Every so often, world timekeepers add a leap second to official clocks. Why do we need it? Isn’t the length of a day based on Earth’s rotation, or spin? And isn’t a day 24 hours long? The fact is that Earth isn’t as reliable a “clock” as you might think. The influence of our nearby large moon causes our planet’s rotation, or spin, to slow down continuously. Roughly every 100 years, the day gets about 1.4 milliseconds longer. That’s 1.4 thousandths of a second. It’s not much, but it adds up.
NASA is involved in timekeeping, because – in our modern world – astronomers use distant astronomical objects as reference points in space, against which to measure Earth’s spin. According to Daniel MacMillan of NASA’s Goddard Space Flight Center in Greenbelt, Maryland:
At the time of the dinosaurs, Earth completed one rotation in about 23 hours. 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.
What’s more an official second isn’t what it used to be. NASA says:
In 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).
There is more than one time standard, however, and keeping the time standards in synch is the reason we need leap second.
Universal Time 1 (UT1) is based not on atomic transitions, but on the rotation of Earth on its axis with respect to the sun. Astronomers compute UT1 from measurements made with a technique called Very Long Baseline Interferometry (VLBI). This technique combines the observations from many telescopes located at various points on Earth’s surface. When it comes to timekeeping, VLBI relies on quasars – the most distant known objects in the universe – to provide a reference point in space. Presumably, like everything else we know in space, quasars are in motion. But they are so distant that they appear essentially motionless when viewed from Earth. NASA says:
Originally, leap seconds were added to provide a UTC time signal that could be used for navigation at sea. This motivation has become obsolete with the development of GPS (Global Positioning System) and other satellite navigation systems. These days, a leap second is inserted in UTC to keep it within 0.9 seconds of UT1.
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.
And therein lies the rub with leap seconds. Opponents speak of the cost of planning for leap seconds and the potential impact of adjusting or turning important systems on and off in synch. That’s why the International Telecommunication Union, a specialized agency of the United Nations, has been debating whether we should abolish the leap second. You can read more about that debate here.
Bottom line: The June 30, 2012 leap second is the 35th leap second to be added and the first since 2008. It’s used to keep UTC (time as measured by electromagnetic transitions in cessium atoms) in synch with UT1 (time as measured by Earth’s rotation with respect to distant quasars, via Very Long Baseline Interferometry).