You would definitely have come across news that June 30 will be one second longer than other days because of addition of an extra second or ‘leap’ second and one of the major reasons behind this is that Earth’s rotation is gradually slowing down and addition of leap seconds is a way to account for this gradual decline in rotation.
For those who are not aware, a day lasts for 86,400 seconds. This is according to the time standard that we follow in our daily lives – Coordinated Universal Time, or UTC as many people know it. UTC is basically “atomic time” wherein the duration of one second is based on extremely predictable electromagnetic transitions in atoms of cesium. These transitions are so reliable that the cesium clock is accurate to one second in 1,400,000 years.
However if we go by actual astronomy, this is not the case as the mean solar day – the average length of a day, based on how long it takes Earth to rotate – is about 86,400.002 seconds long. This extra .002 seconds is because of the fact that our planet is slowing down a bit because of the braking forces in play caused by gravitational tug of war between Earth, the moon and the sun.
According to scientists, the mean solar day hasn’t been 86,400 seconds long since the year 1820 or so.
One would say that 2 milliseconds doesn’t make a whole lot of difference in our day-to-day lives, but if we consider that building up over the years, it would add up to almost a second and if not corrected these seconds would build up into minutes and hours and… – you get the picture.
However, having said that, this doesn’t happen in reality as the length of each individual day varies in an unpredictable way. Rather going into greater details, NASA explains that the length of day is influenced by a range of factors including seasonal and daily weather variations, dynamics of the Earth’s inner core, variations in the atmosphere and oceans, groundwater, and ice storage, and oceanic and atmospheric tides.
El Niño is one such factor that could bring about atmospheric variations that could cause Earth’s rotation to slow down and increasing the length of the day by as much as 1 millisecond.
In a bid to monitor the Earth’s rotation, scientists use extremely precise technique called Very Long Baseline Interferometry (VLBI) which involves measurements to be taken from worldwide network of stations, with Goddard providing essential coordination of VLBI, as well as analyzing and archiving the data collected.
VLBI measurements based time is called the Universal Time 1, or UT1 and as UT1 isn’t as uniform as the cesium clock, UT1 and UTC tend to drift apart. To keep the two time standards within 0.9 seconds of each other leap seconds are added, when needed.
The decision to add leap seconds is made by a unit within the International Earth Rotation and Reference Systems Service.
When are leap seconds added
So why June 30? It has long been practiced that a leap second is inserted either on June 30 or December 31 – because one marks the end of first six months of the year and the other marks the end of the calendar year.
Normally, the clock would move from 23:59:59 to 00:00:00 the next day. But with the leap second on June 30, UTC will move from 23:59:59 to 23:59:60, and then to 00:00:00 on July 1. In practice, many systems are instead turned off for one second.
Previous leap seconds have created challenges for some computer systems and generated some calls to abandon them altogether. One reason is that the need to add a leap second cannot be anticipated far in advance.
“In the short term, leap seconds are not as predictable as everyone would like,” said Chopo Ma, a geophysicist at Goddard and a member of the directing board of the International Earth Rotation and Reference Systems Service. “The modeling of the Earth predicts that more and more leap seconds will be called for in the long-term, but we can’t say that one will be needed every year.”
Proposals have been made to abolish the leap second. No decision about this is expected until late 2015 at the earliest, by the International Telecommunication Union, a specialized agency of the United Nations that addresses issues in information and communication technologies.