According to NASA, June 30 will be one second longer than usual in order to account for changes to the Earth’s rotational speed.
The leap second is to be implemented to account for discrepancies between atomic time and the average length of a solar day.
Leap seconds allow for discrepancies in average length of day
“Earth’s rotation is gradually slowing down a bit, so leap seconds are a way to account for that,” said Daniel MacMillan of NASA’s Goddard Space Flight Center. According to Coordinated Universal Time (UTC), which is based on predictable electromagnetic transitions in atoms of cesium, a day lasts 86,400 seconds.
However according to the speed of the Earth’s rotation, the average day lasts 86,400.002 seconds. The rotation of the Earth has slowed slightly due to the competing gravitational pulls of the Earth, the Moon and the Sun, and scientists believe that the mean solar day has not been 86,400 seconds long since approximately 1820.
A difference of 2 milliseconds may seem negligible, but over the course of the year it adds up to almost a second. The length of each individual day varies due to a number of factors, including the atmosphere. In order to compensate for this, a leap second is inserted on June 30 or December 31.
Adding leap seconds is a controversial practice
Some systems will display 23:59:60 instead of moving directly to 00:00:00, while others will simply be switched off for one second. A leap second can cause problems for computer systems, and the lead up to New Year’s Eve 1999 saw the world whipped up into a frenzy of worry regarding the Y2K bug, which many thought would cause computer systems to crash as the year turned to 2000. However no great problems were caused.
As the world becomes increasingly connected, the potential for disruption caused by the addition of a leap second is on the rise. When the last leap second was added in 2012, outages were suffered on popular websites such as Reddit, Yelp and LinkedIn due to the impact on site stability.
The Qantas check-in system also crashed, grounding over 400 flights and forcing staff to perform check-ins by hand. Other problems were reported with the Linux operating system, as well as computer programs written in Java.
Stock exchanges braced for problems
There are also worries that the leap second could disrupt financial markets. Experts warn that transactions could be delayed if software is not adequately prepared for the addition of the extra second. With global markets already suffering due to the ongoing crisis in Greece, it is imperative that banks prepare themselves in order to avoid further problems.
“There are consequences of tinkering with time,” said Peter Whibberley, Senior Research Scientist in the Time and Frequency group at the National Physical Laboratory (NPL) in the United Kingdom. “Because leap seconds are only introduced sporadically it is difficult to implement them in computers and mistakes can cause systems to fail temporarily,” he continued.
The majority of financial markets across Europe will be closed at the time of the addition, but traders in the United States, Japan, Australia, South Korea and Singapore will all be affected.
Synchronization issues could affect jittery markets
Further complications arise from the fact that various world stock exchanges are taking different approaches to managing the leap second. Asian markets will be open as usual, while U.S. exchanges plan to close early.
“Anyone who conducts transactions with those markets when the change occurs could have a synchronization issue if they are not ready,” said Dr Leon Lobo, Strategic Business Development Manager at NPL.
That said there are methods for minimizing the impact of a leap second, such as modifying servers to add a few milliseconds to updates over the course of the day. Google used this method before, and plans to do so again this June 30.
Despite worries over the addition of a leap second, the process is necessary to correct clocks to the rotation of the Earth. Any changes would take hundreds of years to become apparent, but clocks would eventually display daytime hours during the night if leap seconds were not added.
Opposition to leap seconds is growing due to fears of disruption, but those who defend the practice argue that it is necessary to preserve the connection between the rising and setting of the sun and the human concept of time.
The issue is expected to be the subject of debate at the World Radiocommunication Conference in November 2015. For now we will just have to hope that the June 30 leap second does not cause chaos around the world.