The wait is going to be longer this New Year’s Eve!
You must have heard about a “Leap Year”, a year with 366 days instead of the usual 365 days, with an extra day, February 29, accordingly called “Leap Day”. But this year, an extra second is going to be inserted to the clock as well, and this will be called a “Leap second”.
The U.S. Naval Observatory’s (USNO) Master Clock Facility in Washington, D.C. is responsible for collecting appropriate data for this adjustment.
But before we understand the nitty-gritty’s of it, we need to understand the different type of scientifically accepted time scales such as:
- Coordinated Universal Time (UTC)
- Universal time 1 (UT1)
Both of the above are acceptable standards within “Universal Time (UT)” which has many other versions besides the above two like UT0, UT1R, UT2, etc.
While UTC is based upon International Atomic Time (aka TAI, which is a high-precision atomic time standard based upon more than 400 Caesium clocks, which also defines the SI standard of time), the UT1 is based on Earth's rotation relative to distant celestial objects (like stars and quasars).
Over a period of time, there may arise a difference between the two time standards which may cause serious scientific errors in the long run. Thus, it was decided in 1970 that the maximum permissible difference would be 0.9 seconds. This monitoring exercise is carried out by International Earth Rotation and Reference Systems Service (IERS) and it calls for additional seconds to be added whenever the difference crosses over permissible limit. IERS, in turn, is assisted by USNO which generates required astronomical and computational data by way of Very Long Baseline Interferometry (VLBI) technique to determine relative positions of celestial objects in the observable universe.
In simple terms :
TAI + leap seconds = UTC
If, UTC- UT1 > 0.9 seconds, then, IERS asks USNO to add a leap second.
Since 1972, 26 leap seconds have been added by the IERS orders with the latest on June 30, 2015. Now the 27th leap second will be inserted on December 31, 2016, precisely at 23 hours, 59 minutes and 59 seconds UTC. This is equivalent to 5 hours, 29 minutes, 59 seconds as per Indian Standard Time (IST) on 1st January, 2017.
After the insertion of this new second, the total difference between UTC and TAI will be 37 seconds. And, the same exercise will be carried out in the future as well, probably within the coming 500-750 days.
These new seconds are merely the result of differences in the processes employed for deriving the two standards. While one is dependent on the Earth’s rotation, the other is independent of it. Consequently, a difference occurs if one of the two changes. Experts believe that the Earth’s rotation is slowing down. Despite there being no consensus on the reasons for this, some attribute it to the constant shifting of Earth’s rotational momentum to the Moon’s orbital momentum causing Earth to take a slightly longer time to complete a full rotation on its own axis.
- The Greenwich Meridian Time (GMT), commonly known to us, is a time zone officially used in some European and African countries. It is shown in either 24 hours format or a 12 hour format indicated by a.m. and p.m.
- UTC, on the other hand, is not a time zone, but a time standard that is the basis for civil time and time zones worldwide. Thus, it is not used for setting the local time in any country.
- Like GMT, Indian Standard Time (IST) is also a time zone followed by India and Sri Lanka and it equals UTC + 05:30 hours, instead of GMT + 05:30 hours.
|Time Zone||Time Standard|
|24 hours or 12 hours format||IST = UTC + 05:30 hrs.|