December 31, 2016. Just before the start of a leap year, a leap second is going to be added to the day by the US Naval Observatory. This second is will be added at 23 hours, 59 minutes and 59 seconds Coordinated Universal Time (UTC) or at 6:59:59 pm EST.
This presents that rare opportunity for each of us ‘to be given a second’ more before the end of this year. Maybe we should add another second in the countdown before the New Year! (Of course just for fun)
So, you have heard of leap years, but have you heard of leap seconds?
For those of you that can’t quite put your finger on it, a leap year is called like that because of the addition of another day in February (the 29th), which is called a leap day, and this happens every 4 years.
A leap second on the other hand is the addition of a second to the year every now and then. This addition can vary from six months to seven years and the most recent one was on June 30, 2015.
This time however, it’s been added to December 31, to give us an extra second in the countdown for the New Year.
Why do we need leap seconds?
Time on Earth is based on the movement of Earth relative to other celestial bodies and from this reference frame the second (and its length) was defined. However, with the invention of the more precise “atomic” time scale, we received two time systems in which we measure the time. It was agreed that UTC (which is time on Earth) will always be a second behind TAI (the International Atomic Time).
The International Earth Rotation and Reference Systems Service (IERS) is responsible for monitoring this time difference and for calling for leap seconds, or removing leap seconds from UTC in order to keep the difference between the two times 0.9 seconds apart.
The reason why there are two separate times is because we cannot adhere to the UT1 time easily, as that would mean a ‘false’ time in calculating the day, the position of the Sun in the sky etc. The easier way is by adding or removing these leap seconds every now and then.
When the system was first introduced in 1972, the difference determined between UTC and TAI was 10 seconds. Since then, 26 additional leap seconds have been added and after the addition of the leap second on December 31, the time difference between UTC and TAI will be 37 seconds.