Tuesday to have a 'Leap Second’: Nasa

 A ‘leap second’ will be officially added to June 30 this year to account for the gradual decrease of earth’s rotation, said Daniel MacMillan of Nasa's Goddard Space Flight Centre in Greenbelt.
The slowing down is the result of a form of braking force caused due to the gravitational tug between Earth, the Moon and the Sun.
Earlier, scientists based time calculations on the Earth’s rotation, but after introduction of Very Long Baseline Interferometry (VLBI), that changed. 
The international network of stations that team up to observe and correlate found that the planet's rotation is slowing down overall because of tidal forces between Earth and the moon.
Roughly every 100 years, the day gets about 1.4 milliseconds, or 1.4 thousandths of a second, longer which amount to a large difference if you add up those discrepancies.
"At the time of the dinosaurs, Earth completed one rotation in about 23 hours," says MacMillan.
"In the year 1820, a rotation took exactly 24 hours, or 86,400 standard seconds. Since 1820, the mean solar day has increased by about 2.5 milliseconds."
By the 1950s, scientists were faced with the need for more precise timekeeper than the Earth and in 1967, the second was redefined to be based on predictable measurement made of electromagnetic transitions in atoms of cesium.
These "atomic clocks" are accurate to one second in 1,400,000 years and most people around the world rely on the time standard based on the cesium atom: Coordinated Universal Time (UTC).
Universal Time 1 (UT1) is based on VLBI measurements which rely on astronomical reference points and are precise up to 5 microseconds.
"These reference points are very distant astronomical objects called quasars, which are essentially motionless when viewed from Earth because they are located several billion light years away," says Goddard's Stephen Merkowitz, the Space Geodesy Project manager.
Nasa says although leap seconds were initially added to provide a UTC time signal for sea navigation, introduction of GPS and satellite navigation systems has made it no longer needed.
Now a leap second is added in the UTC to keep it within 0.9 seconds of UTI.
Normally, the clock would move from 23:59:59 to 00:00:00 the next day. Instead, at 23:59:59 on June 30, UTC will move to 23:59:60, and then to 00:00:00 on July 1. In practice, this means that clocks in many systems will be turned off for one second.
Proposals to abolish the leap second and let the two standards drift apart have also been made because of the cost of planning for leap seconds and the potential impact of adjusting or turning important systems on and off in synch.
If they are allowed to go further and further out of synch, the difference will be roughly 25 minutes in 500 years.
In the meantime, leap seconds will continue to be added to the official UTC timekeeping. The 2012 leap second is the 35th leap second to be added and the first since 2008.


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