Keeping the Leap

The Royal Astronomical Society (RAS) wants to keep the leap second, at least for now. On Tuesday, the organization published a statement opposing a proposal by the U.S. delegation to the International Telecommunication Union (ITU) to abolish these occasional extra ticks. The RAS believes that eliminating leap seconds from official time-keeping would lead to large expenses associated with reconfiguring telescope and satellite operations.

The ITU introduced leap seconds in 1971 as a way to have the time measured by high-precision atomic clocks match the time set by the Sun and stars. These two systems tend to diverge, primarily because friction from tides gradually slows the rate that the Earth spins on its axis and causes the length of the day to increase by about 1.4 milliseconds per century. Currently, the day is about 2 milliseconds too long, so every 500 days or so we are due for another leap second. There have been 22 leap seconds inserted so far, with the next one due on 31 December of this year. The frequency of needed leap seconds will increase as the rotation of the Earth continues to slow. Without leap seconds, atomic clocks would drift an hour away from astronomical time by around the year 2650, says Ronald Beard of the U.S. Naval Research Laboratory.

But adding leap seconds comes with its own problems. There is not a uniform way for implementing extra seconds into computer systems. Therefore, lack of synchronization between different networks could cause glitches in time-sensitive activities such as air traffic control and high-speed business transactions.

The U.S. proposal to the ITU, which broadcasts the official Coordinated Universal Time (UTC), is the first formal attempt to eliminate leap seconds. Yet astronomers have voiced their concern because the programs that align telescopes and locate satellites assume that UTC approximates astronomical time. Eliminating the leap second would require a costly overhaul of that software. "Otherwise you could point your telescope in the wrong place," says Mike Hapgood, the geophysical secretary of the RAS. Still, RAS astronomers admit the current system is not perfect. "We are not against change, but we think there needs to be a proper debate," Hapgood says.

A broader issue is that abandoning leap seconds will sever time from its original relation to the sky above. But Beard, who is deputy chair of the U.S. delegation to the ITU, argues that we are all accustomed to time zones and Daylight Savings, which already shift clocks away from a solar basis. "I don't see this to be any major disconnection with the Sun," he says.

Related Sites
U.S. Naval Observatory site on leap seconds
UTC with record of past leap seconds from IERS
Markus Kuhn's leap second site at Cambridge