A New Year’s Eve ‘leap second’ could make your countdown to midnight last 11 seconds instead of 10

Roughly every one and half years, one extra second, or leap second, is added to clocks around the world.

The next leap second will be added to the world’s clocks on December 31, 2016, at 23 hours, 59 minutes and 59 seconds Coordinated Universal Time (UTC). Since the leap second is added at the same time worldwide, people will experience the extra second at a different time depending on their location.

Most of the Western Hemisphere will have one extra second in 2016, while most of the Eastern Hemisphere will have one extra second in 2017.

For anyone in the UTC±00:00 time zone, which includes London and the UK, the official clock will read 23.59.60 right before clicking over into 12.00.00 at midnight. (Normal clocks go from 23.59.59 to midnight.) That, at least conceptually, will make your traditional New Year’s eve countdown last 11 seconds rather than 10 this year. For those in US Eastern Standard Time the clock will read 18:59:60 (6:59:60 p.m.), and for those in Japan Standard Time the clock with read 08:59:60 a.m. on January 1, 2017.

To see how your part of the world will be affected, use this tool provided by Quartz.

Why do we have the leap second?

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 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 last leap second was added on June 30, 2015, and the one before that was June 30, 2012.

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