Leap Second Makes 2016 One Second Longer
Posted on December 30, 2016
Those longing for 2016 to be over will have to wait one extra second longer than usual. A leap second was added to the calendar this year. The photo above shows the numerals 1 and 7 arriving in Times Square as part of the annual ball drop celebration in New York City. Media outlets are reporting the leap second news now that the year is nearing its end. The leap second was first announced in July in a press release from the U.S. Naval Observatory.The press release was entitled, "2016 Will Be One Second Longer." The leap second will be added to the world's clocks on December 31, 2016 at 23 hours, 59 minutes and 59 seconds. An organization named The International Earth Rotation and Reference Systems Service (IERS) determines when a leap second is needed.
The U.S. Naval Observatory - which maintains the Department of Defense USNO Master Clock - says in its release that measurements indicate the the Earth runs slow (on average) compared to atomic time by 1.5 to 2 milliseconds per day.
Measurements show that the Earth, on average, runs slow compared to atomic time, at about 1.5 to 2 milliseconds per day. These data are generated by the USNO using the technique of Very Long Baseline Interferometry (VLBI). VLBI measures the rotation of the Earth by observing the apparent positions of distant objects near the edge of the observable universe. These observations show that after roughly 500 to 750 days, the difference between Earth rotation time and atomic time would be about one second. Instead of allowing this to happen a leap second is inserted to bring the two time-scales closer together. We can easily change the time of an atomic clock, but it is not possible to alter the Earth's rotational speed to match the atomic clocks.A U.S. News story also explains that leap seconds cannot be calculated centuries in advance because the Earth moves erratically. All leap seconds so far have been "positive" additions requiring an extra second to be inserted.
The leap second won't impact the NYC Times Square ball drop according to this entry on Quora. The explanation given is that "the leap second will be adjusted five hours before midnight in New York."
Here's a leap second explanation from PBS Space Time: