Roughly every one and half years, one extra second, or leap second, is added to clocks around the world.
First, the technical bits.
The length of day is determined by Earth’s rotation around its axis, called solar time. In the past, we used this natural clock to measure civil time.
But in the 1950s, we adopted more sophisticated technology, atomic clocks, to keep time.
The advantage of atomic clocks is that they are extremely precise. In fact, some atomic clocks might gain or lose a fraction of a second only once every 100 million years. The atomic clock’s perfect accuracy is great for regulating local times around the world, also known as Universal Coordinated Time (UTC).
The problem is that Earth’s rotation isn’t quite as regular. It actually slows down over time, making the actual length of a day a little longer than the 86,400 seconds of a normal day measured by atomic clocks. About every 1.5 years, the difference between our atomic clocks and solar time adds up to about one second.
Wired UK’s Mark Brown put this in perspective:
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.
To make sure this doesn’t happen, the International Earth Rotation and Reference Systems Service (IERS) in Paris (the agency that regulates the world’s clocks) introduced the leap second in 1972, which is added to UTC at the end of the months of December or June.
The next leap second will be introduced on June 30, 2012.
Here’s a little reminder from IERS:
Photo: International Earth Rotation and Reference Systems Service
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