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To begin with, we need to understand what the leap-second is and under what circumstances it is added to the clock. Prior to the introduction of atomic clocks to world timekeeping, accurate time was a matter of astronomical record. The Earth’s rotation, as measured against the fixed stars, provided the standard for the day and second. This was the method used for navigation and timekeeping for thousands of years, even into the twentieth century. However, both Earth-based observations and observations from space made during the 1940’s-1960’s showed the inherent flaw in this process—a flaw which had been recognized since the time of the ancient Greeks and for which humankind had compensated with such things as the leap-year and the Gregorian calendar reforms.
First was the discovery that the Earth is not, in fact, a perfect sphere. This impacts upon the Earth’s rate of rotation by exacerbating the effect of the Moon’s gravitational pull on the Earth. The Moon would slow down the Earth’s rotation ordinarily even if the Earth were a perfect sphere. The introduction of an imperfection in the roundness of the Earth creates an irregular moment of inertia which is affected by the Moon’s gravity, thus slowing down the Earth’s rotation. Additionally, the discovery of the Van Allen Radiation Belts and the Earth’s extensive magnetic field in the 1950’s led to the discovery that the Earth’s core and surface rotate at slightly different rates, resulting in not only the generation of Earth’s magnetic field but also friction between the Earth’s surface and its interior layers—again, slowing the Earth’s rotation.
These differences, while unnoticeable over a span of centuries and seemingly of little consequence to the common man, have grave implications for navigation: an error of one second can lead to an error in estimating one’s position on the Earth’s surface of at least approximately 0.49 kilometers and as much as 0.66 kilometers. Attempting to locate stranded sailors, or an enemy fleet, or even one’s own home port then becomes a tricky matter. The advent of aviation has made this development even more important, as air travel inherently allows even less time to correct for mistakes.
Two solutions could be posed to resolve this dilemma: the first would be to lengthen the standard second to allow for a uniform length of day. However, the standard second is used for many other purposes beyond navigation; constantly revising the length of the second would render obsolete scientific texts (and even the very charts and maps which prompted the solution!) on almost a yearly basis. Thus is illustrated a basic principle of metrology: that units should be defined from the smallest feasible unit up. The second solution—standardizing the length of the second and periodically revising the length of the day—was deemed more practical in 1972, when the NIST introduced the first leap second.
The standard second was defined in 1965 in terms of the resonance vibration of the cesium-133 atom as the time the atom takes to complete 9,192,631,770 cycles (the irregular number was chosen to correspond with the earlier definition of 1/86,400 of the mean solar day). This time is measured by an atomic clock; or, in the case of the United States, two atomic clocks: one operated by the NIST at their facility in Boulder, Colorado; the other at the U.S. Naval Observatory. Whenever a difference of greater than 0.9 seconds is observed between astronomical time and the atomic clock, the NIST adds a leap-second to their official time signal. Thus, the variance between the two is never greater than 0.9 seconds. A leap-second has been added almost every year since 1972. While it is theoretically possible that a second may have to be subtracted at some point, according to the NIST this is not likely to be necessary any time in the foreseeable future.
At comparatively little expense, metrology and national
standards have helped to preserve the integrity of navigation and commerce—ultimately
saving millions of dollars and possibly untold human lives—through the
use of the leap-second. It is an important application of metrology
which directly impacts our daily lives.
