As we rush about our busy lives, time may seem to speed up, making the days shorter, but in reality days are actually getting slightly longer every year. Tidal forces between the Earth and moon, weather changes and tectonic forces are slowing down the rotation of our planet. According to NASA, a day becomes about 1.4 milliseconds longer approximately every 100 years. While this may sound insignificant, it sure is big enough to cause problems in the highly accurate world of timekeeping.
"One solar day is gradually getting longer than 24 hours due to the Earth's slowing rotation," says SANSA Researcher Dr Pierre Cilliers. "A day measured by atomic clocks is exactly 86,400 seconds. However, the Earth's rotation is not quite so precise. It takes our planet about 86,400.002 seconds to complete one rotation."
The fractions of a second per day slowly add up until the difference between solar time and atomic time mounts up to one second. To make up for this time difference an extra second, known as a leap second, was added to the calendar on 30 June 2015. We are now in synch, for a while!
Just as an extra day is added every leap year to keep the Gregorian Calendar aligned with the Earth's orbit around the sun, a leap second is added every couple of years to make up for the longer solar days.
To understand all of this let's go back in time to 1945 when Isidor Rabi, a physics professor at Columbia University, suggested a clock could be made from a technique he developed in the 1930's called atomic beam magnetic resonance. Scientists realised they needed a more precise timekeeping system than the Earth's rotation could provide and Rabi's new method sounded promising.
In 1967 the atomic clock was officially recognised by the National Institute of Standards and Technology (NIST) as the most accurate form of timekeeping. The definition of a second changed! It was no longer based on the length of a solar day but on extremely predictable electromagnetic vibrations in cesium atoms. The NIST-F1 cesium atomic clock is so precise that it loses only one second in 100 million years.
Most people around the world rely on the time standard based on the cesium atom: Coordinated Universal Time (UTC). "UTC time was adjusted this year by inserting an extra second at the end of the day to accommodate the increasing difference between atomic time and solar time," says Cilliers. "For the first time since 2012, the last second of 30 June 2015 was 23:59:60 instead of the usual 23:59:59."
"At SANSA in Hermanus we mostly rely on GPS receivers and NTP time correction to handle the leap second time shift. Data collection instruments provide a date-stamp with each observation, which is synchronised with UTC. The extra second might generate an additional data point and the data administrators decide if they will use the extra data point or not," said Herman Theron, SANSA ICT Manager. "On systems that only generate one minute values, such as the INTERMAGNET geomagnetic observations and the ionospheric scintillation parameters, there is nothing to worry about. The scripts on our ICT systems normally run at fixed timing intervals (absolute time, not period) – so the extra second is not an issue."
While at SANSA's satellite mission control centre in Hartebeesthoek, accurate synchronisation of local clocks with clocks on board the satellites they control is essential. "If ground control stations and satellites are not synchronised to the required level, measurements of satellite orbits will be incorrect and lead to inaccurately calculated orbital parameters, which have repercussions for successful operations," says Eugene Avenant, SANSA's Chief Engineer. "This can result in antennas pointing incorrectly when trying to track low-Earth orbiting satellites and losing valuable Earth observation or science data. Commands for orbital corrections can also be sent in error or at the wrong time, which causes even greater problems for the spacecraft in orbit."
Earth's rotation has slowed by almost half a minute over the last 50 years. An extra second has been added to the clocks 26 times since 1972, usually on 30 June or 31 December of a leap second year. Scientists at the International Earth Rotation and Reference Systems Service (IERS) in France are responsible for adjusting Earth's time when needed. Leap seconds are announced well ahead of time, typically six months, for businesses to prepare for the adjustment to their clocks.
You may be wondering whether or not anyone would really notice if we were a couple of seconds out? In a world increasingly dependent on information and communications technology, absolutely yes!
Navigation and communication systems all depend on exact timing. Financial systems can experience errors if clocks at the transmitting and receiving end of a transaction are not synchronised. Scientists also use time to determine the position of planets, stars, satellites and other objects in our solar system. Measurements come down to fractions of a second to maintain accuracy and consistency. Some computer systems, however, have difficulty processing a 61-second minute and can't complete normal operations. The 2012 leap second resulted in internet servers of several businesses crashing. Qantas grounded 400 flights when its check-in system went haywire while LinkedIn, Reddit, Mozilla and numerous sites running Linux all experienced technical glitches. The NASDAQ stock exchange closed early on 30 June, to ensure the scheduled leap second didn't occur during trading hours. Google and several other businesses have adopted a "smear" campaign by introducing the leap second in millisecond chunks spread out over the course of the day, rather than in a "massive" one second jump at the end of the day.
The debate about the best way to manage the difference between atomic time and Earth rotation time is on-going in the International Telegraphic Union (ITU). Scientists have suggested that leap seconds should be replaced with leap hours every few centuries, which could be planned for well in advance. The USA leads the case for abolishing leap seconds, supported by Japan, Italy, Mexico and France, while the UK, Germany and Canada want to retain leap seconds. The ITU will consider the abolition of leap seconds at the upcoming World Radio Communication Conference in November 2015.