravitate away from the Earth’s actual time. In a few years we’d be a second out of sync, in hundreds of years we’d be a minute out and after several hundred thousand years we could be eating lunch in the middle of the night.
So time-keepers introduced the leap second. As the atomic clock’s perfect accuracy (known as International Atomic Time, or TAI, from the French name Temps Atomique International) veers farther and farther away from the Earth’s clumsy rotation (called Solar Time), the IERS introduces a leap second to bring them back into perfect parity (known as Coordinated Universal Time, or UTC).
Positive or negative leap seconds (a second added or taken away) are introduced every time atomic and solar time get a second out of sync in either direction (although a negative leap second hasn’t been used in the history of leaps, which kicked off in 1972). They are strictly regimented to be added either on December 31st or June 30th.
The last one was added in December 2008. Since then the Earth has dragged behind, so a positive leap second will be added in June to square everything up. At the chime of midnight on June 30th it will take two seconds to transition into July instead of one, thus delaying UTC by a second.
From January 2009 to July 2012 UTC and TAI are 34 seconds apart (because of the various leap seconds added). From July 2012 onwards, they’ll be 35 seconds apart.
You will probably not notice the change, but people running complicated infrastructure like telecommunications, GPS, air traffic control and the internet are going to have a headache getting everything back in sync.
Because of this irregular change in time there have been several proposals to abolish leap seconds altogether, instead replacing them with entire leap hours every few hundred years. But, with arguments still ongoing, Paris’ time guardians will continue adding an extra second every few years for now. What will you do with your bonus second this June?