Think a second, before leaping! – Part 1
This being a “leap year,” February had an extra day—ending on the “29th,” instead of the 28th. There’s nothing new about “leap days”, of course. They’ve been inserted most fourth-years, since 1582.
What you probably didn’t know, though, is that “leap seconds” are also added to the global timekeeping system occasionally: And that the practice is fueling a worldwide controversy.
Such a minuscule adjustment may seem harmless. But in science, the second is the basic unit of time; and it is fundamental to computer-based systems, such as the Internet, telecommunications and satellite navigation.
The current global time-keeping system is known as Coordinated Universal Time, or UTC. It is derived from calculations based on Earth’s rotation, plus input from 500 or so highly precise atomic clocks, located in various countries and in space. Each GPS satellite contains four such clocks.
Formerly, civil time (for setting watches and clocks) emanated from radio signals broadcast globally, when a certain star-crossed the Greenwich Meridian, in the U.K. Greenwich Mean Time (G.M.T.) became “Universal Time” or, to astronomers, “UT0”. UT0 was later upgraded to UT1 and incorporated into UTC.
UTC consists of Astronomical Time (UT1) and International Atomic Time (TAI). In today’s high-tech world, UT1 alone is unreliable, because it stems ultimately from Earth’s rotation. Our planet wobbles in orbit and spins erratically—slowing down mostly, but also speeding up, occasionally.
The slippage of tectonic plates during the 2011 quake in Japan, for instance, shifted Earth’s crustal mass, causing it to rotate more rapidly—shortening the length of day by 1.6 microseconds. Core-mantle interaction, sloshing ocean waters and atmospheric forces also affect Earth’s rotation.
Overall, though, the planet has been turning slower (with days getting longer) due to the Moon’s tidal tug. According to Vince Summers, of Bright Hub, a day on the early Earth was roughly six hours in duration, compared with 24 now: And day-length continues to increase 1.7 milliseconds each century.
A “millisecond” (or thousandth of a second) seems trifling. But that’s on the scale of the human mind. Advance computing and telecommunication systems, particle accelerators and navigation satellites routinely measure time in millionths, billionths, and trillionths of a second.
Such technology needs precise time-input. Speaking of satellite navigation systems, for example, an Internet blurb of the Thales Group (global security specialists) counsels that “a difference of just one microsecond [a millionth of a second] can lead to a positioning error of 300 metres”.
Likewise, Laura Ost, of the U.S. National Institute of Standards and Technology (NIST), revealed that in 2014 NIST received some eight billion automated requests daily for time signals to synchronize clocks in computers and network devices. Its radio broadcasts updated 50 million watches and clocks per day.
But NIST’s time signals are derived from atomic clocks, rather than Earth’s rotation. Atomic clocks contain detectors, which count the rapid oscillation of certain atoms from one energy state to the other—and use these cycles as units of time. These clocks lose about one second, in 30 million years!
In 1967, the International Bureau of Weights and Measures, in France, defined a second as 9,192,631,770 vibrations of the Cesium 133 atom, as opposed to the astronomically based 1/86,400 of a day.
NIST has since developed even more accurate and stable clocks, including strontium, ytterbium and aluminum quantum logic ion types. The consistency of these devices, which is the basis of UTC, far exceeds that of time systems geared to Earth’s rotation—causing UT1 to lag behind atomic clocks.*
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