Leap-second on December 31, 2016


Ed. Note:  Did you know that today, December 31, 2016, will be longer by one additional second? By the time most of you read this post, it will already have been done. Happy New Year!

From Larry Sessions in EarthSky HUMAN WORLD | December 31, 2016, “Leap second to be added tonight”


Why do we need a leap second? Isn’t the length of our day set by the rotation of the Earth? Like the ancients who insisted that all motion in the heavens must be perfect, uniform and unvarying, many of us today assume that the Earth’s rotation – its spin on its axis – is perfectly steady. We learned, correctly, that the sun, moon, stars and planets parade across our sky because the Earth turns. So it is easy to understand why we assume that the Earth’s rotation is precise and unwavering. Yet Earth’s rotation does not stay perfectly steady.

See why below:

From the U.S. Naval Observatory,


2016 will drag on for just a bit longer than most other years. Yes, it has already stretched an extra day in length thanks to the rules of the Gregorian Calendar which deem it to be a leap-year, but thanks to our modern precision timekeeping methods it will get one extra second added to its length in the last minute before the strike of 00 hours, 00 minutes, and 00 seconds Coordinated Universal Time (UTC) on January 1st, 2017.

This “leap second” is necessary due to the gradual slowing of the Earth’s rotation when compared to the ultra-precise atomic time scale known as International Atomic Time (TAI) that is now observed around the globe. Since 1972 our civil time has been based on TAI, whose second is defined by a very precise atomic frequency. This time-scale exists coincidentally with another, known as UT1, which closely mirrors the length of the mean time between successive transits of the Sun over a standard meridian.

Currently the difference between these two time-scales amounts to about 0.0015 to 0.002 seconds per day, resulting in a cumulative difference between TAI and UT1 of one second over the span of a few years, and when the difference approaches one full second a “leap second” is inserted to coordinate the two time-scales for another few years.

The actual date of insertion for a given “leap second” is based on observations carried out in part by the USNO’s Earth Orientation Department, which keeps close tabs on how the Earth rotates with respect to a distant reference frame made up of thousands of very distant quasars [like those in the Hubble image below]. While 2016 will amount to an extra-log year, at least it won’t be like 1972, which saw the insertion of two “leap seconds” in addition to being a leap year.