If the day seems a little longer than usual on Tuesday, June 30, 2015, that’s because it will be. An extra second, or “leap” second, will be added at midnight to account for the fact that the solar day is gradually getting longer because Earth’s rotation is slowing down ever so slightly.

Scientists know exactly how long it takes Earth to rotate because they have been making that measurement for decades using an extremely precise technique called Very Long Baseline Interferometry (VLBI). VLBI measurements are made daily by an international network of stations that team up to conduct observations at the same time and correlate the results. NASA Goddard provides essential coordination of these measurements, as well as processing and archiving the data collected. And NASA is helping to lead the development of the next generation of VLBI system through the agency’s Space Geodesy Project, led by Goddard.

From VLBI, scientists have learned that Earth is not the most reliable timekeeper. The planet’s rotation is slowing down overall because of tidal forces between Earth and the moon. Roughly every 100 years, the day gets about 1.4 milliseconds, or 1.4 thousandths of a second, longer. Granted, that’s about 100 or 200 times faster than the blink of an eye. But if you add up that small discrepancy every day for years and years, it can make a very big difference indeed.

By the 1950s, scientists had already realized that some scientific measurements and technologies demanded more precise timekeeping than Earth’s rotation could provide. So, in 1967, they officially changed the definition of a second. No longer was it based on the length of a day but on an extremely predictable measurement made of electromagnetic transitions in atoms of cesium. These “atomic clocks” based on cesium are accurate to one second in 1,400,000 years. Most people around the world rely on the time standard based on the cesium atom: Coordinated Universal Time (UTC).

Another time standard, called Universal Time 1 (UT1), is based on the rotation of Earth on its axis with respect to the sun. UT1 is officially computed from VLBI measurements, which rely on astronomical reference points and have a typical precision of 5 microseconds, or 5 millionths of a second, or better.

Originally, leap seconds were added to provide a UTC time signal that could be used for navigation at sea. This motivation has become obsolete with the development of GPS (Global Positioning System) and other satellite navigation systems. These days, a leap second is inserted in UTC to keep it within 0.9 seconds of UT1.

Normally, the clock would move from 23:59:59 to 00:00:00 the next day. Instead, at 23:59:59 on June 30, UTC will move to 23:59:60, and then to 00:00:00 on July 1. In practice, this means that clocks in many systems will be turned off for one second. This will be the 26th leap second added to a calendar year since the practice began in 1972.

A problem with some GPS receivers implementing the extra second caused the U.S. Civil GPS Service Interface Committee (CGSIC) to issue a notice in February.

During preparation of playback scenarios for the upcoming leap-second event, engineers at Racelogic identified a potential pitfall. The BeiDou uses a different “day number” for the date to apply the leap second, compared with GPS and Galileo. GPS and Galileo use 1-7 as week day numbers, and BeiDou uses 0-6. If this fact has been missed during development, then the result is that the leap second may be implemented a day early on GNSS engines that are tracking the BeiDou constellation

But GPS receivers aren’t the only thing that could be affected. In the past, the extra second has messed with computer systems. The last leap second was added in 2012, and it caused problems for big companies like Reddit, LinkedIn, Gizmodo and FourSquare. The problem is that during the leap second, the computer clock shows 60 seconds instead of simply rolling over to the next minute, or shows the 59th second twice. The computer sees a leap second as time going backward. The machine registers this as a system error, and the CPU can overload. Google, to skirt the problem, will add a millisecond to its servers every once in a while throughout the year. This way, the slowed-down servers don’t notice when an extra second is slipped in.

The Wall Street Journal reported that financial regulators and market participants are worried enough about the leap second that they’re planning for potential disruptions. The adjustment could present technical difficulties for traders and exchanges, as some computers might not be programmed to account for the adjustment, according to a Dow Jones report.


Elizabeth Zubritsky, NASA’s Goddard Space Flight Center
GPS World Staff Writer
Kelly Dickerson, Live science