A Leap Second to End the Year


A “leap second” will be added to the world’s official clocks today, December 31, 2016  at 23 hours, 59 minutes and 59 seconds Coordinated Universal Time (UTC). That is 6:59:59 p.m. EST in my neighborhood. I will get an extra second and it will pass in a heartbeat. Easy come, easy go.

Those official world clocks will actually read 23:59:60 before ticking over to midnight. That’s unique.

Why do we have to add a second now and then to those very accurate clocks?  It is because the Earth’s rotation around its own axis is gradually slowing down. I thought I felt things slowing down lately.

Atomic clocks tick away at pretty much the same speed over millions of years. Compared to the Earth’s rotation, atomic clocks are simply too consistent.

We have done this adding of a second since 1972. That first time, UTC was 10 seconds behind Atomic Time. We have added 26 leap seconds so far. I hope to accrue at least a half a minute extra in this lifetime.

It does not mean that the days are any longer. Only on that leap second day (which hardly anyone seems to hold a celebration for) we had 86,401 seconds instead of the usual 86,400 seconds.

Our units of time were defined based on the Earth’s rotation relative to distant celestial bodies. That changed when atomic clocks came into being in the mid-20th century.

Our Moon’s gravitational pull is one reason why the Earth’s spin is slowing down. We lose between 1.5 to 2 milliseconds per day compared to atomic time. We are off by a full second every 500 to 750 days.

Leap seconds are always added on June 30 or December 31 of a particular year. In 1972, they added them on both dates. But, I suppose, this leap second really won’t affect people, though I think it is nice to know and might be a good icebreaker at your New Year’s Eve party tonight.

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Ken Ronkowitz

A lifelong educator on and off the Internet. Random by design and predictably irrational. It's turtles all the way down. Dolce far niente.

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