| Annett Stein |
Berlin (dpa) – Global time will be set slightly forward by a “leap second” this summer in a small step for clocks that may cause a giant headache for the part of mankind that manages big computer systems.
The extra second will be tacked onto the final minute of June 30 to keep Coordinated Universal Time (UTC), which is measured by extremely accurate atomic clocks, in sync with astronomical time (UT1), which is based on the – somewhat variable – rotation of Earth on its axis.
Without the periodic addition of leap seconds, the difference between UTC and UT1 would continuously widen.
Scientists attribute this to the ongoing deceleration of the Earth’s rotation, caused by natural phenomena such as tidal friction.
“At the moment the difference is 0.5 seconds,” said Andreas Bauch, a physicist at the Braunschweig-based PTB (Physikalisch-Technical Bundesanstalt), Germany’s national metrology institute and responsible for weights and measures. “In several weeks it will be 0.6 seconds.”
By international agreement, the difference between UTC and UT1 must never exceed 0.9 seconds.
Leap seconds were introduced in 1972. There have been 25 of them so far. They are added on the last day of June or December and announced a half-year in advance by astronomers at the International Earth Rotation and Reference Systems Service (IERS), who measure Earth’s rotation and compare it with the time kept by atomic clocks in more than 70 national metrology institutes.
Atomic time defines a second as 9,192,631,770 oscillations of a cesium-133 atom, astronomical time as 1/86,400 of a mean solar day.
“If the difference weren’t corrected, at some point the sun would rise in the afternoon,” said Wolfgang Dick from the IERS Central Bureau in Frankfurt.
The correction can cause trouble, however. While most clocks make the leap without stumbling, some computer software don’t. The leap second in 2012 paralysed a number of websites and temporarily knocked out the booking system of the Australian airline Qantas.
“It’s amazing what a little second can do,” Bauch remarked, pointing out that the more processes depend on integrated times, the greater the chance of a missed or faulty correction at a critical juncture.
Adverse chain reactions are conceivable in electricity grids for instance, he said, because calculations of current flow are made with a temporal resolution – or precision – of microseconds, or millionths of a second. The same goes for calculations of grid load.
A leap second causing an incorrect calculation that then signals an error could prompt a power-line shutdown – “an automatic measure meant to protect the high-voltage network”.
A temporal resolution of milliseconds, or thousandths of a second, is used in air-traffic control, money-market transactions and satellite navigation.
“At such enormous orbital speeds, a discrepancy of a second would put (satellites’) positions somewhere else altogether,” Bauch said.
Safety-related systems could also be affected, noted Fiete Wulff, spokesman for Germany’s Network Agency, which regulates and oversees fair competition in the country’s electricity, telecoms and other markets.
“It has to be checked which systems are susceptible,” he said.
For private companies, leap seconds bring the extra costs of having to alter clocks by hand. It’s hardly surprising that leap seconds are not universally beloved, and abandoning them was formally proposed for the first time in 2001 by the United States.
“It’s not normal for a specific issue like this to be on the agenda for 14 years, but the parties were, and are, at loggerheads,” Bauch said.