Why half the world will have to endure 2016 for an extra moment

Published: December 30, 2016

The year 2016 will be a little longer than expected as most of the world is about to experience its 28th leap second.

In Pakistan this means that a second after 4:59:59am on January 1, the clock will read 59:60 before moving on to 5:00:00am. This is because of what is called the ‘leap second.’

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A leap second, applied to Coordinated Universal Time (UTC) adjusts the time to match the mean solar time, which is based on the position of the sun in the sky. This is done to ensure that time calculated by Earth’s rotation does not drift away from atomic time due to irregularities in the Earth’s rate of rotation.

Our planet’s orbit around the sun is not actually a perfect circle – it’s more like an ellipse and the Earth does not maintain its constant speed at a perfect rate every day. For this reason, a leap second is applied to adjust time.

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But because leap seconds are applied at the exact same time around the world, it means that anyone in the UTC±00:00 time zone or west of it—the Western Hemisphere—will have one extra second in 2016. Islamabad, Paris, Moscow, Tokyo, and most of the Eastern Hemisphere, on the other hand, will have one extra second in 2017. There have been 26 leap seconds since 1972.

This article originally appeared on Quartz.

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