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.
So why is the leap second in danger of being dragged out the back door and quietly strangled, metaphorically speaking? And why is the Bush administration, by all accounts, seeking just such a result? Largely, it seems, because many of the timing systems we rely on use a pure atomic standard, with no leavening of leap seconds. These include most notably the American chain of global positioning system satellites. Almost all modern commercial transportation systems now rely on G.P.S. If those advocating an end to leap seconds can be believed, the disparity between atomic-clock-pegged G.P.S. chronometers and the leap-seconds-incorporating U.T.C. clock on your wall complicates navigation and raises the prospect of future catastrophic errors. Particularly because this discrepancy is, of course, increasing. With time.
Those who would retain leap seconds say close to the reverse. Killing leap seconds might well benefit our high technology, but it will be at our expense. Abolishing the leap second will simply export an ever-ratcheting discrepancy elsewhere, raising a host of potentially perilous problems, some foreseeable and some not. And these will also compound as our days and seasons drift free of the clock.
The most vocal advocates of leap seconds include practically all the world’s astronomers, who need their systems to conform scrupulously to the Earth’s rotation. Other threatened constituencies are satellite and deep-space mission controllers, and those whose work links them to the rising and setting of the sun. And then there are people like myself, who simply believe that our corporeal selves are tied to the Earth, or rather the dry bits of a watery planet suspended in a seemingly endless void, and that though our sphere may well turn there at a tempo we’ve recently discovered to be less than flawless, we live here, and not at the subatomic level. We benefit from a sun that has risen at a time determined by that rotation, and no other. To paraphrase Jorge Luis Borges, time may be the substance of which we are made, but the universe, alas, is real – and so are we.
At the very least, there are grounds for concern that a potentially momentous decision might be made with absolutely no input on the issue from the vast majority of time’s “consumers” – meaning us, the non-specialists, the civilian population.
The leap seconds debate is finally freighted with philosophical issues. This temporal marker is a human construct. But it’s positioned strategically between two different orders of scale – the subatomic world of whizzing protons and electrons, and the extensive expanses of solar-system space, in which massive spheres ponderously rotate and wheel. The latter is of course the traditional time-keeping template, the model for the circular faces, orbiting hands and inner wheelwork of the traditional mechanical time-keeper. The former represents the new paradigm that we may be on the verge of shifting to entirely, with scarcely any discussion.
We don’t really know conclusively what will happen if we ignore the evidence of our senses and turn our backs on the cosmos. As one worried astronomer puts it, such a decoupling “would have unknown effects on civil and legal time, and ultimately on people.” But clearly if we take those hourly reaffirmations of civilization’s metronome too much for granted, as Swiss time runs inexorably down to the opening gavel of an obscure meeting of time-keeping mandarins, we do so at our own peril.