Opinion | Just Hang On a Second (original) (raw)

Opinion|Just Hang On a Second

https://www.nytimes.com/2005/11/05/opinion/just-hang-on-a-second.html

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Op-Ed Contributor

Nov. 5, 2005

LJUBLJANA, Slovenia - SOMETIME between the opening seconds of Tuesday and the closing ones of Friday in Geneva, the world's greatest watch-making center, a decision will be made that has profound consequences for our way of telling time. What's in question is the fate of the leap second.

The leap second may seem insignificant -- a chip off a leap year's block -- which is why it has been left in the hands of the bureaucrats at the International Telecommunications Union, the organization in charge of broadcasting international time signals. But what's really at stake is whether we as a civilization, for the first time in history, decide to uncouple our time-keeping from the rotation of the Earth. That would be, to my mind, a serious mistake.

So what is a leap second? It is one way to reconcile the disparity between two very different time-keeping systems. One is International Atomic Time -- or as it is abbreviated by timekeepers, T.A.I. -- which is calculated by measuring the frenetic vibrations of cesium atoms; it is said to be accurate to within one second every 70,000 years. The other has been in force since before history was recorded: astronomical time. It's entirely subservient to the Earth's rotation. We now call it Universal Time 1, or U.T.1.

The macrocosmic, astronomical, U.T.1 definition of a second is that it's one-86,400th of an Earth day. The microcosmic, atomic, T.A.I. definition of a second is that it's "9,192,631,770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the cesium 133 atom." Put that in your clock and wind it.

Leap seconds allow civil timekeeping to benefit from the precision of atomic clocks while also reflecting the reality of a turning Earth. They're necessary in the first place because the Earth's molten core rumbles and sloshes, which creates a variable spin rate. Meanwhile, the gravitational pull of both the Moon and Sun, interacting with the water that covers 70 percent of our misnamed planet, gradually slows the Earth down as the eons pass. Radio interferometry reveals that the planet's spin is slowing by about two milliseconds per day per century.

The first leap second was in 1972, five years after global time-keeping was definitively transferred to a collective aggregate metronome of what are now around 200 atomic clocks worldwide, and there have been 21 leap seconds since. They're typically the 61st second of the last minute of June or December. They're used whenever the disparity between atomic time and astronomical time reaches 0.9 of a second. Our current time-keeping regimen, based on atomic clocks (T.A.I.) but adjusted to the Earth's rotation (U.T.1) using the occasional leap second, is called Coordinated Universal Time (U.T.C). Anyone interested in precision timing had better get used to abbreviations.

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