The Future of Leap Seconds

Scientists will vote to redefine time in 2015. If time is redefined, there will be no more leap seconds

Brief History

Atomic clocks were invented in the mid 1950s as a more stable and accurate way to measure time. However, they needed to be periodically recalibrated because the Earth wobbles and wiggles, therefore causing random fluctuations in its rotation.

The first leap second was added to the atomic clock at 23:59:60 (UTC) on June 30, 1972. Leap seconds are added (and so far none have been subtracted) to compensate for the Earth’s motion. They keep the difference between UTC (Coordinated Universal Time) and UT1 (mean solar time - observed Earth rotation) below 0.9 seconds. timeanddate.com explains in more detail how a leap second works and why it is used.

The International Earth Rotation and Reference System Service (IERS) (IERS) decides when to introduce a leap second in UTC. Still, many have debated about the need for a leap second and if it should continue.

Suggestions to Eliminate Leap Seconds

The IERS distributed a UTC questionnaire in 1999 about a proposal to change UTC’s definition regarding leap second insertions, as well as possibly eliminating their use. Many organizations began to openly discuss if leap seconds should be removed. Various discussions and meetings have been held regarding the future of the leap second.

For example, a meeting, “ITU-R SRG 7A Colloquium on the UTC timescale”, took place in Torino, Italy, in 2003. Suggestions to discontinue leap seconds in UTC in a few years’ time were discussed at the meeting. It was suggested that time should be decoupled from the Earth’s rotation. Moreover, there was no major consensus on if status quo should be maintained or if an alternative should be pursued.

On July 5, 2005, the IERS sent a notice soliciting comments on a US proposal before the ITU-R Study Group 7 to eliminate leap seconds from the UTC broadcast standard before 2008. It was mentioned that leap seconds could be replaced by leap hours. Criticism on the proposal included the lack of consistent public information about the proposal and adequate justification.

Despite a positive leap second to be added in June 2012, the International Bureau of Weights and Measures is considering abolishing leap seconds all together. The issue will be discussed at the the World Radiocommunication Conference in Geneva, Switzerland, from January 23 to February 17, 2012.

Arguments For and Against Leap Seconds

Many people have made, over the years, various suggestions for and against leap seconds. According to Markus Kuhn, of the University of Cambridge, the main arguments for abandoning leap seconds were:

• Leap seconds could cause disruptions where computers are tightly synchronized with UTC.
• Leap seconds are a rare anomaly to deal with, which is a concern with safety-critical real-time systems (eg. new concepts for air-traffic control entirely based on satellite navigation).
• Exact astronomical time is not significant in most people’s daily lives, and those who need to know about UT1 already know where to look it up.

Dr Kuhn also stated that the main reasons to retain leap seconds include:

• There was a lack of credible reports about serious problems caused by leap seconds.
• The assumption that UTC and UT1 differ by no more than a second is hardwired in many deployed systems (eg. antennas that track satellites) that are costly to modify.
• System designers who worry about leap seconds should use International Atomic Time (TAI) instead of UTC. Therefore easy access to TAI references is needed.
• Desktop computers and network servers can cope with leap seconds. Therefore standardized guidelines on how to steer a computer’s clock around them are needed.
• People must not give up the human practice of defining time through Earth’s rotation because of unfounded worries of some air-traffic control engineers.
• Abandoning leap seconds would break sundials.

Despite calls by some people to retain leap seconds, atomic time advocates also argued that leap seconds were a burden because they were unpredictable.

Leap Seconds in the Future

The IERS sends updates on leap seconds, which can be seen in a table that goes as far back as the first introduced leap second in 1972.