Wait Just a Second

The International Earth Rotation and Reference Systems Service announced that June 30, 2012, will contain an extra second. National time-keeping centers around the globe will insert the "leap second" into their master clocks, and all other clocks (including so-called "atomic clocks") that get their time from the master clock, will be updated similarly.

Leap seconds keep our clocks more or less synchronized to the Earth's rotation. But the Earth does not spin at a constant rate. Over a period of 1,000 days, a clock keeping time based on the rotation of the Earth, a time scale called UT1, would lose about 2.3 seconds compared to the world's standard time scale, Coordinated Universal Time (UTC), which is based on the atomic second and referenced to the period of the Earth's rotation about 100 years ago.

To keep UTC to within 0.9 second of UT1, so-called "leap seconds" are periodically added to UTC. The last leap second occurred on December 31, 2008. Previous to that, leap seconds had been added every year or two going all the way back to 1972.

GPS operations are unaffected by the introduction of a leap second because GPS (System) Time is not adjusted for leap seconds. GPS Time was set equal to UTC in 1980 and is currently 15 seconds ahead of it. As of June 30, 2012, this increased to 16 seconds. GPS does provide UTC by transmitting the necessary data in subframe 4, page 18 of its navigation message, permitting a receiver to compute UTC from GPS Time.

The upcoming leap second might be the last. The United States has proposed to a working group of the International Telecommunication Union that leap seconds be abolished, as they are cumbersome, and incorrect use could lead to problems with electronic navigation systems such as GPS. But the debate on the abolition of leap seconds is far from over. [ITU has waffled so far, and does not expect to reach a decison on this question until 2015 at the earliest.]  -- Richard Langley



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