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The real longest day 03.07.15

Despite 21 June being the official Summer Solstice this year, our longest day was actually on Tuesday 30th June.

Did Tuesday 30th June feel longer than your average day? Don’t worry if so, there’s a perfectly good reason for that. Scientists at the National Physical Laboratory (NPL), the UK’s home of measurement, added an extra “leap second” into our time scale, known as Coordinated Universal Time.

Following the directive issued earlier this year by the International Earth Rotation and Reference Systems Service based at the Paris Observatory, the additional second will help keep the international reference time based on atomic clocks in sync with solar time based on the Earth’s rotation.

We need to do this because atomic clocks are more than a million times better at keeping time than the rotation of the Earth, which fluctuates unpredictably and in the long term is slowing down. Leap seconds are inserted to make sure civil time does not drift away from time based on the Earth’s rotation. If not corrected, such a drift would eventually result in clocks showing the middle of the day occurring at night. While it would take hundreds of years for the difference to become obvious to most people, astronomers and celestial navigators rely on time being consistent with the conventional positions of the Sun, moon and stars to within a fraction of a second.

The planned insertion of the leap inspired people and organisations to come up with a range of ideas on what to do with the extra time. For example, Humberside Fire Service encouraged people to test their smoke alarms and US satirical show Last Week Tonight developed a website dedicated to inspiring people on how to use their extra second. In the 24 hours before the leap second was inserted the hashtag #LeapSecond was used more than 7,000 times on Twitter.

Inserting leap seconds can have consequences.  They are only inserted into the time scale sporadically (the previous one was in 2012) so it can be hard to implement them in computers and mistakes can cause temporary systems failures, usually caused by glitches in their code. This year the media reported some minor malfunctions with Twitter and Android as well as crashes for the websites of Instagram, Pinterest, Netflix, Amazon and Apple’s new media streaming service.

Ever since their introduction in 1972, the international community has been split as to the need for leap seconds. The UK Government believes that the consequences of breaking the link between civil time and the Earth’s rotation are not fully understood, and that the problems leap seconds currently cause can be dealt with by technical improvements. The fate of the leap second is expected to be decided at the World Radiocommunication Conference in November 2015.

To learn more about NPL’s work on time, visit:

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