From VLBI, scientists have learned that Earth is not the most reliable timekeeper. 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. Granted, that’s about 100 or 200 times faster than the blink of an eye. But if you add up that small discrepancy every day for years and years, it can make a very big difference indeed.
“At the time of the dinosaurs, Earth completed one rotation in about 23 hours,” says MacMillan, who is a member of the VLBI team at NASA Goddard. “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 had already realized that some scientific measurements and technologies demanded more precise timekeeping than Earth’s rotation could provide. So, in 1967, they officially changed the definition of a second. No longer was it based on the length of a day but on an extremely predictable measurement made of electromagnetic transitions in atoms of cesium. These “atomic clocks” based on cesium are accurate to one second in 1,400,000 years. Most people around the world rely on the time standard based on the cesium atom: Coordinated Universal Time (UTC).
A positive leap second will be introduced at the end of June 2012.
The sequence of dates of the UTC second markers will be:
2012 June 30, 23h 59m 59s
2012 June 30, 23h 59m 60s
2012 July 1, 0h 0m 0s
The difference between UTC and the International Atomic Time TAI is:
from 2009 January 1, 0h UTC, to 2012 July 1 0h UTC : UTC-TAI = - 34s
from 2012 July 1, 0h UTC, until further notice : UTC-TAI = - 35s
Leap seconds can be introduced in UTC at the end of the months of December
or June, depending on the evolution of UT1-TAI. Bulletin C is mailed every
six months, either to announce a time step in UTC or to confirm that there
will be no time step at the next possible date.
Sources: NASA and U.S. Naval Observatory