‘Leap second’ creates extra time as Earth’s rotation slows

‘Leap second’ creates extra time as Earth’s rotation slows


It’s not a leap year, and it’s not even a leap day.

International timekeepers are adding a second to the clock at midnight universal time Saturday, June 30, going into July 1. That’s 8 p.m. EDT Saturday. Universal time will be 11:59:59 and then the unusual reading of 11:59:60 before it hits midnight.  The exact precision of an atomic clock does not take into account the slowing of the Earth’s rotation, say scientists, necessitating the minor change.

The move to add a second is not exactly unusual. According to an International Telecommunication Union statement, “adjustments made in one-second steps have been implemented since 1972 to compensate for variations within the framework of UTC.”

Although it would take millennia for the seconds to accumulate noticeably, the International Earth Rotation and Reference Systems Service (IERS) can introduce leap seconds in Coordinated Universal Time (UTC) “at the end of the months of December and June…” in order to keep time on schedule with the Earth. Experts at the IERS make the adjustment when the planet’s movement falls out of sync with atomic clocks used to measure time.

The time it takes the Earth to rotate on its axis — the definition of a day — is now about two milliseconds longer than it was 100 years ago, said Geoff Chester, spokesman at the U.S. Naval Observatory, keeper of the official U.S. atomic clocks. That’s each day, so it adds up to nearly three-quarters of a second a year.

The last leap second, which occurred in 2008, became a matter of debate within the scientific community, with some scientists touting the nanosecond precision of an International Atomic Time that should replace Greenwich Mean Time (GMT). The article describes how the precision of the atomic clock could change our concept of time and cause problems with the sunrise and sunset rotation schedule. Advocates are mentioned as considering the leap seconds “onerous because they are unpredictable.”

Quoted in the article, David Rooney, the Royal Observatory’s curator of time, says leap seconds should be acknowledged as “the best of both worlds.” He continues to elaborate on the way in which “satellites, physicists, and high-frequency traders…benefit from the accuracy of atomic time while keeping our clocks consistent with the position of the sun in the sky— and with GMT.”

The statement from ITU also mentions deferring the question of replacing GMT with International Atomic Time, stating, “The decision has been reached to ensure that all the technical options have been fully addressed in further studies related to the issue. These studies will involve further discussions within the ITU membership and with other organizations that have an interest in this matter and will be referred to the next Radio-communication Assembly and World Radio-communication Conference scheduled for 2015.”

Meanwhile, the American Astronomical Society is officially neutral in response to the debate; however, a paper from the AAS Division on Dynamical Astronomy Working Group on Time and Coordinate System Standards proposes, “the increase of the maximum time difference allowed between UT1 and [UTC] from 0.9 second to 1 hour, effectively eliminating the leap second.” A change such as this would allow for a compromise. Scientists and timekeepers would make the readjustment, but do so far less frequently.

That said, the logistics of adding a second to the official standard bearer has real-world consequences. The leap second causes a host of timekeeping issues, because no clock can accommodate an extra second. Instead, clocks are traditionally stopped at 23:59:59 for one second—but life goes on, and gets in the way. For venues ranging from airports to GPS satellite , the issue is of vital importance. Navigation systems, for example, work by measuring the time it takes a signal to travel between a known satellite location and a receiver. Such systems require extreme precision on the level of nanoseconds, or billionths of a second.

Earlier this year, official timekeepers from across the world discussed whether to eliminate the practice of adding leap seconds. They decided they needed more time to think and will next debate the issue in 2015.