Do you wish 2016 would last a little longer? You might just get your wish. That’s because an extra second — a leap second — will be added to the final minute of 2016. But why?
Leap seconds, just like leap days, are occasionally inserted to ensure our clocks are in sync with changes in the Earth’s rotation.
How Is Time Kept Accurate?
Something called “International Atomic Time” keeps our clocks across the globe in sync. It is a very accurate and stable time scale; a weighted average of the time kept by about 200 atomic clocks in over 50 national laboratories worldwide. Atomic time is measured through vibrations of atoms in a metal isotope that resembles mercury and can keep time to within a 10th of a billionth of a second a day. The result is extremely accurate time that can be transmitted by radio throughout North America where atomic watches and clocks can receive the signal. Improved time and frequency standards have many applications.
For instance: ultra-precise clocks can be used to improve synchronization in precision navigation and positioning systems, telecommunications networks, and deep-space communications. But from their careful observations of the positions of the stars, astronomers have deduced that our Earth’s rotation is ever so slightly slowing down at a non-uniform rate, probably attributable to changes in the distribution of mass in the Earth’s interior. As a result, Earth falls out of step with our atomic clocks. When the difference between the two amounts to one second, a “leap-second” is inserted into the atomic time scale.
This will happen on Saturday, December 31st, when the final minute of 2016 as measured at the Greenwich meridian time will last 61 seconds. This will correspond to 6:59:60 p.m. Eastern Standard Time (so no, it won’t happen as the ball drops in Times Square).
Have We Added Leap Seconds Before?
The first time a leap second was implemented was in 1972. So not only was that a leap year of 366 days (as is the case also in 2016), but there were not one, but two extra seconds added — one at the end of June 30th, and the other, as will be the case this year, at the end of December 31st.
Now for most of us, this slight alteration of the time scale is not important, but in a very few cases it can affect some jobs: In 1972 the Hayden Planetarium got a phone call from the man who was responsible for dropping the famous lighted ball from atop of what was then called the Allied Chemical Tower in Times Square. In those days, the procedure for lowering the ball was done using manpower and a simple rope and pulley. The caller wanted to get an explanation about the extra second, because (to quote him), “I don’t want to drop that ball one second too soon!”