Scientists at the National Physical Laboratory (NPL) will be adding a leap second at 00:59 BST on 1st July to its atomic clocks, to ensure UK time remains synchronized with international time.

The insertion of the leap second is required as Earth does not rotate at a constant speed, whereas atomic clocks, several of which are located at NPL's site in Teddington, are much better at keeping time. Due to the unpredictable nature of Earth's movement, leap seconds are occasionally required to bring atomic time back into alignment with astronomical time. This procedure ensures that on average the Sun remains overhead at noon.

Peter Whibberley, Senior Research Scientist in NPL's Time and Frequency Group, said: "The purpose of leap seconds is to make sure our time scale based on atomic clocks remains in step with the time based on the Earth's rotation. The Earth is a poor timekeeper compared to our clocks, and its rotation changes unpredictably due to changes in its atmosphere and molten core. The leap second correction to our atomic clocks means we get an extra second of summer time."

Historically, time was measured using the passage of the Sun across the sky -- Earth rotation time is still used by astronomers to track stars and spacecraft. Since the start of the 1960s, an atomic time scale, known as Coordinated Universal Time (UTC), has been the world's official time. The stability and global availability of UTC are essential for the smooth operations of satellite navigation and international telecommunications.

Over the last decade there has been considerable debate about the detrimental effects of inserting a leap second on computers and other equipment needing precise time. One minor effect is that some systems fail to implement a leap second at the correct instant and display an inaccurate time, but there is no agreement on the seriousness of this and other problems attributed to leap seconds.

The decision to introduce this year's leap second was taken by the International Earth Rotation and Reference Systems Service (IERS), and timekeeping experts at NPL and other national timing centres will make the necessary changes to the atomic time scales on 00:00 30th June (UTC).

The future of the leap second is one of keen debate among the official international time measurement community. Some countries, including the US, have called for an end to leap seconds, but other countries disagree, and experts at the International Telecommunication Union (ITU) have delayed a decision on the future of the leap second until 2015.
http://www.sciencedaily.com/releases/2012/...20629142607.htm

The ability to accurately measure a second in time is at the heart of many essential technologies; the most recognizable may be the Global Positioning System (GPS). In a paper accepted for publication in the AIP's journal Review of Scientific Instruments, Judah Levine, a researcher at the National Institutes of Standards and Technology (NIST) and the University of Colorado at Boulder discusses how achieving a stable and coordinated global measure of time requires more than just the world's most accurate timepieces; it also requires approximately 400 atomic clocks working as an ensemble.

According to Levine, however, calculating the average time of an ensemble of clocks is difficult, and complicated statistics are needed to achieve greater accuracy and precision. These statistical calculations are essential to help counter one of the most important challenges in keeping and agreeing on time: distributing data without degrading the performance of the source clocks.

All atomic clocks operate in basically the same way, by comparing an electrical oscillator (a device engineered to keep time) with the transition frequency of an atom (one of nature's intrinsic time keepers). This atomic transition is a "flip" in the spin in the outermost electron of an atom -- an event that is predictable with an accuracy of a few parts per ten quadrillion. Comparing the natural and engineered signals produces the incredibly stable "tick" of an atomic clock.

Several algorithms are then used to estimate the time of the reference clock with respect to the ensemble of clocks. These calculations weed out as much error as possible and establish a reliable reference time. Levine notes that there are strengths and weaknesses in each of these statistical steps, but these weaknesses can be mitigated to some extent by also including retrospective data. So in essence, determining the current time relies on understanding how accurately researchers were able to calculate time in the past.

Even the next generation of atomic clocks and frequency standards are unlikely to eliminate the need for these timescale algorithms. However, keeping time and frequency signals and standards the same in all countries is essential and greatly simplifies international cooperation in areas such as navigation, telecommunication, and electric power distribution.
http://www.sciencedaily.com/releases/2012/...20201181451.htm